Middle–late Holocene river terraces in the Erhjen River Basin, southwestern Taiwan—implications of river response to climate change and active tectonic uplift

Geomorphology 38 Ž2001. 337–372 www.elsevier.nlrlocatergeomorph

Middle–late Holocene river terraces in the Erhjen River Basin, southwestern Taiwan—implications of river response to climate change and active tectonic uplift Meng-Long Hsieh a,b, Peter L.K. Knuepfer a,) a

Department of Geological Sciences, Binghamton UniÕersity, Binghamton, NY 13902-6000, USA b Department of Geology, National Taiwan UniÕersity, Taipei, Taiwan Received 13 July 1999; received in revised form 10 October 2000; accepted 13 October 2000

Abstract We reconstruct the Holocene river history of the Erhjen River Žarea: 140 km2 . by correlating river terraces aided by 28 radiocarbon dates. Multiple terraces developed in the lower Erhjen River since the middle Holocene; they converge downstream to the Coastal Plain. The rates of channel incision into bedrock calculated from these terraces at Yuehshihchieh are 7–8 mmryear during ca. 5.7–2.5 ka, 5 cmryear during 1.5–1.3 ka, and 1 cmryear since 1 ka; the last is close to the average incision rate since middle Holocene. Meanwhile, only a single but wide middle–late Holocene paleo-floodplain was developed in the upper Erhjen River; it was completely abandoned only after 0.8 ka, likely following an episode of base-level fall starting from the Coastal Plain. Based on the apparent downstream and upstream convergence of these dated terraces, we identify a doming structure or anticline within the basin, which results in a tilt rate of 10y6 to 10y7 per year in the lower Erhjen River valley. The major terraces here had different initial long profiles, which implies that a critical Žgraded. long profile may not be a prerequisite for formation of a wide erosional terrace surface. Instead, we propose that these terraces were initiated by a series of catastrophic rainfall events, probably climatic-related, which brought a large amount of bedload from hillslopes to prevent the channel from incising when valley widening was facilitated by high-discharge runoff. We find that not only terrace-surface formation but also channel incision can be strongly controlled by climatic-driven discharge and bedload conditions, as suggested by the contrast of bedrock incision rates we observe at Yuehshihchieh. Apparently, the climate and its effects on the landscape cannot be regarded as constant during the Holocene even in a humid tropical area like Taiwan. However, such a fluctuation of climate could only be recorded in a setting where rivers have a high tendency to incise so that multiple terraces can be created. The lower Erhjen River that is characterized by active tectonic tilting is an example of this setting. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Taiwan; Fluvial geomorphology; River terraces; Incision; Uplift

1. Introduction )

Corresponding author. Fax: q1-607-777-2288. E-mail address: [email protected] ŽP.L.K. Knuepfer..

Erosional river terraces are ubiquitous in the Southwestern Foothills of Taiwan, where prominent

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crustal shortening and uplift are occurring as part of the westward-propagating Taiwan orogeny ŽYu and Chen, 1994; Yu et al., 1997.. Recently, by virtue of ongoing highway construction, study of these river terraces has been facilitated in the Erhjen River Basin, one of the major river basins in the Southwestern Foothills ŽFig. 1.. As first dated by Chen Ž1993. and Lee et al. Ž1994., the extremely AyoungB age of the terraces in this river basin is quite impressive: some abandoned floodplains now more than 30 m above the modern channel are merely ca. 2 ka in age. The rate of channel incision into bedrock measured from these terraces is on the order of centimeters per year, which is probably one of the highest rates documented ŽBurbank et al., 1996; Leland et al., 1998.. The causes of these young river landforms, however, cannot be readily explained by the known concepts of terrace genesis. The dated late Holocene terraces in the Erhjen River Basin are paired and continue through much of the river basin. The contrast between these up-to-kilometer-wide terrace surfaces and the modern narrow, incising channel ought to reflect distinct changes of external environments. However, eustatic sea level has been more or less stable since middle Holocene. In addition, as we will present in this paper, the Holocene tectonic style of the river basin is dominated by broad uplift within the mountains rather than uplift concentrated across range-bounding faults. The formation of these terraces therefore cannot be attributed to episodic faulting that intermittently lowered base level, as envisioned in many other studies ŽBull, 1991.. One might still link these terraces to episodic tectonic activity. However, it is arguable whether the rate of the change of river gradient due to the broad tectonic tilting could be sufficient to trigger the repeated changes of lateral and vertical river erosion that we observed. The creation of these river terraces might involve climatic changes; yet, one may equally wonder if any significant climatic change could have occurred in the Holocene in this humid tropical setting. These uncertainties might reveal our lack of knowledge of river response to AminorB or short-term Žsay, thousands of years. tectonic and climatic processes ŽMerritts et al., 1994. or of the processes themselves. Specifically, although we recognize that

channel incision and increase in sediment supply occur after long-term mountain building and that channel incision occurs in response to increased river slope caused by faulting, it remains unclear about how minor but gradual increases of river slope over the short term should or could affect channel behavior as well as sediment supply. Similarly, while the process of glacial–interglacial climatic change and its effect on river landscape change are reasonably well known, the characteristics and effectiveness of presumably minor climatic changes during the Holocene in a non-glaciated mountain setting like Taiwan have not been well documented ŽMeyer et al., 1995.. This insufficiency also influences how one could apply physically based hypotheses Žor models. to interpret or predict landscape evolution in the time frame of concern to us. This is in part because many variables in the physical models are themselves strongly dependent on either tectonic or climatic factors. For example, channel bedrock incision rate has been formulated to be a function of sediment grain size and supply ŽSklar and Dietrich, 1998.. However, such an equation could hardly be applied to real landscapes until adequate knowledge of how tectonic or climatic factors govern the sediment grain size and supply can be obtained. In addition, equations formulating fluvial processes may not fully reflect the nature of tectonic or climatic processes. For example, stream power, boundary shear stress, or velocity are commonly used to model a river’s ability to incise bedrock ŽHoward and Kerby, 1983; Seidl and Dietrich, 1992; Pazzaglia et al., 1998., laterally erode banks ŽHooke, 1995., and transport sediments ŽYalin, 1977.. Since slope and discharge have a similar numerical contribution to these physical terms, one may wonder if the effects of tectonic uplift Žincreasing slope. and climatic change Žincreasing discharge. on the fluvial system can ever be differentiated. All these insufficiencies in fact point to the lack of field evidence to demonstrate how a river responds to a given short-term tectonic andror climatic change or how a change of tectonic andror climatic process could have driven a given landscape evolution. The Erhjen River terraces have the potential to help us address these problems. These terraces are all underlain by weakly resistant mudrock, some of

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Fig. 1. Location and geological setting of the study area. Ža. Geodynamic framework of Taiwan. Open arrow with rate shows the present movement of the Philippine Sea plate relative to the Chinese continental margin ŽYu et al., 1997.. Thick lines with triangle barbs represent subduction zones with barbs on overriding side; thick dashed lines with triangle barbs mark the approximate deformation front of the collision zone Žmodified from Ho, 1986.. Žb. The Erhjen River system. This study focuses on the main-stem draining the hill area Žheavily shaded area.. Structures underlying the Coastal Plain are based on Sun Ž1964..

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which even forms badlands. Tropical typhoons commonly bring disastrous floods to the basin. The basin is undergoing extremely rapid tectonic uplift. Given these conditions, it is not surprising that drastic river changes have occurred in this basin in the recent past. Apparently, the landforms of the Erhjen River Basin are sensitive to even minor environmental changes, which provides two potential benefits in the study of river terraces ŽHoward and Kerby, 1983.. First, minor environmental changes and their nature may be suitably recorded by these young terraces. Second, it should be possible to detect the river’s response to co-existing environmental changes Žbase-level, tectonic, and climatic changes., if any. We realize that how much we can learn from these terraces, which likely had complicated origins, depends on how well we can independently derive river evolutionary history, base-level, tectonic, and paleo-climatic information for this river basin. In this paper, we present the complexity of the sequence of river terraces in the Erhjen River Basin. The Holocene history of the river and that of the Coastal Plain adjacent to the mountain front of the basin are reconstructed by examining these terraces, aided by 28 radiocarbon dates, including those previously reported by Chen Ž1993. and Lee et al. Ž1994.. We evaluate the regional tectonic pattern from the shape and sequence of these terraces. We do not assume that the tectonic uplift rates are the same as the channel bedrock incision rates we obtain, but we do assume that they are close Žespecially on the long term.. This information allows us to quantitatively assess the tectonic effect of changing river slope Žtilting rate. on fluvial behavior, which can be compared with the effects of such climatically influenced variables as changing river discharge and sediment load on the fluvial system. Specifically, we focus on answering the following questions: Ž1. Must the initiation of the major Žwide and continuous. terrace surfaces in the Erhjen River Basin require a change from unstable to stable base levels, or from active to relatively inactive tectonics, or an attainment of a specific slope or long profile, as proposed in some studies ŽBull, 1990, 1991.? Ž2. Could a particular climatic-driven hydraulic and bedload condition be responsible for the initiation of these terrace surfaces? Ž3. What causes the abandonment of these major terrace surfaces? Are they necessarily related

to those that initiated development of the terrace surfaces? We then use the answers of these questions to generalize guidelines for interpreting river terraces of different origins. Our ultimate goal is to use these young terraces to achieve greater understanding of river incision and lateral-erosion processes in response to base-level, tectonic, and climatic changes —knowledge that may not be as readily obtained from landforms in a relatively immobile setting or formed by small rates of processes.

2. Study area—geological and geomorphic setting 2.1. Geological setting The Taiwan orogeny is the result of arc–continent collision between the Philippine Sea plate and the Chinese continental margin that began about 4–5 ma ago ŽFig. 1a; Chai, 1972; Biq, 1973; Bowin et al., 1978; Ho, 1986; Teng, 1990.. The Southwestern Foothills are situated on the frontal zone of this orogenic belt ŽFig. 1a.. It is a hillyrmountainous region bounded to the east by the Central Range up to 3000 m in elevation and to the west by the Coastal Plain generally no more than several tens of meters above sea level ŽFig. 1b.. The Southwestern Foothills are underlain by folded and thrusted Neogene sedimentary rocks, in contrast to the pre-Neogene metamorphosed Central Range and the relatively undeformed Coastal Plain ŽHo, 1982, 1986.. The westward younging and decreasing deformational intensity of the strata thus indicate westward propagation of the orogenic belt. The Erhjen River originates in the southern part of the Southwestern Foothills, flowing westward across the Coastal Plain before entering the Taiwan Strait ŽFig. 1b.. The hill region of the Erhjen River consists of two sets of sedimentary rocks: pre-collisional, late Miocene sandstone and shale formed on the stable continental margin and a syn-collisional Plio–Pleistocene foreland sequence; the latter is dominated by mudstone at least 4 km in thickness ŽFig. 2; Ho, 1986; Lin, 1991.. These rocks are cut by four major west-verging reverse faults trending in N–S to NNE–SSW directions ŽFig. 2.. Three of them Žthe Lungchuan, Pingchi, and Chishan faults. thrust Miocene rocks in the hanging wall over Plio–

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Fig. 2. Geological and geomorphological framework of the Erhjen River basin Žfor location, see Fig. 1.. Location of the mud volcanoes is based on Shih Ž1967.; structures are according to Chinese Petroleum Ž1971.; rock chronology is based on Lin Ž1991..

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Pleistocene strata in the footwall; one Žthe Kutingkeng fault. is developed entirely within the Plio– Pleistocene rocks ŽFig. 2.. A Pleistocene limestone ridge Žthe Takangshan Ridge. rises above the southern bank of the main-stem Erhjen River in the mountain-front area ŽFig. 2.. This ridge, along with similar limestone ridges on the Coastal Plain, is underlain by Plio–Pleistocene mudrock. Huang Ž1993. argued that these mudrocks were domed by diapirs Žor by thrust faults according to Lacombe et al., 1997. to form submarine mounts on which the lime reefs grew. Analogous structures are found today offshore of southwestern Taiwan; some appear to link to the limestone ridges inland ŽHuang, 1993; Liu et al., 1993.. A notable feature of the Erhjen River Basin and the adjacent Coastal Plain is the occurrence of active mud volcanoes that line up approximately along some of the major reverse faults ŽFig. 2; Shih, 1967.. These mud volcanoes indicate that the fine-grained material underlying the region still has high fluid pressure.

roughly by the Lungchuan fault. The upstream valley has a maximum width of 2.5 km measured from the major river terraces, which is double that of the downstream valley ŽFig. 2.. The upstream valley is also characterized by linear valley walls and terrace scarps, in contrast with those in the downstream valley that are all outlined by meander bends ŽFig. 2.. 2.2.2. Climate The Erhjen River Basin has a tropical monsoon climate. The region is dry in winter as moisture is blocked by the lofty Central Range. Precipitation comes mainly from summer rainfall, especially associated with tropical typhoons ŽWu, 1992.. The major drainage divide of the basin has a mean annual rainfall of 2000–2500 mm, decreasing toward the Coastal Plain ŽHydrological Year Book of Taiwan, R.O.C.. to ca. 1750 mm in the Tainan area ŽFig. 1.. The annual precipitation record of the Tainan area since 1900 is shown in Fig. 3 ŽCentral Weather Bureau climatic data: Tainan Station..

2.2. Geomorphic setting 2.2.1. OÕerÕiew The main stem of the Erhjen River has a drainage basin of 140 km2 within the Foothills ŽFig. 1b.. The highest peak of the drainage divide is about 450 m in elevation, and the mean gradient of the main-stem channel is 1:75. The river carries abundant finegrained sediments. The mean suspended load of ca. 1.3 = 10 7 tryear, or 9.3 = 10 4 trkm2ryear, is the highest among the major rivers in Taiwan ŽHydrological Year Book of Taiwan, R.O.C... The Erhjen River also carries a gravel bedload, presumably from Miocene sandstonershale ridges. The morphology of the Erhjen River Basin is strongly controlled by underlying bedrock lithology. The major drainage divides that exceed 250 m in elevation are all composed of relatively resistant Miocene rock Žexcept the Takangshan Ridge which is composed of limestone. ŽFig. 2.. The lower areas surrounding these ridges are underlain by Plio–Pleistocene mudstone ŽFig. 2.. This mudrock is either poorly consolidated or densely fractured and in many places forms badlands—the reason why the river carries such a high suspended load. The main stem Erhjen River valley can be separated into upstream and downstream parts,

2.2.3. Coastal plain The Coastal Plain controls the base level of the Erhjen River in the hills. The Holocene relative sea-level history is the net result of eustatic sea-level movement and tectonic uplift. Thus, although the eustatic sea-level probably reached a maximum of q2 m around 4 ka in Taiwan ŽChen and Liu, 1996., part of the Coastal Plain bounding the mountain front may have started to emerge before the middle Holocene when the rate of tectonic uplift exceeded the rate of the eustatic sea-level rise ŽChen, 1993.. The inner part of the Coastal Plain between the Tsengwen and Erhjen Rivers ŽFig. 1. generally has an elevation of 40 m. Here, marine shells 7–12 m below the surface have been dated from 8400 to 9200 YBP 1 ŽHashimoto, 1972; Chen, 1993; Lee et al., 1994.; shells 4 m below the surface have been dated at 6660 YBP ŽHsu et al., 1968.. The timing of the emergence of this portion of the Coastal Plain is estimated as 6–7 ka, and the tectonic uplift rate as

1 All radiocarbon dates are reported by calibration to calendar years BP following Stuiver and Riemer Ž1993., unless otherwise noted, and are marked as YBP for simplicity.

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Fig. 3. Annual rainfall record of Tainan Station for 1900–1996 ŽCentral Weather Bureau climatic data..

5–7 mmryear, based on the Holocene eustatic sealevel curve of Chen and Liu Ž1996. and assuming steady tectonic uplift ŽChen, 1993.. The Coastal Plain bounding the southern part of the Southwestern Foothills is not a uniformly declining surface ŽFig. 1; Sun, 1964.. Two distinct topographic features occur across the lower Erhjen River

ŽFigs. 1 and 4.. One is the steep gradient of 0.005 of the surface bounding the mountain front between altitudes of ca. 45–25 m ŽFig. 4.. The other is the rather flat Chungchou marine-terrace surface in the middle part of the plain with an elevation of 15–25 m above sea level; it drops relatively abruptly on the west Žwith a gradient of 0.008. before it grades to the

X

Fig. 4. Profile across the Coastal Plain. All surfaces are projected to the lines parallel to the general dip of the surfaces on the southern ŽAA . X Y and northern ŽAA . banks of the main channel Žfor locations, see Figs. 1 and 5.. Height and length of each rectangle represent the uncertainty in determining the surface elevation Žfor terraces. and distance between successive 5-m contours Žfor modern channel.; in cases where the projected terraces partly overlap, only one rectangle covering the projected terraces is shown. Position 0 km is located at the Nanhsiung Bridge ŽFig. 5..

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modern shoreline ŽFig. 4.. It is uncertain, however, to what extent these two morphological features are caused by tectonic deformation andror by marine depositionalrerosional processes. Whatever the cause, the long-term, overall incision of the Erhjen River into the Coastal Plain should reflect the progressive steepening of the entire Coastal Plain.

3. Method of surveying and terrace correlation We mapped terraces and searched for surface faults from aerial photographs and plotted them onto photo based, 1:5000 scale topographic maps issued by the Department of Agriculture and Forestry of Taiwan. The elevations of the terrace surfaces were determined directly from these maps. As the maps have a 5-m contour interval, we assigned an uncertainty of 5 m to each elevation. The elevation information in the mountain front area is also aided by highway construction maps with 1-m contour interval Žfrom the Taiwan Area National Expressway Engineering Bureau.. Hand leveling and other survey methods were used in the field to measure or estimate the thickness of sediments and the height of other targets Ždated samples, bedrock surfaces. above local datum Že.g., modern channel surface.. Such measurements, generally having values of less than 20 m, are assumed to be more accurate than those determined from the topographic maps; in most cases, the former do fall into the uncertainty of the latter. Terrace correlation is based on continuity of terrace geometry Žpattern and relief. from aerial photographs, aided by radiocarbon dates. For two neighboring terraces to be correlated, they must have relative relief of less than a few meters and generally be less than a few hundred meters apart along the valley ŽFigs. 4–9.. This sometimes allows more than one possible correlation of terrace remnants before obtaining chronological constraints, if terraces with different levels are close together in both elevation and along-valley position. We do not attempt to correlate across significant position gaps between terrace remnants, unless additional chronological data are available. The major terrace surfaces and the modern floodplains are used to define the mid-valley line by connecting the midpoints between their boundary walls. All the terraces are projected to this

mid-valley line for defining their long profiles ŽFigs. 4, 8 and 9.. Ideally, the terraces are projected perpendicular to this line; however, where the line significantly curves andror valley width changes, such a projection becomes uncertain or subjective. In these cases, the projection is done to minimize the distortion of the terraces. We sought wood or plant fragments within the terrace deposits for radiocarbon dating. All of the radiocarbon ages are calibrated to calendar ages using the program of Stuiver and Reimer Ž1993.. Ages are presented in the text as calibrated age ranges incorporating one standard deviation. In cases of more than one possibility for each age range, only the minimum and maximum values of these multiple ranges are presented; for brevity, the centroid of the calibrated age range is used on some figures ŽFigs. 8 and 9.. The significant uncertainty Ž5 m. in determining terrace elevation precludes resolution of secondary terraces that differ in elevation by less than a few meters. The correlation of terraces in this study thus can only be regarded as first order. The available radiocarbon dates Žsee below. indicate that terracesurface formation and incision are very rapid Žfor example, centimeter-order channel incision rates.. Many of the secondary terraces therefore could have been created in a few hundred years or less. Such a time interval may fall within the intrinsic uncertainty of the radiocarbon ages, considering the occurrence of the dated samples. We doubt that these secondary terraces can be correlated with confidence at present even with more precise elevation data. Instead, we use a few hundred years as an uncertainty for the formation time of each mapped terrace surface which can have a vertical irregularity of up to 5 m. Resolution of multiple terraces within this time frame therefore requires extremely rapid channel incision after formation of each terrace surface.

4. River terraces in the Erhjen River Basin 4.1. Morphology and sequence The distribution and long profiles of the river terraces in the Erhjen River are shown in Figs. 2 and 5–9. The interpretation and correlation of the se-

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quences of the terraces are complicated by the varied numbers along the river. For the convenience of mapping, four types or groups of terraces are defined, based on either their relative elevation above the modern channels or their along-valley continuity. Low terraces Žor modern floodplains. are those adjacent to and within several meters above the modern channels. Major terraces are those that can be correlated throughout a significant portion of the river valley. Where more than one possible correlation exists, they are correlated to make the terrace long profile as smooth as possible ŽFigs. 8 and 9.. The remaining terrace remnants are referred to as minor terraces when they are close to or lower than the major terraces Žlabeled by small letters. or high terraces when they are higher than the surrounding major and minor terraces Žlabeled by capital H.. In addition to the elevation contrast, the minor and high terraces can also be differentiated by the valley morphology they delineate. The paleo-valley boundary outlined by major terraces is well defined. The minor terraces are located inside this boundary and have little influence on the valley morphology defined by the major terraces ŽFig. 6.. In contrast, high terraces commonly occur as flat ridges or mounts; the valley boundary outlined by them is usually vague—a result of extensive erosion after incision below these surfaces. High terraces therefore are located outside the valley boundary defined by the major terraces ŽFig. 6.. 4.1.1. Low terraces (modern floodplains) The modern channel of the Erhjen River is mainly confined by vegetated or mudrock banks. Instead of AactiveB floodplains, low terraces Žup to more than 5 m in height. have developed above the modern channel. Some of these low terraces, including the abandoned meander bends in the mountain-front area ŽFig. 5, profile BBX . and at Yuehshihchieh ŽFig. 6, profile CCX ., were recorded as having been abandoned during 1904–1926 following meander cutoffs ŽChang et al., 1996.. These surfaces were still frequently flooded until the middle 1980s, according to conversations with local residents. Floods in 1997 and 1998 inundated parts of these surfaces again. We therefore regard these low-level surfaces as modern floodplains ŽFigs. 5–7. that have been formed during the last few hundred years and still can be occupied

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by infrequent floods. Terraces with similar heights above the modern channel are also commonly developed in other river basins in the Southwestern Foothills. Several unpublished radiocarbon dates indicate that these terraces, even densely vegetated and with heights of up to 10 m above the modern channel, are no older than 300 YBP.

4.1.2. Major terraces Most of the major terraces are paired; the paleofloodplains defined by them are at least a few hundred meters in width ŽFigs. 6 and 7.. We identified four major terraces along the main stem: KT1 and KT2 downstream of Kuting, CP1 upstream of the Lungchuan fault, and CP2 upstream of Kuting ŽFigs. 6 and 7.. The differentiation of Terrace CP2 from terraces KT1 and KT2 in Kuting reflects the uncertainty in correlation, as Terrace CP2 could continue downstream to either Terrace KT1 or KT2 ŽFig. 8.. The most likely interpretation is that Terrace CP2 diverges downstream to terraces KT1 and KT2 ŽFig. 8.. Terraces CP1 and CP2 converge upstream from Chungpu on the right bank, resulting in an inclined surface more than 1 km in width, although we still map them as two terraces based on their trend along the valley ŽFig. 7, profile DDX .. Other important observations of these major terraces include: Ži. The long profiles of terraces KT1 and KT2 are parallel between Chungte and Kuting, and both terraces increase in height above the modern channel upstream ŽFig. 8.. However, the two terraces gradually converge downstream toward the Coastal Plain downstream from Chungte ŽFig. 8.. Žii. Terraces KT1 and KT2 have significantly higher gradients than those of terraces CP1 and CP2 ŽFig. 8. —part of the reason why we separate Terrace CP2 from terraces KT1 and KT2 . Žiii. Terrace CP2 shows no discernible offset or deformation across the Lungchuan fault; neither are terraces KT1 and KT2 offset or deformed across the mountain front ŽFig. 8.. Živ. Terrace CP1 appears to be tilted in the upstream direction along the 2-km long reach upstream of the Lungchuan fault and converges with Terrace CP2 farther upstream ŽFig. 8.. Žv. Terrace CP1 has an asymmetric cross-valley profile. This terrace, including an alluvial fan, is

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Fig. 5. Morphology of the Coastal Plain, distribution of the river terraces adjacent to the mountain front Žfor location, see Fig. 2., and X Y radiocarbon dates and sites. AA profile is parallel to the general dip of the Coastal Plain and high-terrace surfaces in the right bank of the main channel; dates are projected onto the cross section following the general strike Žcontour lines. of the surfaces. Stratigraphic boundaries in both sections are shown by solid lines where exposed or by dotted lines where inferred.

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Fig. 6. Distribution of river terraces between the Lungchuan fault and the mountain front Žfor location see Fig. 2. and radiocarbon dates and sites.

wider and higher on the true right bank than on the opposite bank ŽFig. 7, Profile DDX .. In this valley,

which is characterized by straight walls and terrace scarps, the terraces occupy a linear position centered

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Fig. 7. Distribution of river terraces upstream of the Lungchuan fault Žfor location, see Fig. 2. and radiocarbon dates and sites. Terraces CP1 and CP2 merge upstream of Chungpu but their contact is still shown as a dashed line. Other notes as in Fig. 5.

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on the trend of Tributary 27 rather than a sinuous position centered on the mid-valley line ŽFig. 7.. The valley sharply narrows upstream from the junction of Tributary 27, Terrace CP2 terminates, and only Terrace CP1 continues farther upstream ŽFig. 7.. Near Hengshan, another terrace ŽTerrace CP1a . is mapped parallel to Terrace CP1 but - 5 m higher; Terrace CP1 disappears as the valley narrows upstream ŽFig. 7.. Terrace CP1a extends toward the headwaters and eventually converges with the modern floodplains ŽFigs. 7 and 8.. Žvi. These major terraces continue upstream into some of the major tributaries: Terrace KT1 extends to both tributaries 1 and 4 ŽFigs. 2 and 9.; terraces CP1 and CP2 extend to Tributary 17 ŽFig. 7.; Terrace CP2 continues into Tributary 27 ŽFig. 7.. Interestingly, Tributary 8, a major tributary with a basin area Ž9.7 km2 . smaller than tributaries 1 and 4 Žboth 19.8 km2 . but larger than Tributary 17 Ž8.3 km2 . and Tributary 27 Ž4.5 km2 ., has no river terraces ŽFig. 2..

4.1.4. Underfit low-order Õalleys Valley bottoms with uniform widths of more than 50 m are also developed in many headwater valleys underlain by Plio–Pleistocene mudstone Žexamples are shown in Tributary 6 in Fig. 6; marked as modern floodplains.. These valley bottoms lack active channels Žalthough they are mapped as modern floodplains in Fig. 6., which may be due Žin part. to cultivation. Their widths, even greater than many of the floodplains of high-order rivers, appear to be too large for their extremely small drainage basin areas Žon the order of 0.1 km2 .. Furthermore, these valley bottoms merge downstream into narrow, steep channels that join the trunk rivers, which leads us to interpret that they are relict features formed before the current headward incision of the low-order tributaries. In many cases, these valley bottoms project downstream to the major terraces in the trunk rivers.

4.1.3. Minor and high terraces Most of the minor terraces are developed in the main stem between Chungte and the Lungchuan fault ŽFig. 6.; most of them are preserved only locally. Only terraces e and f in the Yuehshihchieh–Kuting area could be correlated for more than 1 km along the valley ŽFigs. 6 and 8.. Although high terraces cannot be correlated as extensively as major terraces, Terrace H H along the downstream part of Tributary 1 is impressive ŽFigs. 2 and 9.. This terrace appears to be the up-valley continuation of the Coastal Plain ŽFigs. 2 and 6.; it widens to about 1 km in Hsinhsing and serves as the drainage divide separating the Erhjen River Basin from the Akungtien River Basin to the south ŽFigs. 1 and 2.. Other relatively extensive high terraces include Terrace H K at Kuting ŽFig. 6. and Terrace H C upstream of Chungpu ŽFig. 7.; the latter is composed of a series of incised alluvial fans. Some of the high terrace surfaces are not apparent because they are preserved as ridges or mounts, and their recognition relies mainly on exposures of fluvial deposits in the field. Thus, we may not have identified all of the high terraces in this study. Nonetheless, at one time, these higher and older surfaces may have been even wider than the present major terraces.

4.2.1. Pre–middle Holocene fluÕial and marine sediments in the mountain-front area Fluvial sediments with a maximum thickness of at least 25 m are exposed underlying the Coastal Plain and its correlated high terraces near the basin outlet of the Erhjen River. They are dominantly planar stratified silts or alternating siltsrmuds and commonly incorporate plant debris, burrows, and root casts. We interpret these fine-grained sediments as overbank deposits. Dark, organic-rich beds Žeach tens of centimeters thick. are also associated with these overbank deposits; they are probably paleosols. Cross-bedded sandy beds, as thick as 10 m, which we interpret as channel deposits, are only observed in the northern tip of the Takangshan Ridge Žsample 1 dating site; Fig. 5.. Available dates from these fluvial sediments are all earlier than middle Holocene in age ŽTable 1; samples 1–5; Fig. 5.. The oldest age, 13,169–13,360 YBP Žsample 1., is obtained from sandy channel deposits exposed in the northern tip of the Takangshan Ridge Žthe outlet of the Erhjen River Basin. ŽFig. 5.. The sample is an aggregate of herbaceous fragments collected about 24 m below the terrace surface Žor ca. 10 m above the modern channel level.. Nearby, wood fragments within overbank muds 12 m and 9 m below the terrace surface

4.2. Terrace deposits and chronology

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are dated at 9276–9444 YBP Žsample 4. and 8582– 8954 YBP Žsample 5., respectively ŽTable 1; Fig. 5..

Two pre-Holocene dates are also obtained between the main-stem Erhjen River and its tributary the

M.-L. Hsieh, P.L.K. Knuepferr Geomorphology 38 (2001) 337–372

Sungtzuchiao River ŽFigs. 2 and 5.: one, 11,269– 12,054 YBP Žsample 2., is obtained from an assemblage of wood fragments associated with basal gravels of 0.5 m in thickness; the other, 10,228–10,385 YBP Žsample 3., is from a wood fragment within overbank muds. Both are sampled about 17 m below the local terrace surface. These pre–middle Holocene fluvial sediments in the mountain-front area are replaced shoreward by marine sediments. We recognize two marine sedimentary units. One consists of planarrcross-bedded sands with Žoccasionally. marine shells, which we interpret as a beachrshoreface deposit. The sands, up to 5 m in thickness, are uniform across outcrops more than tens of meters in length ŽFig. 5, profile BBX .. This unit is coarser, much better sorted, and has an overall tabular geometry in outcrop, all of which distinguish it from adjacent fluvial deposits. The other unit consists of structureless muds with marine shells. The shells, although fragile, are commonly articulated and well preserved, suggesting that they were buried in growth position. We interpret these deposits as of lagoonalrestuarine origin. Locally they contain wood, which is especially abundant at the sample 7 radiocarbon site ŽFig. 5. where some appears to be buried in growth position. It is difficult to distinguish this lagoonalrestuarine mud unit from fluvial overbank sediments in the field without marine shells, so it is difficult to constrain the extent of this unit. However, massive mud with maximum thickness of 12 m is exposed filling an erosional trough underlain by stratified fluvial sediments at one site ŽFig. 5, profile BBX .. The available dates from these marine sediments are all early Holocene in age ŽTable 1; samples 6–11; Fig. 5., stratigraphically consistent with those obtained from fluvial sediments landward ŽFig. 5, X Y . and those previously reported from the profile AA Coastal Plain to the north ŽHsu et al., 1968;

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Hashimoto, 1972; Chen, 1993.. Ages of 9048–9369 YBP Žsample 6. and 8492–8589 YBP Žsample 7. are obtained from wood fragments within lagoonalr estuarine muds about 30 m and 38 m in elevation, respectively ŽFig. 5, profile BBX .. An age of 7940– 8101 YBP Žsample 8. is obtained from an assemblage of shells within lagoonalrestuarine muds about 41 m in elevation in the Kueitung River valley ŽFigs. 2 and 5.. These ages are consistent with that of 8557–8941 YBP Žsample 9. dated by Chen Ž1993. within beachrshoreface deposits about 37 m in elevation exposed in the Sungtzuchiao River ŽFig. 5.. Chen Ž1993. also obtained two ages, 8979–9143 YBP Žsample 10. and 6572–6728 YBP Žsample 11. from the Coastal Plain 2 km from the mountain front ŽFig. 5.; the former is sampled from lagoonalr estuarine muds about 16 m in elevation, and the latter is from beachrshoreface sands about 28 m in elevation Ž1 m below the surface.. A similar woodbearing lagoonalrestuarine mud sequence was also described by Chen Ž1993. in the Coastal Plain ca. 4 km to the north and has an age identical to our sample 6 Ž9048–9369 YBP.. 4.2.2. 4.5-ka FluÕial sediments in the mountain-front area Fluvial sediments 15 m thick and 50 m wide were exposed in a roadcut at a side ridge bounding a high terrace that consists of beachrshoreface and lagoonalrestuarine deposits Žsample 24 dating site; Fig. 5, profile BBX .. They are mainly composed of stacks of ) 10 m scale cross-bedded strata that can be further subdivided into at least three fining-upward sequences. Each sequence starts at the base with a thin Ž- 1 m. layer of cross-bedded pebbles and fine sands and terminates at the top by an organic-rich, paleosol-like mud bed. We interpret these deposits as the products of progressive lateral migration and aggradation of channels. Two dates,

Fig. 8. Ža. River terrace long profiles of the main-stem Erhjen River Žfor distribution, see Figs. 5–7.. Terraces are projected along the valley defined by the terraces lower than high terraces; other notes are as in Fig. 4. Only terraces on the right bank of the main channel upstream of location 16 km upstream are shown to avoid confusion and complication caused by the projection of the asymmetric terraces with respect to the valley axis. Žb. Interpretation of the river terrace sequences in the main-stem Erhjen River. Radiocarbon dates Žcal. year BP. are shown on the top of the dated terraces.

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352

Table 1 Radiocarbon ages, sample occurrence and associated terrace information in the Erhjen River and the adjacent Coastal Plain Sample Lab. no.a

Terrace Altitude Žm. C-14 age Sampler Bedrock surf.r Žyear BP. terrace modern surface channel

1 2 3 4 5

NTU2369 NTU2387 NTU2589 NTU2523 NTU2321

C.P.d C.P. C.P. C.P. C.P.

18r42 33r50 31r48 31r43 34r43

- 15r8 33r14 28r7 - 25r8 - 25r8

6 7 8 9 10 11 12 13 14 15 16

NTU2358 NTU2311 NTU2615 NTU1465 NTU1447 NTU1194 WK4111 NTU2614 WK6132 NTU2427 NTU1337

C.P. C.P. C.P. C.P. C.P. C.P. HY HT HC KT1 KT1

30r38 38r44 41r46 37r44 16r29 28r29 76r81 84r96 88r95 34r38 51r57

17 18 19 20 21 22 23 24 25 26 27 28 29 30

NTU2375 NTU1968 NTU2361 NTU1320 NTU2341 NTU1945 NZA6116 NTU2340 NTU2390 NTU2335 NTU2391 NTU2376 NTU2370 NTU2336

KT1 KT1 KT1 KT1 KT1 KT1 KT1 – – KT2

52r59 28r38 20r35 40r47 25r37 40r47 41r47 24r35 24r35 35r41 26r32 72r77 75r80 70r80

e

CP1 KT1 KT1

Calibrated ageŽs. with 1 s range b Žcal. year BP.

11,350 " 70 10,150 " 70 9370 " 50 8390 " 60 7950 " 60

Occurrence c Reference

B1 A2 A4 A4 A4

this study this study this study this study this study

- 15r8 - 18r8 - 37r30 17r8 - 16r10 - 16r10 75r15 84r51 - 87r60 28r11 50r15

13,169 Ž13,260. 13,360 11,269 Ž11,840. 12,054 10,228 Ž10,359. 10,385 9276 Ž9390, 9410, 9430. 9444 8582 Ž8680, 8720, 8800, 8920, 8940. 8954 8260 " 60 9048 Ž9240. 9369 7820 " 60 8492 Ž8550. 8589 7240 " 60 7940 Ž7990. 8101 7900 " 50 8557 Ž8610, 8629, 8643. 8941 8110 " 50 8979 Ž8989. 9143 5840 " 50 6572 Ž6668. 6728 5060 " 140 5648 Ž5760, 5830, 5880. 5935 5490 " 50 6215 Ž6291. 6306 8580 " 63 e 9482 Ž9500. 9535 1680 " 40 1530 Ž1553. 1611 1710 " 40 1543 Ž1576, 1590, 1603. 1691

D D C1 C2 D C2 B1 A1 B2 A4 A3

51r17 28r11 18r8 39r13 25r10 39r13 39r13 20r8 20r8 35r16 26r14 - 70r62 65r51 65r51

2020 " 40 2160 " 50 2220 " 50 2240 " 40 2270 " 50 2410 " 40 5155 " 83 e 4020 " 50 7060 " 60 1340 " 50 1000 " 45 870 " 40 1290 " 50 1650 " 50

A1 A1 A3 A1 A2 A1 B2 f A3 B2 f A1 A1 B2 B2 A3

this study this study this study Chen Ž1993. Chen Ž1993. Chen Ž1993. this study this study this study this study Shea, K.S., unpublished data this study Lee et al. Ž1994. this study Chen Ž1993. this study Lee et al. Ž1994. this study this study this study this study this study this study this study this study

1897 Ž1951. 1994 2062 Ž2140. 2297 2141 Ž2157, 2253, 2302. 2321 2150 Ž2310. 2326 2156 Ž2321. 2340 2350 Ž2360. 2468 5763 Ž5920. 5984 4415 Ž4446, 4479, 4507. 4531 7785 Ž7830, 7860, 7890. 7913 1194 Ž1279. 1294 910 Ž928. 943 724 Ž755. 789 1167 Ž1195, 1204, 1239. 1280 1510 Ž1535. 1570

a

Sample number assigned at dating laboratory: NTU s National Taiwan University; WK s The University of Waikato; NZA s Institute of Geological and Nuclear Sciences, New Zealand. b Calibrated using program and calibration curveŽs. of Stuiver and Reimer Ž1993.. Central valueŽs. of distribution given in parentheses. c A: Wood in mud bed Žor matrix. associated with gravely channel deposits ŽA1., in gravely channel deposits ŽA2., in sandy–silty channel deposits ŽA3. or in silty–muddy overbank deposits ŽA4.; B: plant fragments, stems and leaves in siltyrmuddy channel deposits ŽB1. or overbank deposits ŽB2.; C: shell in lagoonalrestuarine muds ŽC1., or beach sands ŽC2.; D: wood in lagoonalrestuarine muds. d C.P.s Coastal Plain and its correlated high terraces. e Dated by accelerator mass spectrometry. f Reworked.

4415–4531 YBP Žsample 24. and 7785–7913 YBP Žsample 25., are obtained from these deposits ŽFig. 5; Table 1.. The former is derived from a log in silty channel deposits, and the latter is from an aggregate

of plant debris within a suspected paleosol. Both are sampled about 4 m above bedrock surface, or 24 m in elevation. The age of 7785–7913 YBP Žsample 25. is slightly younger than those obtained from

M.-L. Hsieh, P.L.K. Knuepferr Geomorphology 38 (2001) 337–372

fluvial and marine sediments in the nearby Coastal Plain and high terraces Ž7940–9444 YBP; samples 4–9., but these high-terrace deposits are at elevations of 38–46 m above sea level ŽTable 1.. Very likely, then, part or all of sample 25 was reworked from the adjacent Coastal Plain and high terraces that contain abundant wood. We therefore chose the date of 4415–4531 YBP Žsample 24. to represent the timing of deposition of these fluvial sediments in this side ridge. 4.2.3. RiÕer terraces in the lower Erhjen RiÕer Õalley Plio–Pleistocene mudrock crops out almost continuously in the lower portion of the Erhjen River downstream of the Lungchuan fault. In contrast, fluvial sediments that cap the terraces are exposed only intermittently. Nonetheless, as the exposed terrace deposits commonly show an overall sheet or broad-channel geometry regardless of the orientation of the exposure relative to the valley trend, we assume that the terraces are underlain by somewhat planar bedrock surfaces with irregularity of - 5 m Žour uncertainty in determining terrace elevation.. The thicknesses of these deposits are about 10 m for Terrace KT1 , high terraces, and minor terraces a and c and are generally 5–8 m for the lower KT2 , CP2 , and other minor terraces. These terrace deposits appear to thicken in the mountain-front area. Here, one KT2 terrace exposure consists of 12 m of sediments; the KT1 terrace may have a maximum thickness of 17 m ŽFig. 5, profile BBX .. Nearby, even the sediments of the modern floodplain are as thick as 10 m, as indicated by unpublished drilling data from highway construction. The terrace sediments in this portion of the river Žand also others elsewhere in the river basin. generally show a single fining-upward sequence, with lower channel deposits consisting of structureless gravels andror cross-bedded gravelsrsandsrsilts and upper overbank deposits characterized by planarstratified siltsrmuds. Scour-fill muds are also included in the channel deposits, from which most of the carbon samples for dating in this study are obtained. The channel deposits range from - 1 to 6 m in thickness. The upper, overbank deposits are usually a few meters thick and are up to 10 m thick in some KT1 and high terrace exposures; they are often thicker than the lower, coarser-grained channel

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deposits. These terrace deposits are distinct from the 4.5-ka sediments in the mountain-front area, which are characterized by multiple fining-upward sequences associated with paleosol-like strata. The KT1 terrace exposed in the mountain-front area also consists of much coarser Žcobble-sized. and thicker Žabout 3 m. basal gravels than those of the 4.5-ka sediments. There is only one date available for the high terraces in this portion of the river, 5648–5935 YBP Žsample 12. obtained from Terrace H Y at Yuehshihchieh ŽFig. 6.. The date is derived from an aggregate of plant fragments Žmany of them are herbaceous species. from sandy channel beds. Eight dates, ranging from 1530 to 2468 YBP ŽTable 1; samples 15–22; Fig. 6., are available for Terrace KT1 Žincluding those previously reported by Chen, 1993, Lee et al., 1994, and K.S. Shea, unpublished data.. These dates are all obtained from wood fragments Žtrunks or aggregate of twigs.; five of them are associated with basal gravels Žsamples 17, 18, 20– 22., two within sandy channel beds Žsamples 16 and 19., and one within overbank silts Žsample 15.. The date obtained from overbank deposits Ž1530–1611 YBP; sample 15. was sampled 6 m above basal channel deposits dated at 2062–2297 YBP ŽFig. 9; sample 18.. One date of 5763–5984 YBP Žsample 23. from Terrace KT1 is inconsistent with others Žsamples 15–22. obtained from the same terrace ŽTable 1.. This age, obtained by the AMS method, is derived from an aggregate of plant fragments within silty overbank strata at the same site as samples dated at 2150–2326 and 2350–2468 YBP Žsamples 20 and 22, respectively; Table 1.. It is highly unlikely that Terrace KT1 could have begun to form in the middle Holocene given that the nearby high terrace H Y yields a date of 5648–5935 YBP Žsample 12; Fig. 6, profile CCX .. We therefore interpret sample 23 as reworked, probably from the nearby H Y terrace. Dates from Terrace KT2 and minor terraces are available only in the Yuehshihchieh area ŽFig. 6.. Here, a log in Terrace KT2 is dated at 1194–1294 YBP Žsample 26., and an aggregate of twigs in minor terrace e below Terrace KT2 is dated at 910–943 YBP Žsample 27; Table 1; Fig. 6, profile CCX .. Both of the dated materials are associated with basal gravels.

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M.-L. Hsieh, P.L.K. Knuepferr Geomorphology 38 (2001) 337–372

Fig. 9. River terrace long profiles and radiocarbon dates Žcal. year BP; superscripts refer to sample numbers in Table 1. of Tributary 1 Žfor distribution see Fig. 2.. Position 0 km is located at the junction with the main stem. Only the terraces on the left bank of the channel between positions 2 and 3.5 km are shown. Shaded arrow indicates the position of the inferred synclinal axis. Locations of radiocarbon dates shown in proper stratigraphic position. Symbols and other notes as in Fig. 4.

4.2.4. RiÕer terraces in the upper Erhjen RiÕer Õalley Thickness and character of terrace deposits do not seem to change between the CP1 terrace upstream of the Lungchuan fault and Terrace KT1 downstream, nor do the CP2 terrace deposits change across the Lungchuan fault. Farther upstream, outcrops of the wide, combined CP1rCP2 terrace are rare ŽFig. 7.. Nonetheless, sporadically exposed bedrock along the modern channel indicates that at least in some places the maximum thickness of the CP1rCP2 terrace deposits is less than 20 m. The best outcrop in this upstream valley, at Hengshan, exposes at least 10 m of Žoverbank?. mud deposits of Terrace CP1a ŽFig. 7, profile EEX .. These fine-grained sediments are bounded vertically on one side by steeply inclined sandstone that forms an isolated mount between terraces CP1a and CP1 ŽFig. 7, profile EEX .. There are only two dates available for the river terraces in this upstream valley. One, 9482–9535 YBP Žsample 14., is obtained from high terrace H C

and dated by the AMS method ŽFig. 7; Table 1.. The date is derived from an aggregate of plant debris within locally exposed Žoverbank?. muds 7 m below the terrace surface ŽFig. 7, profile DDX .. The other date, 724–789 YBP Žsample 28., is obtained from Terrace CP1 at Hengshan ŽFig. 7; Table 1.. The age is derived from a combination of herbaceous debris within silty overbank deposits ranging from 5 to 6 m below the surface ŽFig. 7, profile EEX .. 4.2.5. RiÕer terraces in Tributary 1 Terrace deposits in Tributary 1 are similar in nature and thickness to those exposed in the mainstem valley Žand other major tributaries.. One KT1 outcrop at Tienliao ŽFig. 2. exposes abundant freshwater gastropods associated with overbank deposits, indicating that the floodplain was once occupied by pond water. Three dates are available for terraces in this tributary. One, 6215–6306 YBP Žsample 13., is obtained from an aggregate of twigs within channelfill basal gravels in high terrace H T at Tienliao

M.-L. Hsieh, P.L.K. Knuepferr Geomorphology 38 (2001) 337–372

ŽTable 1; Figs. 2 and 9.. Below this high terrace, we obtained two dates, 1167–1280 YBP Žsample 29. and 1510–1570 YBP Žsample 30., from Terrace KT1 ŽTable 1; Figs. 2 and 9.. The former is derived from an aggregate of herbaceous stems within overbank muds 5 m below the terrace surface, and the latter from a trunk in growth position within silty channel deposits 10 m below the surface Žor 19 m above the modern channel.. 4.3. Summary of Holocene riÕer history 4.3.1. Early Holocene sedimentation adjacent to the mountain front Fluvial sedimentation, dominantly overbank silts and muds, has prevailed in the Coastal Plain adjacent to the mountain front since the latest Pleistocene. The oldest radiocarbon age we obtained from these sediments, now 18 m above sea level, is about 13 ka X Y .. The deposition rates were ŽFig. 5, profile AA 3.3–3.5 mmryear during 13–8 ka Žcalculated from samples 1 and 5; Table 1. and 3.5–9.3 mmryear in the early–middle Holocene Žcalculated from samples 4 and 5; Table 1.. This deposition terminated at 8–6 ka, estimated by extrapolating the Holocene deposition rate to the surface of the terraces now 40–45 m in elevation. Meanwhile, marine transgression occurred shoreward, depositing beachrshoreface sands and lagoonalrestuarine muds on part of these fluvial X Y sediments ŽFig. 5, profiles AA and BBX .. Incursion of seawater killed coastal vegetation, as recorded by a distinct, wood-bearing lagoonalrestuarine sequence deposited during 9.4–8.5 ka Žconstrained by samples 6, 7, and 10.. The culmination of this marine transgression occurred no earlier than 8.0 ka Žconstrained by sample 8.; it inundated the entire Coastal Plain and drowned the valley of the Kueitung River ŽFig. 5.. This marine transgression episode, and perhaps also the fluvial aggradation, was caused by the eustatic sea-level rise since the last glacial maximum ŽChen, 1993.. Subsequent falling of relative Žlocal. sea level, evidenced by westward progradation of beach and fluvial sediments, may reflect tectonic uplift of the Coastal Plain after the eustatic sea-level rise had decelerated and become stable ŽChen, 1993.. By ca. 6.7 ka, the shoreline had retreated about 2 km from the mountain front Žcon-

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strained by sample 11.. The uplift eventually triggered channel incision, terminating the fluvial sedimentation on the inner part of the Coastal Plain. 4.3.2. Early–middle Holocene riÕer actiÕity A series of alluvial fans was developed on the eastern slope of the Lungchuan Ridge bounding the upstream Erhjen River valley during the early Holocene, as indicated by the age of 9482–9535 YBP Žsample 14. from high terrace H C ŽFig. 7.. This age suggests that formation of the wide upstream Erhjen River valley that consists of terraces CP1rCP2 could have started at least by the middle Holocene. Early Holocene fluvial sediments are not preserved in the lower Erhjen River Basin. The highest terraces at Yuehshihchieh ŽH Y . and Tienliao ŽH T . are middle Holocene in age Žsamples 12 and 13; Figs. 6 and 9.. The closeness of these two ages suggests that part of the high-terrace remnants in the lower Erhjen River Basin may be correlative. Thus, an extensive floodplain may have existed in the lower Erhjen River Basin during the middle Holocene, and it may have continued upstream to the CP1rCP2 floodplains. 4.3.3. FluÕial sedimentation at 4.5 ka in the mountain-front area A channel aggradational episode around 4.5 ka is suggested by the 15-m thick fluvial sediments exposed in the mountain-front area where a date of 4415–4531 YBP Žsample 24. is obtained ŽFig. 5.. These sediments are different from those exposed in the KT1 terraces in both age and sedimentary characteristics. This leads us to separate them from KT1 terrace deposits, although the surface of the side ridge where they crop out has an elevation similar to that of the nearby KT1 terrace ŽFig. 5, profile BBX .. This channel aggradational episode, however, appears restricted only to the mountain-front area. 4.3.4. Late Holocene riÕer actiÕity Channel incision dominated in the lower Erhjen River Basin after abandonment of the high terraces in the middle Holocene Žwith a rate of 7–8 mmryear calculated at Yuehshihchieh; Fig. 10.. It was not until 2.5 ka that the main-stem channel started laterally eroding bedrock and aggrading floodplain sediments, creating the KT1 floodplain hundreds of me-

356

M.-L. Hsieh, P.L.K. Knuepferr Geomorphology 38 (2001) 337–372

Fig. 10. River bedrock incision history at Ža. Yuehshihchieh Žmain-stem Erhjen River. and Žb. Tienliao ŽTributary 1.. Uncertainties of bedrock surface elevation Ž5 m. and calibrated age Žwith 1 s range. are represented by the height and width of the shaded rectangl,e respectively. The inferred duration of occupation of the KT1 terrace is shown by the width of the dotted–line rectangle. The calculated channel bedrock incision rates between terraces are also shown.

ters to 1 km wide ŽFig. 6.. We obtained eight dates Žsamples 15–22. from this deposit along the 8-km long valley ŽFig. 6., which constrained the termination of this episode of channel lateral erosion to 1.5 ka. During this 1000-year period, some 10 m of sediments accumulated, yielding an average deposition rate of 1 cmryear. This depositional episode is

even more prominent in the mountain-front area, as the KT1 terrace deposits are thicker and coarser than those deposited around 4.5 ka ŽFig. 5, profile BBX .. The bedrock there, however, was only slightly incised Ž2 m. during the period ca. 4.5–2.5 ka ŽFig. 5, profile BBX ., which yields a bedrock incision rate of 1 mmryear. A series of incision and lateral erosion events has occurred since abandonment of Terrace KT1 , creating Terrace KT2 and several minor terraces in the lower Erhjen River. The channel incision right after the abandonment of Terrace KT1 was very rapid, with a rate of 5 cmryear calculated at Yuehshihchieh Žfrom samples 16 and 26; Fig. 10.. The KT2 floodplain was formed at about 1.3 ka Žsample 26. at Yuehshihchieh, with duration of occupation likely only a few hundred years at most, constrained by the dates obtained from the next higher and lower terraces. The rate of channel incision into bedrock between abandonment of the KT2 terrace and formation of Terrace e is calculated as 2–3 cmryear at Yuehshihchieh Žfrom samples 26 and 27.. The rate reduced to 1.2 cmryear during the last 1 ka as the channel continued to incise from Terrace e to the modern channel Žfrom sample 27.. This rate, interestingly, is close to the average rate of incision calculated from Terrace H Y to the modern channel Žfrom sample 12; Fig. 10.. An episode of channel lateral erosion and floodplain sedimentation also occurred in Tributary 1 during the late Holocene. The deposition rate of the KT1 terrace deposits was 1.2–2.2 cmryear at Tienliao Žcalculated from samples 29 and 30.. The formation of the KT1 floodplain here thus may have covered the interval of 2.0–0.8 ka, estimated by extrapolating this deposition rate to the base and top of the terrace deposits. The timing of initiation of Terrace KT1 in the upstream Tributary 1 may thus have been later than that in the main river. It is clear that while the Erhjen River at Yuehshihchieh was undergoing repeated rapid incision and lateral erosion, which created Terrace KT2 , the KT1 floodplain in the upstream Tributary 1 was still frequently occupied by overbank floods. In contrast to the lower Erhjen River Basin, the combined CP1 and CP2 floodplains in the upstream valley continued widening and aggrading throughout much of the late Holocene Žconstrained by sample

M.-L. Hsieh, P.L.K. Knuepferr Geomorphology 38 (2001) 337–372

28.. It was not until sometime after 0.8 ka that the CP1 floodplain near the headwaters of the basin was completely abandoned and incised. 5. Holocene tectonic activity 5.1. Faulting and synclinal folding in the upstream Erhjen RiÕer Õalley We identify two Holocene faults in the wide upstream Erhjen River valley. One is delineated by linear scarps separating terraces CP1a and H C from Terrace CP1 on the western side of the valley, and the other by scarps separating Terrace CP2 from Terrace CP1 on the eastern side of the valley ŽFig. 7.. The fault origin of these scarps is suggested by their

357

linearity and continuity, which are too great to be created by fluvial processes, especially by the prevailing meandering channels. Straight, continuous terrace scarps could be erosional remnants controlled by underlying bedrock lithology and structures. However, given that many of these scarps have heights Ž- 5 m. smaller than the known thickness of the terrace deposits throughout the basin Ž5–15 m., it is likely that they developed mostly or entirely within fluvial sediments and should not be affected by any steps or fractures within the underlying bedrock. The aligned terrace scarps on the western side of the valley increase in height southward from - 5 between terraces CP1a and CP1 to 15 m between terraces H C and CP1. Farther south, the scarp is buried by an alluvial fan on the CP1 terrace surface ŽFigs. 7

Fig. 11. Summary of Holocene structures Žlight-gray symbols; arrows indicate down-dip direction. in the Erhjen River basin inferred from river terrace evidence. The fold axes are assumed to be parallel to nearby mapped bedrock fold axes Ždotted lines; see also Fig. 2..

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and 8a.. The linear terrace scarps on the eastern side of the valley, less than 5 m in height, extend only about 1 km and merge on both sides to valley walls that are also linear ŽFig. 7.. The nature of these two inferred faults is unknown, although their downthrow side must be toward the center of the valley ŽFig. 7, profiles DDX and EEX .. Nonetheless, they are probably high angle, judging from the straightness of the scarps. We also interpret a Holocene synclinal structure in this upstream valley ŽFig. 11.. This syncline is asymmetric with an axis approximately along the trend of the main-stem channel downstream of the junction of Tributary 27 ŽFigs. 7 and 11, profile DDX .. This syncline is suggested by the morphology of the CP1 and CP2 terrace surfaces, which are consistently centered away from the mid-valley line ŽFig. 7.; we interpret this terrace asymmetry as resulting from folding.

5.2. Tilting of the Coastal Plain adjacent to the mountain front We estimate the Holocene average tectonic uplift rate of the inner Coastal Plain from the marine sediments we dated in this study. We apply the Holocene eustatic sea-level curve of Taiwan established by Chen Ž1993. and Chen and Liu Ž2000. and derive relative sea-level curves of the area by multiplying the eustatic sea-level curve by any given constant tectonic uplift ŽFig. 12.. We plot our dated marine samples Žsamples 6–11. on these curves ŽFig. 12.. We consider that samples obtained from beachrshoreface sediments should have been deposited close to the position of the paleo-sea level, but that samples obtained from lagoonalrestuarine sediments may have been deposited up to several meters below the paleo-sea level Žwe assign a water depth of 3 m.. The resulting plot gives a wide range

Fig. 12. Holocene relative sea-level curves as a function of an assumed eustatic sea-level curve for Taiwan Ži.e., the 0 mmryear curve; Chen, 1993. and a constant tectonic uplift rate Žmarked above each curve.. Radiocarbon dates from marine deposits are plotted to estimate the long-term average tectonic uplift rate of the site Žshaded rectangles.; the height and width of the rectangles represent the uncertainty of sample elevations and calibrated ages with 1 s range, respectively; each number is the sample number in Table 1. We assume that samples derived from beachrshoreface sediments Žsamples 9 and 11. were deposited at sea level and that lagoonalrestuarine samples Žsamples 6, 7, 8 and 10. were deposited 3 m below the contemporary sea level.

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of tectonic uplift rates ŽFig. 12., which may be due, in part, to our assumptions and the accuracy of the eustatic sea-level curve we apply ŽChen, 1993; Chen and Liu, 2000.. Still, we can differentiate different tectonic activity between the portion of the Coastal

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Plain adjacent to the mountain front Žwith an uplift rate of 6–8 mmryear. and the portion 2 km west of the mountain front Žwith rate of 4–5 q mmryear. ŽFig. 12.. This different tectonic uplift is consistent with the abnormally steep gradient of this portion of

Table 2 Summary of tectonic uplift and tilting rates Assumption

Parameter a

Rate b Žper year.

1. Holocene eustatic sea-level curve of Taiwan ŽChen, 1993.. 2. Beachrshore face deposits form at sea level; lagoonalr estuarine deposits form at water depth of 3 m. as above

Žsee Fig. 12.

4.3–5.4 mm

Žsee Fig. 12.

5.9–7.7 mm

Long-term bedrock incision rate equals tectonic uplift rate.

h B s 75 m, h B s 15 m, a s 5.8 ka

10.3 mm

Different uplift rates

–

2.5–17 = 10y7

Yuehshihchiehr Mountain front

Divergence of high terrace surface and modern river

Yuehshihchiehr Mountain front

Divergence of high terrace and Terrace KT1 surfaces

1. H Y terrace can be correlated to H H terrace. 2. H Y and H H terrace surfaces were abandoned at 5.8 ka. 3. Original river Žor floodplain, bedrock surface. slope formed at time a is the same as that X formed at time a . as above

u B s 4.3–5.4 mmryear, uA s 5.9–7.7 mmryear, d s 2 km X h B s 46 m; h B s 8 m, X hA s 81 m; hA s 15 m, X a s 5.8 ka; a s 0 ka, d s 6.5 km

Yuehshihchiehr Mountain front

Divergence of Terrace KT1 surface and modern river Divergence of Terrace KT1 bedrock surface and modern river Different uplift rates

Location A relative to location B ŽArB.

Criterion

Tectonic uplift rate at a site (A s B) Coastal Plain 2 km Marine deposits from mountain front elevation and age

Inner Coastal Plain Žmountain front. Yuehshihchieh

Tilting rate Mountain frontr Chungchou terrace

Yuehshihchiehr Mountain front

Yuehshihchiehr Mountain front a b

Marine deposits elevation and age Bedrock incision rate

Original river Žor floodplain, bedrock surface. slope formed at time a is the same as that X formed at time a . Ž Original river or floodplain, bedrock surface. slope formed at time a is the same as that X formed at time a . –

X

X

h B s 46 m; h B s 35 m, X hA s 81 m; hA s 57 m, X a s 5.8 ka; a s 1.5–2.5 ka, d s 6.5 km X h B s 35 m; h B s 8 m, X hA s 57 m; hA s 15 m, X a s 1.5–2.5 ka; a s 0 ka, d s 6.5 km X h B s 18 m; h B s 8 m, X hA s 50 m; hA s 15 m, X a s 1.5–2.5 ka; a s 0 ka, d s 6.5 km u B s 5.9–7.7 mmryear, uA s 10.3 mmryear, d s 6.5 km

uA and u B , inferred uplift rates in locations A and B respectively; see Fig. 13 for definition of other terms. X X X Tilting rate s wŽ hA y hA . y Ž h B y h B .xrd Ž a y a ., or Ž uA y u B .rd.

7.4 = 10y7

4.7–6.1 = 10y7

9.2–15 = 10y7

1.5–2.6 = 10y6

4.3–5.4 = 10y7

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the Coastal Plain ŽFigs. 4 and 5.. We conclude that this steep gradient is caused, at least in part, by tectonic tilting with a tilt rate calculated as 2.5 = 10y7 to 1.7 = 10y6 per year ŽTable 2; Fig. 13.. 5.3. Doming in the downstream Erhjen RiÕer Õalley A doming structure across the downstream Erhjen River valley is suggested by downstream convergence of terraces KT1 , KT2 , and the modern channel; by steeper long profiles of terraces KT1 and KT2 than Terrace CP2 ; by apparent upstream tilting of Terrace CP1 upstream of the Lungchuan fault; and by upstream convergence of terraces CP1 and CP2 ŽFig. 8.. The crest of this structure, based on the geometry of these long profiles, is located between Kuting and the Lungchuan fault ŽFigs. 8b and 11.. This interpretation is further supported by eight radiocarbon dates Ž2.5–1.5 ka. from multiple sites of Terrace KT1 , which confirm our correlation of this terrace based on topographic criteria and thus confirm the downstream convergence between this terrace and the modern channel. In addition, the ca. 5.8 ka high terrace H Y at Yuehshihchieh is about 66 m above

Fig. 13. Definition of parameters for computing tectonic tilting ŽTable 2.. A and B are reference locations ŽA upstream.; a and aX X are ages of two surfaces Ž a older than a .; hA and h B represent the elevations of the surface formed at time a at locations A and X X X B, respectively; hA and h B are the elevations of the a surface at A and B, respectively; d is the along-valley distance between A and B.

the modern channel Žor 25 m above the KT1 terrace., whereas the Coastal Plain and its correlated high terrace in the mountain-front area, dated no younger than 6 ka, are 35 m above the modern channel Žor 10–15 m above the KT1 terrace. ŽFig. 8.. Furthermore, Terrace H Y Žca. 80 m in elevation. is higher than much of the combined CP1rCP2 terrace upstream Ž65–80 m in elevation.; Terrace CP1rCP2 likely began to form around the time of Terrace H Y ŽFig. 8., so the initial H Y elevation should have been much lower as it is downstream. We calculate tilting rates, ranging from 4.3 = 10y7 to 2.6 = 10y6 per year, for the river valley between Yuehshihchieh and the mountain front based on different criteria ŽTable 2.. We assume that the original gradients of the terrace surfaces were the same as the modern channel gradient along the valley. The rates Ž9.2 = 10y7 to 2.6 = 10y6 per year. based on the convergence of Terrace KT1 Žterrace surface or bedrock surface. and the modern channel are highest. The rate derived from the difference of inferred tectonic uplift rates between Yuehshihchieh and the mountain-front area yields a minimum value of 4.3– 5.4 = 10y7 per year ŽTable 2., if we assume that the tectonic uplift rate at Yuehshihchieh equals the average post–middle Holocene bedrock incision rate of 10.3 mmryear derived from Terrace H Y ŽFig. 10; Table 2.. The orientation of this doming structure, however, is poorly constrained due to limited distribution of the terraces. We assume that its axis has the same trend as regional structures, a NNS–SSW direction, and plunges to the south downstream from Kuting ŽFig. 11.. We delineate this axis within the main valley of the downstream Erhjen River, somewhat coinciding with the mapped Kutingkeng fault along which active mud volcanoes are aligned ŽFigs. 2 and 11.. This axis does not appear to extend as far south as Tributary 4 ŽFig. 11. because the long profile of the KT1 terraces in this tributary does not display any discernible warping. This axis must extend northward at least to the Lungchuan Ridge because terraces converge upstream of the Lungchuan fault where the river would flow through the eastern limb of this anticlinal structure. The growth of Lungchuan Ridge appears to affect the upstream Erhjen River valley bounding the ridge, resulting in an asymmetric cross-section of the valley ŽFig. 7..

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5.4. Synclinal folding near Hsinhsing A synclinal structure in the Tributary 1 valley is suggested by divergence of terraces KT1 and H H both upstream and downstream of Hsinhsing ŽFigs. 9 and 11.. We assume that this structure has an axis coincident with that of the bedrock syncline on the published geological map ŽChinese Petroleum, 1971. ŽFig. 11..

6. Discussion: origin of river terraces in the Erhjen River 6.1. PreÕious terrace studies The formation of a flight of erosional river terraces requires repeated channel lateral erosion superimposed on incision in an otherwise V-shaped valley. As suggested by the terrace landform itself, channel lateral erosion and incision appear to occur as separate processes under certain circumstances over geomorphic time. Gilbert Ž1877, p. 126. was probably the first to address this point: Adownward wear ceases when the load equals the capacity for transportation. Whenever the load reduces the downward corrasion to little or nothing, lateral corrasion becomes relatively and actually of importance.B Creation of a wide, continuous erosional surface Žstrath. thus should require a period of stability in a river’s vertical position ŽRitter et al., 1995, p. 244.. This stability condition later evolved to be the concept of grade or equilibrium ŽDavis, 1902; Mackin, 1948; Knox, 1975; Leopold and Bull, 1979.. Bull Ž1979, 1990, 1991. formulated this equilibrium condition as the balance between power available Žstream power. and power needed Žresisting power. for entrainment and transport of bedload: Stream powerrResisting power s 1.0

Ž 1.

Stream power Ž v . is the time rate of conversion of potential gravitational energy into kinetic turbulent eddy energy by water flowing in the channel and is a function of river slope Ž S . and discharge Ž Q . ŽBagnold, 1960, 1977.:

v s r gQS

Ž 2.

where r is the density of water. Resisting power is

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mainly controlled by bedload characteristics, channel morphology, and bed roughness ŽBull, 1991.. It is generally believed that high-energy rivers with sufficient power are able to achieve grade or equilibrium and create wide floodplains Žwhich may become terrace surfaces., provided that prevailing base-level, tectonic, and climatic conditions are stable. Subsequent incision that displaces the river from equilibrium and leaves the wide floodplain as a terrace surface therefore implies a change of baselevel, tectonic, andror climatic conditions that is too abrupt or of too great a magnitude for the river to adjust to internally. For example, other things being equal, sudden tectonic uplift or an increased uplift rate may increase stream power so that it exceeds the resisting power, resulting in channel incision that abandons a terrace surface ŽBull, 1990, 1991.. It is worth noting that the initial concept of grade or equilibrium of rivers, as envisioned by Gilbert Ž1877., is dictated as a condition in which neither channel incision nor deposition occurs—that is, a stability in the river’s vertical position with respect to a datum ŽKnox, 1975; Leopold and Bull, 1979; Bloom, 1991.. However, much attention has been explicitly or implicitly focused on defining the graded condition as a critical river slope or long profile or an AabsoluteB stability in a river bed’s vertical position ŽDavis, 1902; Mackin, 1948; Howard et al., 1994; Pazzaglia et al., 1998.. The departure of a river from the graded or equilibrium condition thus has been widely attributed to external change of river slope, especially due to base-level change ŽDavis, 1902; Chorley, 1963; Pazzaglia and Gardner, 1993; Pazzaglia et al., 1998.. Bull Ž1990, 1991. proposed that formation of a wide strath surface in a region of tectonic uplift requires the river to incise, keeping pace with the tectonic uplift and maintaining a graded long profile ŽType 1 dynamic equilibrium.. The terrace thus created after abandonment is called a Atectonic terraceB, in contrast to the Aclimatic terraceB that is primarily the result of climate-induced river aggradation before incision ŽBull, 1990, 1991.. A river terrace, however, may also develop from internal complex response of incising rivers, adjusting to long-term tectonic uplift, short-term climatic change, or even a single event such as a large flood ŽBorn and Ritter, 1970; Schumm and Parker, 1973; Womack and Schumm, 1977; Boison and Patton,

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1985; Bull and Knuepfer, 1987; Germanoski and Harvey, 1993.. The formation of these terrace surfaces need not imply grade or equilibrium nor any changes in external forcing variables. They are, however, generally minor and discontinuous. River terraces in the Erhjen River are complicated in sequence. Still, we are able to separate major terraces ŽKT1 , KT2 , CP1 and CP2 . from minor ones Ž a y j .; the former are paired and continuous for much of the river valley, whereas the latter are not. We currently are unable to explain the origin of each minor terrace with confidence, though internal responses likely are important. Rather, we consider that the formation of the major terraces—a contrast between the up-to-kilometer-wide terrace surfaces and the present narrow, incising channel—reflects distinct changes of external variables. In the following discussion, we first focus on Terrace KT1 in the lower Erhjen River Basin—the terrace best chronologically constrained in this study. Terrace KT1 represents an interval of valley widening that followed incision below middle Holocene high terraces. We ask what triggered this change in river behavior from incision-dominated to lateral-erosion-dominated modes: whether it requires a change from unstable to stable base levels; or from active to relatively inactive tectonics so that a specific slope or long profile can be attained Žas proposed by Bull, 1990, 1991.; or if it was more likely related to climatic-driven hydraulic and bedload conditions that were different from present. We then discuss and assess the possible factors that could contribute to the abandonment of this terrace surface Ži.e., change in river behavior from lateral erosion to incision.. We discuss the initiation of this terrace surface and its abandonment separately as we find that the agents that govern these two stages of terrace development are different and perhaps separable. Finally, we use this knowledge to explain the terrace configuration we observe in the entire Erhjen River Basin. 6.2. Possible role of base-leÕel and tectonic change on initiation of KT1 terrace surface We first examine if the initiation of Terrace KT1 was related to a change in base level. The downstream convergence of terraces KT1 , KT2 , and the inner part of the Coastal Plain to the Chungchou

marine-terrace surface ŽFigs. 4 and 8. shows that not only Terrace KT1 but also KT2 and high terrace H Y Žwhich is apparently younger than the Coastal Plain surface. were created under a more or less stable base level Ži.e., no discernible base-level change occurred before, during, or after the formation of the KT1 terrace surface.. The progradation of the flat Chungchou marine-terrace surface may have caused a slight base-level rise after middle Holocene. This may be the reason for the river aggradation in the mountain-front area starting about 4.5 ka ŽFig. 5, profile BBX .. This aggradational episode, however, seems to have been restricted to the mountain-front area; meanwhile, the channel in the lower Erhjen River Basin was still dominated by incision ŽFig. 6, profile CCX .. Our interpretation is concordant with the observation of Merritts et al. Ž1994. in the Mattole River in California, who showed that base-level rise affected only the lower reach of the river basin. The stability of base level before the formation of the KT1 terrace surface is also manifested by the minor bedrock incision after the termination of the 4.5-ka river aggradation ŽFig. 5, profile BBX .. The channel bedrock incision rate here was about 1 mmryear from ca. 4.5 to 2.5 ka, in contrast to the 7–8 mmryear bedrock incision rate at Yuehshihchieh during the same time period ŽFig. 10.. We next examine if the initiation of Terrace KT1 could be related to deceleration of tectonic uplift— that is, if valley widening can be triggered by a reduction in uplift rate. The annual change of river slope due to a reduction in uplift rate is very minor in these rivers compared to other factors that affect stream power. The average rates of change of river slope, derived from terracerchannel divergence data Žand which are used to estimate the tectonic tilting rates in Table 2., are on the order of 10y6 to 10y7 per year, or - 0.1% per year of the modern floodplain gradient of 1.3 = 10y3 in the downstream Erhjen River. The change of stream power produced by such an annual change in river slope is negligible, especially compared with that produced by the variation in annual discharge Že.g., Fig. 3. Žnot to mention the change in stream power from a storm event.. The river slope could be gradually decreased by continuing incision if tectonic tilting decelerated or stopped. Thus, for deceleration of tectonic uplift to be able to trigger valley widening, a threshold slope Žor a spe-

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cific long profile. must exist below which channel lateral erosion dominates ŽBull, 1990, 1991.. If this is the case, one may expect that higher and older terraces should be steeper than lower and younger ones since they presumably had the same initial long profiles before being tilted. The latest Holocene Erhjen River terraces, however, do not show this ŽFig. 8.. The KT1 terrace surface has a gradient similar to not only KT2 but also to the subsequently developed minor terraces Ž e and f . in the river reach upstream of Chungte ŽFig. 8. despite the extremely rapid bedrock incision Ž5 cmryear. between the KT1 and KT2 terrace surfaces at Yuehshihchieh ŽFig. 10.. Note also that the paleo-channels may have meandered across these terrace surfaces; the wider the surface, then, the lower the channel gradient may have been, as they could have large meander amplitude. It is possible, then, that the paleo-channel that flowed across the KT1 terrace surface had a gentler gradient than those across the lower terrace surfaces, as the terraces from KT1 to f become progressively narrower and more discontinuous ŽFig. 6.. This suggests that the initiation of a major terrace surface may not require an attainment of a specific long profile or that such a long profile itself Ži.e., a threshold river slope. is so strongly affected by contemporary discharge and bedload conditions that it can vary from one terrace Žtime period. to another. 6.3. Climatic interpretation for formation of the KT1 terrace surface We next consider how climatic changes might interact with tectonic uplift and tilting to initiate development of the extensive KT1 terrace surface. We apply the concept that a major episode of channel lateral erosion that creates a wide erosional surface requires a period of stability in a river’s relative vertical position Žor grade. ŽGilbert, 1877. —that is, when the river’s stream power equals resisting power ŽEq. Ž1.; Bull, 1979, 1990, 1991.. The incision before the formation of Terrace KT1 indicates that the channel had sufficient stream power not only for transporting its sediment load but also for eroding bedrock beneath the sediments Ži.e., under capacity, or stream power ) resisting power.. The subsequent river planation therefore required either a decrease in stream power or an increase in resisting power so

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that channel incision could be retarded. We have already argued that a decrease in river slope is unlikely to decrease stream power sufficiently by itself. A decrease in discharge also seems unlikely, because the KT1 terrace Žand high terraces. is capped by thick fine-grained floodplain deposits, which implies frequent overbank floods during the formation of the terrace surface. The common 10-m thickness of the KT1 terrace deposits is comparable with the known maximum thickness of the modern floodplain deposits in the mountain-front area. If the maximum thickness of the modern floodplain deposits in the hill area Žfor which we have no direct data. is similar, the paleo-flood stage that corresponds to the deposition of the KT1 terrace sediments should have been no lower than the maximum modern floods. Alternatively, the termination of channel incision could occur through an increase of resisting power that overcomes the available stream power. An increase of resisting power could be initiated by an increase of bedload yield andror size in a narrow mountain valley where the river cannot readily change pattern or sinuosity. This is possible in the Erhjen River as an abrupt increase of sediment yield Žand size. could result from severe rainfall events such as typhoons in this humid setting—an agent that is powerful in removing vegetation and triggering landslides and debris-flow events ŽKelsey, 1980; Benda, 1990; Pitlick, 1993; Trimble, 1995.. Bedload would be increased because of an increase in sediment yield from sandstone ridges. An important aspect of this floodplain-genesis scenario is that not only is the bedload yield increased but so too is the channel discharge during large-magnitude storms. This increased discharge implies an increase of available stream power—which should be more effective in floodplain widening than when stream power is decreased ŽMerritts and Vincent, 1989; Merritts et al., 1994.. An increase of discharge also likely would produce more frequent overbank floods, which increases the chance for streamflow to contact and erode the valley walls. A positive feedback mechanism also could be involved in this floodplain widening process. Once the floodplain started widening, it provided room for the channel to shift laterally. The channel thus would become flexible in adjusting its horizontal parameters Žresisting power. to changing stream power and tend to maintain a

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constant level responding to contemporary increases of valley slope due to tectonic tilting ŽOuchi, 1985; Schumm, 1993., which in turn would facilitate floodplain widening ŽYoxall, 1969.. Support for this idea that extensive slope failure produced an increase in bedload yield is provided by the alluvial fans preserved on the CP1 and H C terrace surfaces beside the eastern slope of the Lungchuan Ridge ŽFigs. 2 and 7.. These alluvial fans are now densely vegetated and stable; the hillslope environment today must be different from that in the past. It is of interest to note that Tributary 8, which has a basin area even larger than tributaries 17 and 27, has no river terraces ŽFig. 2.. This can be readily explained by the lack of sandstone ridges in its catchment ŽFig. 2., so that bedload supply likely would not be a factor to affect river behavior. The existence of the low-order AunderfitB valleys cutting into mudrock also is suggestive of high-discharge runoff in the past, as they were likely formed by extremely large discharge events taking advantage of the weakly resistant valley walls. Valley or channel widening following storm events that brought abundant bedload into the system has been documented in modern rivers flowing through mountainous valleys ŽKelsey, 1980; Pitlick, 1993. and alluvial plains ŽSchumm and Lichty, 1963.. In terms of hydraulic geometry, this widening could simply reflect the river’s adjustment to an increase of prevailing discharge and bedload ŽSchumm, 1968.. Some workers ŽSugai, 1993; Meyer et al., 1995. also interpreted that river terraces formed as a result of such a change of hydraulic conditions. Meyer et al. Ž1995. provided a notable example from the rivers of Yellowstone National Park in the western United States. Numerous radiocarbon ages and detailed climatic data show that floodplain widening consistently occurred in wetter periods, associated with increased bedload supply as the axial streams trimmed back valley-side alluvial fans ŽMeyer et al., 1995.. The difference between the terrace genesis that we interpret in the Erhjen River and that in Yellowstone is the main way bedload sediments are generated. In a semi-arid setting like Yellowstone, the major bedload influx or alluvial fan aggradation results from fire-related sedimentation in the drier periods ŽFlorsheim et al., 1991; Meyer et al., 1995.. A significant time lag exists between the generation

of these sediments and their transport by axial streams in the wetter periods. In contrast, in a humid setting like the Erhjen River, abrupt flushing of debris Žor mass wasting. is usually associated with disastrous rainfalls; sufficient stream power apparently causes the generation and transport of these sediments at more or less the same time. We propose, then, that the KT1 terrace surface was initiated by a series of catastrophic rainfall events which began about 2.5 ka. These rainfall events caused regional instability of Miocene sandstone ridges and drastically increased bedload available for transport. Channel incision was prevented during these powerful rainfall Žand flood. events Žand also intervening moderate floods. by increased bedload; instead, valley widening proceeded taking advantage of the weakly resistant valley walls. We also interpret the initiation of the younger KT2 and middle Holocene high-terrace surfaces ŽH Y and H T . in the same manner. Catastrophic rainfall events in Taiwan are usually associated with tropical typhoons ŽWu, 1992.. Whether typhoons generated in the western Pacific Ocean impact Taiwan is mainly determined by the relative intensity and position of the Siberian High to the west and the West Pacific High to the east: typhoons tend to invade Taiwan when the Siberian High is weaker andror retreats to higher latitudes. Otherwise, typhoons would be restricted to lower latitudes and miss the island. The intensity and position of these two high pressure systems in fact also control the intensity of the Asian monsoon, which strongly affects the climatic pattern of Taiwan: Taiwan tends to be warmer and wetter when the Siberian High is weaker andror retreats to higher latitudes, which allows stronger summer monsoons from the tropical Pacific Ocean onto the island ŽP.M. Liew, personal communication.. We therefore propose that the KT1 Žand also KT2 , H Y , and H T . terrace was initiated during a relatively warm and wet climatic period. This interpretation is supported by pollen studies in northern and central Taiwan, which indicate a relatively warm and wet climate in the early and middle Holocene, a cool and dry climate after 4–5 ka, and a resumed warm Žand perhaps also wet. climate in the late Holocene ŽChen and Liew, 1990; Kuo, 1994; Liew and Huang, 1994; Lu, 1996.. The timing of these climatic changes corresponds roughly

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with the formation of high terraces H Y and H T , incision of them, and then formation of terraces KT1 and KT2 and other lower terraces. 6.4. Causes for the abandonment of the KT1 terrace surface We have inferred that the KT1 terrace surface was initiated when the available stream power was essentially balanced by the resisting power ŽEq. Ž1... The resumption of channel incision and the abandonment of the KT1 terrace surface implies an increase in stream power andror a decrease in resisting power. As proposed above, we believe that the formation of the KT1 terrace surface was related to an increase of bedload supply triggered by frequent catastrophic rainfall events Žor tropical typhoons.. It would seem natural that the abandonment of the KT1 terrace surface was due to a decrease of bedload supply resulting from a decrease of frequency of catastrophic rainfall events. This, however, is not supported by available pollen evidence, which does not show a late Holocene change to a significantly cooler and drier climate, unlike the middle–late Holocene transition ŽLiew and Huang, 1994.. Perhaps this proposed late Holocene climatic change was so minor that it cannot be resolved by pollen evidence, or the frequency Žand intensity. of tropical typhoons is in fact controlled by factors more complicated than those controlling the general climatic pattern of the island. However, the abandonment of the KT1 terrace surface can be related, at least in part, to the following two causes even without any significant climatic change. Ži. Oversteepening of river slope. Tectonic tilting would gradually increase valley slope during the period of valley widening and sediment accumulation that forms terraces. We estimate the accumulated tectonic tilting in the downstream Erhjen River by assigning a tilt rate of 4.3–5.4 = 10y7 per year— the minimum rate we calculate in this study ŽTable 2.. This would increase the valley slope by 4.3–5.4 = 10y4 after the 1000-year formation time of the KT1 terrace surface in the main river, a 30–40% increase assuming that the initial gradient of the KT1 surface was the same as the modern floodplain Ž1.3 = 10y3 .. If discharge remained constant, this should increase stream power by the same amount; channel

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incision would occur unless the resisting power also could increase. Žii. Decrease of bedload supply caused by internal factors. After repeated mass movement triggered by catastrophic rainfall events, the slope of the Miocene sandstone ridges should stabilize as the availability of loose rock fragments was exhausted andror the slope of the ridges was reduced ŽBull, 1991.. Such an internal adjustment of bedload supply could vary from one drainage basin to another, depending on watershed characteristics Že.g., vegetation, morphology, geology, and distribution of the sandstone ridges.. This may explain the different river behavior between the main Erhjen River and Tributary 1 during the late Holocene: while the main river underwent repeated lateral erosion and incision that created terraces KT1 and KT2 , the channel in the upstream part of Tributary 1 continued to aggrade a single KT1 terrace. The hydrological and climatic conditions that initiated the KT1 terrace surfaces probably continued at least as late as 0.9 ka Žthe time of formation of Terrace e .; bedload apparently was continuously supplied in the catchment of Tributary 1, but it was not in the main river. During periods without sufficient Žor excess. bedload supply, the main river thus would incise Žrapidly. during highdischarge events. Our interpretation of the abandonment of the KT1 terrace surface is different from the traditional view that attributes the abandonment of major river terraces to a change of external environments Žfor example, another climatic change or an increase of tectonic activity.. The causes for abandonment of the KT1 terrace surface are in fact more like those that create minor, Acomplex-responseB terraces. Thus, we recognize that the causes we use to explain the abandonment of the KT1 terrace surface may not universally apply to the rest of the terraces.

6.5. Synthesis: riÕer terrace origin Channels tend to incise in response to an increase of river slope caused by either base-level fall or faulting when such steepening of river is too abrupt or of too great a magnitude for the channels to adjust laterally ŽSchumm et al., 1987.. This is especially

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true for an incising channel confined in bedrock, where the channel has little room to shift laterally in response to steepening ŽSchumm, 1993.. Thus, it would seem natural that formation of an erosional terrace surface that shifts the river from incision to lateral erosion requires a decrease of river slope or an achievement of a river long profile ŽBull, 1990, 1991. that is just steep enough to transport bedload supplied from upstream ŽMackin, 1948.. For this to be true, there are two fundamental assumptions. First, other factors, especially climate-driven discharge and bedload conditions, are of secondary importance during formation of river terraces so that adjustment or maintenance of river slope would be the primary control on a river in equilibrium or grade. Second, major rivers in a humid setting usually have sufficient power so that once a critical river long profile is achieved after incision that reduces the river slope Žand thus stream power and capacity. rivers still can efficiently widen the valley. Our evidence, however, challenges these two assumptions. First, the KT1 and KT2 floodplains must have had different initial gradients or long profiles. Spatial and temporal change in discharge andror bedload conditions apparently dominated over the effect of river slope to determine if the river would be in equilibrium. Such climate-driven river discharge andror bedload conditions cannot be assumed to have been stable during Holocene even in a humid tropical setting like Taiwan. Our evidence cannot directly disprove the second assumption. However, the variations in a river’s ability to shape the landscape is clearly demonstrated by the 6- to 7-fold difference of the channel bedrock incision rates before and after the formation of the KT1 terrace surface at Yuehshihchieh ŽFig. 10.. The rate of channel incision into bedrock is commonly formulated as proportional to stream power ŽHoward and Kerby, 1983; Seidl and Dietrich, 1992; Howard et al., 1994; Seidl et al., 1994; Pazzaglia et al., 1998. or as a complex interaction between stream power Žor its related physical parameters. and resisting power Žprevailing bedload supply and grain size. for a given bedrock ŽSklar and Dietrich, 1998.. We propose that the much greater incision rates after abandonment of the KT1 terrace than before its formation must be due in part to higher discharge andror frequency of flood events Žor wetter climate.

of the post KT1 terrace period. The controls on rate of channel lateral erosion into bedrock are less discussed in the literature, but lateral erosion also should depend on the force acting on the bank relative to resistance ŽHooke, 1995. and thus be positively related to stream power Žor its related physical parameters. ŽPizzuto, 1984; Thompson, 1986; Howard, 1992. and affected by bedload characteristics. Although the role of bedload in both the bedrock incision and lateral-erosion processes remains unclear in the Erhjen River, we argue that like the 6- to 7-fold contrast of bedrock incision rates, the bedrock lateral-erosion rates can also be significantly reduced under a drier climate Žwith less frequent floods., even after the river has achieved a critical long profile Žor the tectonic tilting has stopped.. We suggest that the reduction of river slope Žand therefore stream power. that may hinder the channel from incision need not produce channel lateral erosion ŽMerritts and Vincent, 1989; Merritts et al., 1994., which should also depend on prevailing discharge and bedload factors—conditions that are not constant in a humid tropical mountain setting. We do not mean to deny the importance of river slope in controlling terrace genesis. Rivers do tend to incise in response to an increase of river slope. Thus, although river slope may not directly contribute to the stream power and bedload condition needed for initiation of floodplain widening as much as discharge does, slope controls channel incision, which indirectly determines if channel lateral erosion can proceed. Indeed, the formation of the KT1 Žand KT2 , H Y . terrace surface still relies on a more or less stable base level and a rate of tectonic tilting that is small compared with the rate of changing discharge and bedload yield. The increased valley slope due to tectonic tilting, however, can be accumulated if the channel does not immediately incise in response. Since bedload yield Žor resisting power. cannot infinitely increase, channel incision is unavoidable after long-term tectonic tilting that gradually increases stream power. This accumulated valley slope thus would promote the tendency of a river to incise; that is, other things being equal, a steeper river should have a higher tendency to incise than a gentler river as the threshold for channel incision is more easily crossed in a steeper river given any environmental perturbation ŽBegin and Schumm, 1984; Schumm,

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1991.. This helps explain why multiple levels of terraces ŽH Y , KT1 , KT2 , and the minor terraces. were developed across the inferred Holocene doming structure, while the channel upstream of this structure still occupied a single-level floodplain Žthe CP1rCP2 surface. ŽFig. 8.. Although we planned to AgeneralizeB guidelines for interpreting terrace genesis, we end with a picture even more complicated than the traditional view. The genesis of river terraces should be a result of complex interaction between factors that initiate terrace surfaces Žmainly climate-driven. and factors that contribute to subsequent abandonment of the surface by channel incision Žincluding all base-level, tectonic. and climatic factors.. We strongly suggest that one should consider both stages in terrace development separately before designating the origin of a terrace. Accordingly, we consider two end-member types of terrace genesis. In the first type, we assume that a river has a high probability of incision for reasons such as active tectonic uplift or base-level fall. For a terrace to be created, the factors that can initiate valley widening must overcome this tendency for incision. In this situation, the terrace development is likely initiated by a climatic change or by one or a series of flood events. The causes that abandon this terrace surface, however, could be complex and multiple or merely internally driven. This would be a good mechanism to produce a Acomplex-responseB terrace. The duration of channel occupation on such a terrace surface would be relatively short, so that the resulting terrace is likely minor and discontinuous. However, given sufficient stream power and weakly resistant bedrock, this type of terrace can be extensive, as terraces KT1 and KT2 show. In the second type, changes in the factors that control channel incision are much more important than those that create terrace surfaces. This requires that Ži. over the long term the river has a low tendency toward incision, which would be mainly due to the very gentle slope of the river reach under a relatively stable tectonic environment under which the channel maintains its Abase level of erosionB as proposed by Bull Ž1991.; and Žii. the factors Žprimarily river slope. that cause channel incision fluctuate infrequently—that is, the duration of the river’s vertical stability is much longer than cycles of climatic change. In this situation, a single terrace sur-

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face could be created over multiple climatic cycles Žor flood events.; whether a terrace would develop thus is determined by whether a base-level fall or tectonic perturbation occurs to induce channel incision. The CP1rCP2 terrace is an example as its abandonment requires downstream channel incision overcoming the inferred doming. The terrace generated under this situation would more fit with the concept of grade and the traditional notion regarding the river slope as the primary control of terrace formation. In addition, such a terrace surface is likely to be extensive and classified as a major terrace, because of the relatively long duration of channel occupation during its formation. In the real world, however, the formation of river terraces usually falls between these two end-member types—that is, when the frequencies of change in factors that initiate terrace surfaces and those that abandon terraces are comparable. One could hardly designate a cause or causes of the formation of these terraces without independently derived tectonic, base-level and climatic information. The tectonic and base-level histories that we derived from the terrace morphology, sequence, and radiocarbon dates allow us to interpret the genesis of the river terraces in the Erhjen River with confidence. The creation of the terrace surfaces in the downstream Erhjen River ŽKT1 , KT2 , and H Y . mainly reflects climatic changes or flood events Žthe first type terrace., whereas the contemporary CP1rCP2 terrace surface upstream mainly reflects the more or less constant valley slope Žas tectonic tilting is relatively minor in this part of the valley. and stable base level Ždue to the balance between downstream channel incision and the inferred doming. Žthe second type terrace.. The causes of abandonment of these terraces, as mentioned above, are complicated ŽFig. 14.. We propose that tectonic uplift Žand tilting., either steady or intermittent, is a prerequisite for long-term channel incision Žalthough we do not know if coseismic uplift has occurred in the Southwestern Foothills and affected the channel.. Short-term channel incision is strongly affected by climatic factors Ždischarge and bedload conditions, which also control formation of the terrace surface when there is no incision.. We interpret that the slower bedrock incision between the creation of the H Y and KT1 terrace surfaces is due, at least in part,

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Fig. 14. Schematic long profiles summarizing the inferred effects of long-term tectonic uplift and short-term climatic and base-level changes on the channel incision in Ža. Tributary 1 and Žb. main stem. Uplift and incision rates calculated in Table 2 and Fig. 10 are also shown.

to the drier climate Žor less frequent catastrophic rainfall events. than that during and between the creation of the KT1 and KT2 terrace surfaces. This also explains the slower bedrock incision between the creation of the H T and KT1 terrace surfaces in Tributary 1 ŽFigs. 10b and 14. and the finer grain size of the ca. 4.5 ka river deposition in the mountain-front area. Sometime after the abandonment of the KT2 terrace surface, a basin-wide channel incision occurred that also abandoned the CP1rCP2 terrace surface upstream. We propose that this episode of incision was triggered mainly by base-level fall following the abandonment of the Chungchou marine terrace surface in the Coastal Plain Žalthough the incision rate is also affected by climatic factors. ŽFig. 14.. This base-level fall then migrated upstream; this explains why the abandonment of the CP1rCP2 terrace surface in the upstream valley was significantly later than that of the terrace surfaces downstream ŽFig. 8b.. The incision of the trunk rivers caused base-level fall of the tributaries and eventually rejuvenated the whole basin. This also explains why most of the

modern channels in the Erhjen River, even those cutting through homogeneous mudrock, have knickpoints ŽFigs. 8 and 9.. This is true also in those reaches that connect narrow, deeply incised channels and the AunderfitB low-order valleys.

7. Conclusion We reconstruct the Holocene river history of the Erhjen River by correlating river terraces aided by 28 radiocarbon dates at multiple sites. Fluvial and marine deposition occurred since the latest Pleistocene in the basin outlet, apparently related mostly to the eustatic sea-level rise since the last glacial maximum. This deposition terminated no later than the middle Holocene Ž8–6 ka., followed by marine regression that resulted in emergence of the Coastal Plain. After that, the channel in the mountain-front area underwent repeated incision and deposition until the latest Holocene. The incision in this period carved mainly into Holocene sediments and little into

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bedrock, which may reflect the then-stable base level controlled by the development of the flat Chungchou marine terrace surface shoreward. In the lower Erhjen River Basin, the oldest river terraces, middle Holocene in age, are preserved as flat ridges or isolated mounts. The highest one above the modern channel Žca. 66 m. is found at Yuehshihchieh, which yields an average bedrock incision rate of 1 cmryear since middle Holocene. Channel incision dominated in the lower Erhjen River Basin after abandonment of these middle Holocene terraces, with a bedrock incision rate of 7–8 mmryear at Yuehshihchieh during ca. 5–2.5 ka. After ca. 2.5 ka, two major river terraces with widths of hundreds of meters were created in the main river; they converge downstream to the Chungchou marine terrace surface. The bedrock incision rate between the formation of these two terrace surfaces was about 5 cmryear at Yuehshihchieh. Meanwhile, only one late Holocene major terrace was developed in the major tributaries. While the lower Erhjen River was undergoing repeated channel incision and lateral erosion that created middle and late Holocene terraces, the river in the upper Erhjen River valley maintained a relatively constant level and developed a paleo-floodplain up to 2.5 km in width. This paleo-floodplain was completely abandoned only after 0.8 ka, following an episode of base-level fall starting from the Coastal Plain. Based on the apparent downstream and upstream convergence of these river terraces, we interpret a doming structure in the middle part of the basin, which results in a tilting rate of 10y6 to 10y7 per year in the downstream part of the Erhjen River valley. Rivers tend to incise in response to oversteepening of river slope. Thus, it would seem natural that the initiation of an erosional terrace surface that shifts the river from incision to lateral erosion requires a decrease of river slope or an achievement of a critical river long profile. Our evidence from the Erhjen River, however, suggests that the initiation of the major terrace surfaces in the lower Erhjen River Basin is more dependent on increases in bedload yield from hillslopes, triggered by catastrophic rainfall events that balance the prevailing channel capacity. Such an understanding, in fact, was envisioned by Gilbert Ž1877, p. 126.: it is the AloadB Žnot the reduction in slope. that reduces the downward corra-

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sion of a channel which facilitates the channel’s lateral erosion. We find that not only terrace-surface formation but also channel incision processes are strongly controlled by such climatic-driven discharge and bedload conditions, as suggested by the contrast of bedrock incision rates we observe at Yuehshihchieh. We do not deny the importance of river slope in controlling terrace genesis. An increase of river slope either by tectonic tilting or base-level fall should increase the tendency of a river to incise. River terrace surfaces thus cannot be created if the river is too steep. Still, we regard tectonic uplift as the fundamental agent for long-term channel incision, although in the short term, channel incision is strongly affected by climatic and base-level factors. This explains why at the same time that multiple levels of terraces were developed in the lower Erhjen River valley on the downstream limb of the doming structure the river upstream of this structure still maintained a relatively constant level without incision. We conclude that genesis of river terraces is a result of complex interaction between factors that initiate terrace surfaces Žmainly climate-driven. and factors that contribute to subsequent abandonment of the surfaces by channel incision Žincluding all baselevel, tectonic, and climatic factors.. We suggest that one must consider both stages in terrace development separately before designating the origin Žor origins. for a terrace.

Acknowledgements We thank Yao-Wen Lee for his helpful guidance to the Erhjen River, and Dr. Ping-Mei Liew of National Taiwan University for her helpful discussions and logistical support for field work in Taiwan. We greatly appreciate the assistance of Dr. Yu-Kao Chen of National Taiwan University, and the Central Geological Survey, R.O.C. who sponsored much of the radiocarbon dating. We also thank Kai-Shyuan Shea of the Central Geological Survey, R.O.C., who provided one unpublished radiocarbon date, and Tung-Sheng Shih of the Central Geological Survey, R.O.C., for his timely assistance in the field. This

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work was supported by National Science Foundation grant EAR93-15041 to Knuepfer.

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