Fluvial response to Late Quaternary climatic and environmental change, Edwards Plateau, Texas

Palaeogeography, Palaeoclh~Tatology. Palaeoecologv, 108 (1994): 1 21

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Elsevier Science B.V.. Amsterdanr

Fluvial response to Late Quaternary climatic and environmental change, Edwards Plateau, Texas Michael D. Blum a, R i c k a r d S. T o o m e y III b a n d S a l v a t o r e Valastro Jr. c ~Department of Geology, Southern Illinois University, Carbondale, IL 62901, USA bQuaternary Studies Program, Illinois" State Museum, Springfield, IL 62703, USA ~Radiocarbon Laboratoo,, University o/'Texas at Austin, Austin, TX 78753, USA (Received F e b r u a r y 9, 1993 revised a n d a c c e p t e d S e p t e m b e r 8, 1993)

A BSTRACT Blum, M.D., Toomey III, R.S. and Valastro Jr., S., 1994 Fluvial response to Late Quaternary climatic and environmental change~ Edwards Plateau, Texas. Palaeogeogr., Palaeoclimatol., Palaeoecol., 108: 1-2l. Radiocarbon-controlled late Pleistocene to modern stratigraphic frameworks for fluvial systems that drain the Edwards Plateau, westcentral Texas, coupled with reconstruction of climatic and environmental changes, permit development of a model for the evolution of fluvial landscapes during the past 20,000 years. During this time, large valleys of the Edwards Plateau were characterized by channel aggradat,on and sediment storage from 20 to 14,000 yrs B.P., deep excavation of bedrock valleys from ca. 14 to 11,000 yr B.P. and deposition of extensive and complex valley fills during the last 11,000 yr. Valley fills contain three distinct unconformity-bounded allostratigraphic units representing episodes of channel aggradation and floodplain construction during the early to middle (ca. 11,000 5000 yr B.P.) and late Holocene (ca. 5000-1000 yr B.P.), and development of the incised channels and associated narrow floodplains of the last 1000 years. Early to middle Holocene alluvial deposits were derived from proximal sources within the respective drainage basins, whereas late Holocene deposits include sediments derived from distal sources. Sediment supply to major valleys axes has been extremely limited during the last 1000 years. Alluvial sequences of the Edwards Plateau record fluvial responses to climatically-driven changes in discharge regimes and the concentration of sediments along valley axes. Allostratigraphic units define time periods when the concentration of sediment exceeded stream power, resulting in sediment storage, whereas unconformities record widespread morphological and sedimentary adjustments. Unconformities between late Pleistocene alluvium and Holocene valley fills record bedrock valley incision in response ~o decreased sediment loads associated with slope stability in the uplands. By contrast, unconformities within the Holocene valley fill record floodplain abandonment accompanied by continued channel migration and sediment storage, but little additional bedrock valley cutting. Episodes of floodplain abandonment occurred as a result of decreased flood magnitudes following shifts to drier climatic conditions at ca. 5000 and 1000 yr B.P. Fluvial responses to climatic change were conditioned by a progressive degradation of upland soils that caused increases through time in the flashiness of flood events, which in turn led to changes in processes of floodplain construction. Flood events on late Pleistocene and early to middle Holocene rivers were, for the most part, contained within channel perimeters, and floodplains were constructed by lateral migration. By contrast, late Holocene rivers were characterized by deep overbank flooding and floodplain construction by vertical accretion. High magnitude floods were most significant from ca. 2500 to 1000 yr B.P. when large chute channels were cut and filled on floodplain surfaces, and soils developed on previously stable terrace surfaces were buried by up to 2 m of fine sands and muds.

Introduction Documentation of Quaternary environmental change generally follows two complimentary paths. The first examines proxy records that define the state of climatic and biological systems at various times in the past, and provides empirical data SSDI 0 0 3 1 - 0 1 8 2 ( 9 3 ) E 0 1 3 9 - K

necessary for testing and refinement of paleoclimatic models (e.g. COHMAP, 1988). The second focuses on reconstructing the response of surface process systems to climatic and environmental change (Rutter et al., 1990). Such research should be undertaken at regional scales before interregional and global variability can be addressed.

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Moreover, to avoid interpretation of data within the context of pre-existing explanatory models, studies should rest on development of chronologically-controlled stratigraphic frameworks, and comparison with independently-derived records of climatic and environmental change. This is especially true for fluvial systems, where responses to climatic change may be physiographically circumscribed due to geologic controls, and nondeterministic due to antecedent conditions and internal complex response mechanisms (Butzer, 1980; Knox, 1983; Bull, 1991). Well-established traditions of research in fluvial response to climatic and environmental change exist for many areas (e.g. Knox et al., 1981; Knox, 1983; Starkel, 1983; Bull, 1991), but other parts of the earth's surface remain poorly understood. One such area is the south-central United States where prior to a few years ago investigations consisted of site specific descriptions of paleontological or archaeological sites, or relatively small-scale mapping of surficial deposits veneering pre-Quaternary bedrock. These efforts generally resulted in differentiation of Holocene floodplain alluvium from a number of terraces that were presumed to be Pleistocene in age. This paucity of more detailed examinations was evident in a comprehensive review of Holocene alluvial sequences by Knox (1983), wherethe lack of good stratigraphic and chronometric data from the south-central United States made comparison with other regions difficult. Recent studies have begun to define the record from this area. Examples include Hall's (1990) summary of late Holocene alluvial chronologies in Texas and Oklahoma, systematic investigation of the Holocene stratigraphy of small drainages on the Southern High Plains of Texas (Holliday, in press), and work on the Late Quaternary stratigraphy of the upper Trinity valley in north Texas (Ferring, 1990). This paper first summarizes geomorphological, sedimentological, stratigraphic, and geochronologic studies in one part of the south-central United States, the Edwards Plateau of west-central Texas (Fig. 1), to illustrate development of fluvial landscapes during the late Pleistocene and Holocene. Each valley axis studied drains physiographically and hydrologically sim-

M.D. B L U M ET AL.

ilar terrains. Moreover, the major components of these alluvial sequences appear to be regional in scope, implying that larger drainages of the Edwards Plateau share a common late Pleistocene and Holocene history. In this tectonically-inactive continental interior setting, far removed from the effects of base level change, such widespread changes in fluvial activity suggest climatic controls. The discussion therefore concludes by examining the alluvial stratigraphic record within the context of recent paleoclimatic reconstructions to suggest a model for regional fluvial response to climatic change.

General setting The Edwards Plateau of west and central Texas represents the southernmost extension of the Great Plains physiographic province of North America. The Plateau is bounded to the north by the Rolling Plains of north-central Texas, and merges topographically with the Southern High Plains of west Texas and eastern New Mexico to the northwest. The Basin and Range province and the Pecos-Rio Grande drainage form its western and southwestern boundaries, while the Balcones Escarpment separates the deeply dissected eastern and southern margins from the Gulf Coastal Plain. The Colorado River and its major tributaries, the Concho, San Saba, Llano, and Pedernales Rivers drain the northern part of the Plateau, while the Guadalupe, San Antonio, and Nueces Rivers and their tributaries drain the southern margins (Fig. 1).

Climate and hydrology The present climate of the Edwards Plateau is continental-semiarid to subtropical-subhumid and characterized by long, hot summers and cool winters (Larkin and Bomar, 1983). Mean monthly temperatures range from a low of 9-10°C in January to a high of 27-28°C in August (Fig. 2a). Relatively steep E-W precipitation gradients exist across the Plateau, with annual means of 81 cm along the Balcones Escarpment and 51 cm at the western escarpment leading into the Pecos drainage (Larkin and Bomar, 1983). Precipitation maxima

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occur in both late spring and early fall, with a primary minimum during the winter months and a secondary low during July and August (Larkin and Bomar, 1983; Fig. 2b). The present climatic regime supports a vegetation mosaic that ranges from a Juniper Oak Mesquite woodland in the east and south along the Balcones Escarpment to a more open Oak-Mesquite savanna farther west (Kier et al., 1977; McMahan et al., 1984; Riskind and Diamond, 1988). Hydrologic characteristics of major rivers on the Edwards Plateau reflect the seasonal and interannual variability in the precipitation regime. In addition, steep bedrock slopes with scarce vegetation and soil cover promote rapid concentration of surface runoff into stream channels (Baker, 1977). Flow duration curves are strongly right-

skewed and reflect long periods of low flow punctuated by short periods of high-magnitude discharge. Mean annual floods on larger perennial streams typically are more than two orders of magnitude greater than mean annual discharge, and maximum peak discharges recorded during the period of historical monitoring are three orders of magnitude greater (Blum, 1992). In contrast to the morphodynamics of smaller bedrock streams from this area, which reflect infrequent extreme high magnitude floods (e.g. Baker and Kochel, 1987), cross-sections of larger alluvial streams are adjusted to transmit discharges with recurrence intervals of 2 5 years, and channels undergo constant adjustment to, and recovery from, relatively frequent moderate- to high- magnitude floods (Blum, 1992). Cold fronts associated with midlatitude cyclones

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have produced some of the largest events on record. In most years, significant flood events are rare during the winter months when fronts pass too rapidly to permit advection of moist airmasses from source areas in the Gulf of Mexico and North Pacific this far inland. Along larger streams, floods rarely occur during summer months when midlatitude cyclones track farther north and precipitation results from more localized but intense convective thunderstorms (Bomar, 1983).

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Fig. 2. A. Mean monthly temperatures at Fredericksburg, Texas (period of record 1951-1980), located in central part of the plateau within the Pedernales drainage (after Larkin and Bomar, 1983). B. Mean monthly precipitation at Fredericksburg, Texas (period of record 1951-1980; after Larkin and Bomar, 1983). C. Monthly distribution of flood events in the annual duration series of the Pedernales River at Johnson City, Texas (period of record 1939-1988), and the upper Colorado River at Ballinger, Texas (period of record 1908-1967). Original data courtesy of United States Geological Survey, Austin, Texas.

are especially important to discharge regimes of larger rivers on the Edwards Plateau because they distribute precipitation over significant portions of the drainage basin, and track west to east in the same direction that surface runoff is routed through drainage networks. More than 90% of discharge events in the annual duration series of larger streams have occurred during spring or fall when such storms are responsible for the majority of precipitation (Fig. 2c). Tropical storms add to the frequency of occurrence of high magnitude floods during the late summer and early fall, and

The Edwards Plateau is an extensive tableland underlain by the relatively flat-lying Lower Cretaceous Edwards Limestone, with older Cretaceous and pre-Cretaceous rocks exposed in major river valleys. The Glen Rose Limestone (L. Cretaceous) dominates valley side walls and valley floors in the southern part of the Plateau, and constitutes the uppermost rock unit along the deeply dissected Balcones Escarpment (Barnes, 1983). In the Pedernales valley the arkosic Hensel Sandstone (L. Cretaceous) underlies the Glen Rose and makes up the valley floor in the upper half of the drainage (Barnes, 1981), whereas the upper Colorado valley is incised completely through the Cretaceous section, and exposes reddish Triassic and Permian siliciclastics, Permian carbonates, and Pennsylvanian carbonates and clastics (Barnes, 1986). Farther downstream the Colorado River enters the Llano topographic basin, where deformed Lower Paleozoic sedimentary rocks crop out around a core of Proterozoic igneous and metamorphic basement, then crosses the Balcones Escarpment in a deep canyon cut through Cretaceous carbonates (Barnes, 1981). The Edwards Plateau is a large-scale erosional landscape dominated by flat to gently rolling bedrock uplands and steep valley slopes with little soil cover. Major valleys contain flights of alluvial terraces in various degrees of dissection and/or preservation. Alluvial deposits in the Nueces and Guadalupe drainages consist of coarse limestone and chert gravel and calcareous mud derived from the Edwards and Glen Rose Limestones. By contrast, alluvial deposits in the Pedernales, Llano, San Saba, Concho, and upper Colorado Rivers

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include this same upland Edwards Plateau suite, but more importantly also contain easily identified sediments derived from older Cretaceous and preCretaceous rocks exposed in valley floors. Quaternary fluvial deposits are less extensive to absent in areas with bedrock- and/or structurallycontrolled steep channel gradients and high unit stream power, for example where the Colorado River and its tributaries cut through Paleozoic and older rocks of the Llano basin, and where major rivers cross the Balcones Escarpment in steep bedrock canyons. Mo(tern fluvial depositional systems of the Edwards Plateau consist of laterally-accreting chute channcl and chute bar modified, coarsegrained point bars, and high relief (5 7 m) floodplains constructed by vertical accretion. Lenticular bodies of medium to coarse sands and gravels, most commc, nly 3 5 m in thickness, dominate point and channel bar facies, whereas vertical accretion facies consist of 2 3 m of interbedded line sands and muds (Blum and Valastro, 1989; Gustavson, 19781.

Quaternary stratigraphy Previous studies of alluvial deposits on the Edwards Plateau were limited to general geologic mapping and identification of morphostratigraphic units. Following tradition the modern floodplain was assigned to the Holocene and all terraces were assumed to be Pleistocene in age. Mear's (1953) work along the Sabinal River in the Nueces system on the Plateau's southern margins, represents an important exception since two terraces and underlying fills of Holocene age were identified based on relationships with archaeological materials. Recent studies of the Pedernales River, a tributary in the Colorado system which drains the central part of the Plateau (Blum, 1989; Blum and Valastro, 1989), and the upper Colorado which drains the northern margins (Blum and Valastro. 1992), have resulted in the development of detailed alluvial sequences. Each is based on field and photogeologic mapping of geomorphic and stratigraphic relations, and field and laboratory documentation ,of the relative degree of soil development. Radiocarbon ages on organic-rich

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sediments and soils, in conjunction with temporally-diagnostic archaeological materials, provide chronological control for late Pleistocene and Holocene deposits. As noted above, observations in other valley axes across the Edwards Plateau suggest major elements of alluvial sequences identified in the Pedernales and upper Colorado Rivers are present there as well. The t\)llowing discussion outlines the alluvial stratigraphy for major valleys of the Edwards Plateau. drawing mostly on examples from the Pedernales and upper Colorado Rivel>,, Temporally distinct fluvial depositional landt\~rms and associated sedimentary facies are treated as informal, unconformity-bounded alloslratigraphic units (North American ('ommission ~m Stratigraphic Nomenclature. 1983: see also Autm, 1992). Nomenclature follows Birkeland (1984)and Machette (1985) for soils, Folk (1980) lk)r sediment texture, and McGowan and Garner (19701, Gustavson (1977), and Blum and Valastro (1989) for sedimentary facies. Figures 3 and 4 present a geomorphic map and schematic cross-section of the Pedernales wtlley, which illustrate characteristic spatial distributions and stratigraphic relationships for allostratigraphic units of the Edwards Plateau. Figure 5 summarizes key, data from soil profiles characteristic of late Pleistocene and Holocene allostratigraphic units of the Pedernales and uppcr Colorado Rivers. E a r l y to M M d l e P l e i s t o c e n e terrace r e m m m t s

In most large valleys on the Edwards Plateau a series of high, partially dissected to topographically isolated terrace remnants have been identified in previous general mapping efforts as Pleistocenc "High Gravels" (e.g. Barnes, 1981, 1983, 1986). Basal unconformities with underlying bedrock range from 15 to 30 m above the modern low water channel. Exposures typically show 2 5 m of calcite-cemented, horizontally-stratified gravel, horizontally laminated sand, and trough crossstratified gravel and sand that grade upwards into strongly indurated stage IV V petrocalcic soil horizons some I 2 m in thickness (e.g. Machette (1985). Thin ( < 50 cm) non-calcareous solums with well-developed Bt horizons may be present above

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petrocalcic horizons in various stages of degradation and erosion. Petrocalcic horizons with this degree of development formed over multiples of l0 s yr elsewhere in the arid to semiarid western United States (Machette, 1985). Although calcic soil develop-

ment is perhaps more rapid on the Edwards Plateau due to the abundant supply of calcium in limestone-derived parent materials, and a higher leaching rate, these terrace remnants and underlying deposits are still considered to be early to middle Pleistocene age. Similar high terraces of

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Fig. 5. Principal soil horizons, d i s t r i b u t i o n o f c a l c i u m c a r b o n a t e , and d i s t r i b u t i o n of clay-size fraction (Pleistocene only) for soils on different a l l o s t r a t i g r a p h i c units of the P e d e r n a l e s River.(A) and u p p e r C o l o r a d o River (B). Soils d e v e l o p e d within alluvial sequences of the two river systems have similar profiles a n d show similar trends with age, but the d e p t h o f leaching of p r i m a r y c a r b o n a t e rock fragments, and the d e p t h of d e v e l o p m e n t o f calcic h o r i z o n s differs in a c c o r d a n c e with the p r e c i p i t a t i o n g r a d i e n t across the E d w a r d s Plateau. being greater for the Pedernales River. N o t e t h a t for soils d e v e l o p e d in early to m i d d l e H o l o c e n e a l l u v i u m , depth indicales d e p t h below surface o f buried soil. Original d a t a f r o m Blum (1989) and Blum and Valastro (1992), except d a t a for soil developed in earl~, H o l o c e n e a l l u v i u m of the Pedernales River, which is courtesy of A n n e Kerr, Texas A r c h a e o l o g i c a l Research L a b o r a t o r y , University o f Texas at Austin.

presumed early to middle Pleistocene age occur on the Lampasas Cut Plain (see Fig. 1), an eastern outlier of the Edwards Plateau drained by the Brazos River and its tributaries (see Hayward, 1990).

Late Pleistocene deposits Two terraces and associated allostratigraphic units of late Pleistocene age have been mapped and described along the Pedernales and upper Colorado Rivers (Blum, 1989; Blum and Valastro, 1989, 1992), and appear to be present along most

major valley axes on the Edwards Plateau. The two units differ in terms of geomorphic and stratigraphic relations and the degree of development of argillic and calcic soil horizons (Fig. 5). Correlative late Pleistocene deposits appear to be less extensive or absent in smaller tributary valleys. Basal unconformities for the older of the two units rest on bedrock at elevations of 5 8 m above present low water channels, with average fill thicknesses of 4-10 m, and moderately dissected terrace surfaces at 12 15 m above present channels. Most sections consist of 3 6 m of horizontally and crossstratified gravel and sand, overlain by 3 5 m of

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fine sand and mud with occasional interbedded lenses of cross-stratified gravel and/or coarse sand. The lower half of most sections is strongly cemented by calcite precipitated from ground waters. Soils developed on terrace surfaces consist of well-developed, non-calcareous Bt horizons overlying Stage I I I - I I I + B k horizons (Fig. 5). A single radiocarbon age of 33,020 _+ 1620 yr B.P. (TX-5542) was obtained from organic rich muds in the Pedernales valley (Blum, 1989), but due to the large error term and proximity to the limits of radiocarbon dating this is considered to be a minimum age. Younger late Pleistocene allostratigraphic units are more extensive and well-preserved. They typically rest on bedrock at 2-5 m above present low water channels, with fill thicknesses of 8 10 m, and relatively undissected terrace surfaces at 12 14 m above low water channels. Most sections are dominated by channel-related facies, consisting of 5-8 m of horizontally and cross-stratified gravel and sand overlain by lenticular bodies of interbedded sand and mud some 2 3 m in thickness. Tabular vertical accretion-style floodplain facies are rare. Soil profiles developed on terrace surfaces vary a great deal as a function of depositional facies. Those developed in siliceous sandy or gravely facies on the Pedernales and upper Colorado Rivers consist of non-calcareous A and Bt horizons overlying Stage I I - I I + Bk horizons (Fig. 5). By contrast, in muddy channel-fill facies of the Pedernales and Colorado Rivers, and all facies along rivers like the Concho, Guadalupe, and Nueces which drain only carbonate terrain, mildly calcareous A and Bt horizons overlie stage I I - I I + Bk horizons (Wiedenfield et al., 1970; Botts et al., 1974; Allison et al., 1975; Clower and Dowell, 1988). Four radiocarbon ages provide constraints on the timing of deposition and subsequent soil formation. A finite age of 17,670_+230 yr B.P. was obtained from bulk organic material within a mud lens in the middle of this unit within the Pedernales valley (Blum, 1989), and represents an approximate time of deposition. In the upper Colorado valley, a minimum age of 14,300_+1190 yr B.P. was obtained from the lower part of a calcic horizon developed in muddy facies. Additional minimum

M.D. BLUM ET AL.

radiocarbon ages of 11,430+ 540 and 10,360+ 150 yr B.P. were obtained from carbonate nodules within calcic horizons that developed in alluvial deposits prior to erosional truncation and burial by non-calcareous eolian sands (Blum and Valastro, 1992). Although more chronological control is needed, available evidence suggests these alluvial deposits were emplaced from ca. 20-14,000 yr B.P., with terrace formation and soil development after that time. Nordt and Hallmark (1992) describe fluvial deposits in the Leon drainage on the Lampasas Cut Plain with similar soilgeomorphic and stratigraphic relations, and which produced a radiocarbon age of 15,270_+ 270 yr B.P.

Holocene valley fill." general setting Major streams on the Edwards Plateau began incising bedrock valleys ca. 14,000 yr B.P., a process that continued until ca. 11,000 yr ago when present valley depths were essentially established. Bedrock valleys have since been widened by lateral channel migration and thick valley fills have been emplaced. Valley fills of the Pedernales and upper Colorado Rivers contain three distinct allostratigraphic units (Fig. 6): (i) an early through middle Holocene unit, deposited from ca. 11,000 to 5000 yr B.P.; (2) a late Holocene unit, deposited from ca. 5000 to 1000 yr B.P. or shortly thereafter; and (3) the modern incised channel- and floodplain-related depositional environments which represent the last 800 years or so. Similar valley fills characterize the Concho, San Saba, and Llano Rivers before they join the Colorado trunk stream, as well as major valleys in the Guadalupe, San Antonio, and Nueces drainages above the Balcones Escarpment (e.g. Mear, 1953). Nordt and Hallmark (1990) document an extensive Holocene valley fill in the Lampasas Cut Plain as well. The earlier part of the sequence appears to be more complex, but the late Holocene record corresponds to that found on the Edwards Plateau.

Early to Middle Holocene alluvium Early to middle Holocene fills rest on bedrock near present low water channels, and may be 6-10 m or more in thickness. Upper boundaries consist of soil profiles or laterally traceable erosional

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Fig. 6. Cross-sections of Holocene valley fill of the upper Colorado River at (A) Simon Freese Dam near Coleman. Texas. some 10 km below the confluence with the Concho River, and (B) 2 km downstream from the confluence with the Cotlcho Rixer, illustrating soil-geomorphic and stratigraphic relations and positions of key radiocarbon ages (after Blum and Valastro, 1992). Qc.~eolian sand sheets: LP- late Pleistocene alluvium: EH= early to middle Holocene alluvium; LH- late Holocene alluvium: M modern depositional system. (C) Photograph illustrating typical stratigraphic relations between weathered early to middle I lolocenc (EH) and unweathered late Holocene (LH) alluvium of the Pedernales River near Fredericksburg, Texas, as well as positions of" radiocarbon ages (after Blum and Valastro, 1989). Terrace surface is 9.5 m above low water channel and modern point bar surlhce in foreground. Radiocarbon ages given in uncorrected and uncalibrated years before present.

10

unconformities with late Holocene alluvium. Along larger rivers like the Pedernales and upper Colorado, soil profiles are usually buried by up to 2 m of late Holocene alluvium. Radiocarbon ages from the upper Colorado River indicate that soilformation occurred from ca. 5000 yr B.P., when floodplains were abandoned as active depositional environments, to ca. 2500 2000 yr B.P. when pedogenesis was halted by burial or erosion. Archaeological data support this time range, since up to three thousand years of cultural activity is commonly associated with this soil profile (Blum and Valastro, 1992; Hester, 1971; Hester, et al., 1989). Soils typically consist of partially leached mollic A horizons overlying Bw and Stage 1 to 1 + Bk horizons (Fig. 5). In small tributary valleys, soil profiles most often were not buried by late Holocene alluvium (e.g. Mear, 1953; Collins et al., 1990). Deposition of early to middle Holocene allostratigraphic units was broadly contemporaneous in major valley axes across the Edwards Plateau, but alluvial facies differ in texture and composition. Along the Pedernales River most exposures consist of 4-7 m of interbedded sand and mud (Fig. 6c), horizontally-stratified to massive but with largescale lenticular geometry that represent deposition in low-relief channel margins with well-defined ridge and swale topography. These relatively finegrained sediments were derived from erosion of soils in the uplands and from the Lower Cretaceous Hensel Sands exposed along the valley floor. Transport of limestone and chert gravel to the valley axis from high in the drainage network was insignificant during this time period. By contrast, most sections along the upper Colorado River consist of 4-7 m of horizontally-stratified gravel and sandy gravel overlain by 2 m of sand and mud. Here, limestone clasts derived from local bedrock or reworking of older alluvial deposits dominate channel-related facies, and reddish sands and muds transported from far upstream reaches of the drainage basin are volumetrically less significant. Late Holocene alluvium

In larger valleys late Holocene fills rest on bedrock at or slightly below present day low water

"M.D. B L U M ET AL.

channels, and occur inset against early to middle Holocene alluvium. Along the Pedernales, upper Colorado and other major rivers, fill thicknesses exceed 9 11 m, and late Holocene sediments have completely buried soil profiles developed in early to middle Holocene alluvium. Upper boundaries to this unit consist of weakly developed soils, either with thin calcareous A horizons overlying cambic B horizons in sandy facies (Fig. 5), or with cumulic A horizons overlying cambic B horizons in muddy facies (Wiedenfield et al., 1970; Botts et al., 1974; Allison et al., 1975; Stevens and Richmond, 1971; Clower and Dowell, 1988). Secondary carbonates occur as films and filaments on ped faces in the lowermost part of the B horizon, hut rarely in sufficient quantity to warrant calcic horizon designation. Along smaller tributars; streams late Holocene sediments constitute a volumetrically minor component of Holocene valley fills, with fill thicknesses generally less than 5 6 m. Deposition of late Holocene allostratigraphic units was contemporaneous in major valley axes, but again alluvial facies differ in texture and composition. Channel-related facies along the Pedernales River consist of 1 2 m of horizontally bedded gravels overlain by 4-6 m of cross-stratified gravels and coarse sands, often with slip faces in excess of 1 m in height (Blum and Valastro, 1989). Abundant gravelly facies reflect delivery of limestone and chert gravels from sources in the upper part of the drainage, whereas large-scale crossstrata indicate the frequent occurrence of deep, high magnitude flows. By contrast, channel facies along the upper Colorado River consist of 2-3 m of horizontally bedded and cross-stratified gravel and coarse sand overlain by 3 4 m of interbedded, reddish cross-stratified medium to fine sand. Here, the abundance of reddish sand suggests frequent large floods capable of transporting sediments from Permian and Triassic source rocks that crop out in far upstream reaches of the drainage basin. Floodplain facies comprise the upper 3 4 m of most sections along both streams (see Figure 6c), and consist of interbedded fine sands and muds deposited by vertical accretion in high-relief floodplain settings (Blum and Valastro, 1989, 1992). Floodplain facies also contain inset largescale lenticular chute channels and channel fills,

FI tJVIAL RF.SPONSE I O LAI t¢ QL A T E R N A R Y C L I M A T I C A N D E N V I R O N M E N T A L C H A N G E , E D W A R D S P L A T E A | , I I X A S

indicating high magnitude flooding and scouring of floodplain surfaces (Fig. 7), especially from ca. 2500 to 1000 yr B.P. It is during this same time period that soils developed in early to middle Holocene alluvium were buried by terrace veneer facies (Blum and Valastro, 1992).

Modern depositional environments Volumetrically minor allostratigraphic units produced by the incised and underfit streams of the last millennium consist of gravely and sandy point and channel bars with 2 3 m of relief, and horizontally-stratified muddy and sandy facies that underlie narrow ( < 100 m in width) constructional floodplains at 4 6 m above low water channels. Along larger streams like the Pedernales and upper Colorado Rivers, modern floodplains rest 3-5 m lower in elevation than abandoned late Holocene counterparts, and demonstrably inset. Thin accumulations of recent sediments veneer Holocene terrace surfaces at tributary-trunk stream junctions or other localities where flow cross-sections become

] I

constricted. Sediments from the last millennium can be distinguished on the basis of preserved sedimentary structures, the absence of soil development, and in some cases by the presence of historic artifacts (Blum and Valastro, 1989, 1992). The youthfulness of high terrace surfaces and inset active channel- and floodplain-related depositional environments is not unique to the Edwards Plateau, since channel entrenchment and/or floodplain abandonment occurred ca. 1000 yr B.P. throughout much of the south-central United States (Hall, 1990). But little is known concerning processes of fluvial adjustment since most investigations were focused solely on identification of stratigraphic relationships. In the Pedernales drainage excellent preservation and exposure of late Holocene and modern allostratigraphic units provides an opportunity to examine morphological and sedimentary adjustments in detail (Blum and Valastro, 1989). When compared to late Holocenc counterparts prior to ca. 1000 yr B.P., the modern Pedernales River maintains a distinctly underfit

Fig. 7. Air photo of area surrounding the confluence of the Concho and upper Colorado Rivers, west Texas (location l in Fig. I), illustrating chute channels developed on now-abandoned late Holocene floodplains by high magnitude floods. Photograph represents a black and white reproduction of a color-infrared original.

12

M.D. B L U M ET AL.

channel with a lower w i d t h - t o - d e p t h ratio, as well as a channel cross-sectional a r e a a d j u s t e d to smaller b a n k f u l l discharges, a n d t r a n s p o r t s a greatly d i m i n i s h e d a n d g r a v e l - p o o r s e d i m e n t l o a d (Fig. 8 a n d Fig. 9). M o r p h o l o g i c a l a n d sedim e n t a r y a d j u s t m e n t s a l o n g o t h e r streams on the E d w a r d s P l a t e a u r e m a i n difficult to establish, b u t sediment delivery to m a j o r valley axes has been extremely limited d u r i n g the p a s t 1000 years, a n d resultant a l l o s t r a t i g r a p h i c units are v o l u m e t r i c a l l y m i n o r c o m p o n e n t s o f H o l o c e n e valley fills.

Fluvial response to climatic and environmental changes

A n u m b e r o f writers discuss f u n d a m e n t a l concepts that underlie i n t e r p r e t a t i o n o f the response o f fluvial systems to climatic and e n v i r o n m e n t a l c h a n g e (e.g. Butzer, 1980; K n o x , 1976, 1983; Starkel, 1983; Bull, 1991). M o s t focus on changes in the r e l a t i o n s h i p between discharge regimes a n d the c o n c e n t r a t i o n o f s e d i m e n t a l o n g valley axes, a n d the effect changes in this r e l a t i o n s h i p have on

Pedernales River

A.

gravels,

sandy gravels|~:~,:~*~1longltUCIlnaloars, ChUtechannel tills gravelly~ chute channel fills, sands [ ' ~. Idunes, sand waves sands ~ dunes, sand waves, t~.-Z :qplane beds muddy sands,/Z.Z.Z.].Irinnl,~ 1,,~in~t,.~l ~

Pedernales River

Low Water Channel

sandymuds'.t

/

...

muds [ ~

B.

f .........

)//

Fig. 8. Schematic block diagrams illustrating morphological and sedimentary adjustments by the Pedernales River near Fredericksburg, Texas. A. The modern Pedernales River. B. The late Holocene Pedernales River prior to ca. 1000 yr B.P. (after Blum and Valastro, 1989). Details of facies shown in B underlie late Holocene terrace surfaces shown in A.

t I I Vl/\I P.FISP()N%II IO I.A IF ()[I,\qERNAR'~ CLIMA]IC AND |!NVIRONMEN'IAL CHANGE, EI)WAR[)S PI~ATI-A[. IEX*XS

13

A. Bankfull Width

5 ~--

-~

Bankfull Width

~'1

4

"-" 3 ~D

2 1O0

25

50

I

I

I

Width (m) VE = 8x

B. Bankfull Width

54..=1 ..a

321

,, !

I

/

Bankfull Cross-Sectional Area = 290 sq. m Estimated Bankfull Discharge = 363 cms Bankfull Width-To-Depth

R a t i o = 35

Fig. 9. A. Measured channel cross-section of the modern Pedernales River, combined with estimated bankfull discharge. B. Reconstructed cross-section of the late Holocene Pedernales River based on measurements along lateral accretion surfaces exposed in sand and gravel quarries, combined with estimated bankfull discharge (after Blum and Valastro, 1989).

alluvial channels, morphogenesis of fluvial landforms, and the development of stratigraphic sequences. In general, alluvial channels place sediment into storage when sediment supply exceeds stream power, and incise bedrock or remove sedi-ment from storage if the reverse is true. Changes in patterns of sediment storage or removal along valley axes actually occur via adjustments in channel morphology and sedimentation style (e.g. Knox. 1976, 1983; Starkel, 1983; Bull, 1991). Extended periods of net sediment storage result in alluvial stratigraphic units, whereas widespread morphological and sedimentary adjustments produce bounding unconformities in alluvial sequences (see Autin, 1992). Although widely recognized that climatic change can drive changes in discharge regimes and sediment supply, no generally accepted deterministic model for fluvial response to climatic change presently exists. The response of an individual drainage basin to changes in climate may, for example, vary in upstream and downstream reaches, or between tributaries and trunk streams, as energy and mate-

rials are cycled through the drainage network. Moreover, due to variability in geologic controls and antecedent conditions, and differential sensitivities to change, fluvial systems in different physiographic regions may exhibit divergent response to the same climatic change or convergent response to different controls (e.g. Butzer, 1980; Schumm and Brackenridge, 1987: Bull, 1991). Hence, chronologically-controlled stratigraphic frameworks such as that presented above should be interpreted within the context of independently-derived records of climatic and environmental change. Record o f Late Quaternary climatic and ,n ~,ironmen tal change

Synthesis of fossil vertebrate, pollen, and plant macrofossil data from the Edwards Plateau and south-central United States, combined with the results of paleoclimatic modeling experiments (Kutzbach and Guetter, 1986; C O H M A P Project Members, 1988), permits reconstruction of changes in the temperature and moisture regimes, storm

M.D. BLUM ET AL.

14

types, vegetation, and upland soil mantles over the past 20,000 years (Toomey et al., 1993; Fig. 10). Proxy data shows that late Pleistocene temperatures were significantly cooler than those o f today, especially during the s u m m e r months, but by ca. 13,000 yr B.P. s u m m e r temperatures were within 2 - 3 ° C o f present values. M o r e significant have been changes in the seasonality o f temperature, with minimal seasonality from ca. 20,000-14,000 yr B.P., maximum seasonality from ca. 11,000-5000 yr B.P., and essentially m o d e r n thermal regimes since then. P r o x y data also provide information on changes in the moisture regime. There was, for example, considerably m o r e effective moisture between ca. 20,000-14,000 yr B.P. than at any time since then. F r o m ca. 14,000-10,500 yr B.P., effective moisture first decreased then increased, while the early to middle Holocene was d o m i n a t e d by a protracted decrease in effective moisture. This long-term trend culminated in post-glacial m i n i m u m s in effective moisture during the early part o f the late Holocene from ca. 5000 to 2500 yr B.P. Between ca. 2500

CLIMATE Eft. Moisture high low high

Temperature 0-

low

and 1000 yr B.P., effective moisture was greater than at present, while the m o d e r n d r o u g h t - p r o n e climate has characterized the last millennium. Changes in climate system b o u n d a r y conditions p r o b a b l y resulted in changes in storm types and the seasonality o f rainfall inputs. Midlatitude cyclones would have p r o d u c e d the majority o f precipitation t h r o u g h the late Pleistocene, with isolated convectional storms less frequent due to cooler summer temperatures, and tropical storms rare due to the greatly diminished size and reduced temperatures in the G u l f o f Mexico. By contrast, increased summer insolation, coupled with greatly reduced continental ice volumes and glacio-eustatic sea level rise, favored greater m o n s o o n a l flow during the early t h r o u g h middle Holocene, with the summer half-year dominated by maritime tropical airmasses and isolated but high intensity convectional storms. The present seasonal distribution o f precipitation events p r o b a b l y took hold in the middle to late Holocene when seasonality o f temperature a p p r o a c h e d m o d e r n values and the G u l f o f Mexico reached its present size and temperature.

TYPESOF STORMS Front. Conv. Trop. W, Sp, F

S

S, F

5 Sp, F ~. 10

I ]/

~. . . . F

mixed grasses

savanna with

S

savanna with mixed grasses

%"x~ ~

vN,,N

UPLAND SOILS min. max. thin to non-existent thin, brown stony soils moderately deep reddish-brown loamy soils

deep red

o~

15

UPLAND VEGETATION savanna with mixed grasses short grasses and scrub

W, Sp,

F

/

1

Sp, S. . . . . . . F

---

clayey soils

savanna with mixed grasses

2 0~hveCraegtury 20 Fig. 10. Summary of climatic and environmental changes on the Edwards Plateau, Texas during the last 20,000 years. Mean annual temperature curve is based on faunal and pollen records, whereas inferred seasonality is based on orbital forcing and lag effects due to ice sheet. Effective moisture is based primarily on faunal records, but supported by pollen data. Inferred storm types and seasonality is based on changes in climate system boundary conditions and climate model results (Front.= midlatitude cyclonic or frontal storms; Cony.= high-intensity convectional storms; Trop.= tropical storms and hurricane; S = summer, F = fall, W= winter, Sp= spring). Changes in upland vegetation and soil mantles on the Edwards Plateau are based on faunal records and localities with preserved cave fill sequences (from Toomey et al., 1993).

FLUVIAl RESPONSE TO L A I E Q U A T E R N A R Y CLIMATIC A N D E N V I R O N M E N T A l ( ' H A N G [ , El)WARDS P L A T E A t

Vertebrate faunas and cave fill sediments provide critical information on changes in vegetation cover and soils present on upland surfaces of the Edwards Plateau. During the late Pleistocene upland landscapes were covered by an open savanna with a mixed tall and short grass understory, and more importantly, by deep red soils. Changes to strongly seasonal temperature regimes and enhanced monsoonal flow promoted a diminished vegetation cover, and the increased importance of high intensity summer convectional storms, which together favored the gradual degradation of soil profiles. Hence middle Holocene upland soils were thinner, stonier, and darker in color than late Pleistocene to early Holocene counterparts. Minimum effective moisture during the earlier part of the late Holocene, ca. 5000-2500 yr B.P., resulted in upland landscapes that were covered by a mixture of short grasses and scrub vegetation, and the near complete removal of the remaining soil mantle. Vegetation changes during the last 2500 years are poorly known, but the upland landscape consisted of exposed limestone bedrock with little soil cover.

Responses o/"Edwards Plateau fluvial systems Allostratigraphic units and bounding unconformities in alluvial sequences of the Edwards Plateau record changes in patterns of sediment storage and removal along major valley axes (Fig. 11). We suggest that discontinuities in the late Pleistocene through Holocene part of the record roughly correlate with climatic changes discussed above, and represent a series of responses to climaticallydriven changes in discharge regimes and/or the concentration of bedload sediments. Fluvial responses to changes in climate were less than straightforward, however, as they were conditioned by a protracted degradation of soil mantles in upland settings which produced changes in the hydrological response of the drainage network and depositional processes.

Late Pleistocene fluvial systems From 20,000-14,000 yr B.P. significant precipitation inputs on the Edwards Plateau most likely occurred in association with the slow passage of

IIXAS

[5

midlatitude cyclones. With grassland vegetation and deep soil profiles in the uplands, resultant flood hydrographs probably were less flashy and broader-based than today due to increased infiltration. Major streams on the Edwards Plateau record a protracted episode of channel aggradation, lateral migration, and sediment storage, indicating the concentration of bedload sediments exceeded stream power (Fig. l la). Thin to nonexistent vertical accretion facies within late Pleistocene alluvial fills suggest that tloodplam construction proceeded by lateral migration, with most flood events contained within channel perimeters. Sediments for alluvial fills along major valley axes may have been supplied by clearing of upper reaches of the drainage network, since deposits of this age are rare in tributary valleys. From ca. 14,000 11,000 yr B.P., the Edwards Plateau was characterized by increasing seasonality in the temperature regime, available moisture decreased, and grassland vegetation with deep soil and weathering profiles persisted on upland surfaces. During this time, channels incised bedrock valleys in response to substantial decreases in bedload sediment supply (Fig. 1 l b) that apparently resulted from near-complete clearing of sediment stored in smaller tributary valleys and continued stability of soil mantles in the uplands. Hence even though available moisture decreased, stream power was more than sufficient to transport a greatly diminished sediment load, and no preserved depositional record exists for this time period.

Early to Middle Holocene.fluvial systems The early to middle Holocene was characterized by a protracted warming and drying trend, a strongly monsoonal atmospheric circulation, and a precipitation regime dominated by high-intensity but relatively localized convectional storms during the summer half-year. Such storms probably initiated the gradual removal of deeply weathered soils on the uplands, and promoted the introduction of proximal slope-derived sediments into channel networks. Although efficient at removing sediments from hillslope source areas, high intensity, localized convectional storms would have produced small flood discharges in larger streams. Early to middle Holocene rivers on the Edwards

16

M.D. BLUMET AL. ca. 20-14,000 yrs BP

A.

ca. 14-11,000 yrs BP

- bedload sediment supply exceeds transport capacity - s l o w l a t e r a l m i g r a t i o n and valley widening - deposition of late Pleistocene fills

- bedload sediment supply l e s s than t r a n s p o r t capacity - f l o o d p l a i n a b a n d o n m e n t and t e r r a c e f o r m a t i o n

- deep incision of bedrock valleys - no preserved depositional record

B. ~

~

Terrace

ca. 11-5000 yrs BP

Terrace

./

- bedload sediment supply exceeds transport capacity - s l o w l a t e r a l m i g r a t i o n and valley widening

C.

- deposition of early .to middle Holocene fills

Plateau were characterized by slow lateral migration and valley widening, with very slow channel aggradation, floodplain construction, and sediment storage (Fig. 11c). Resultant allostratigraphic units are prominent components of valley fill sequences in small tributary valleys and along all major valley axes. The texture and composition of alluvial facies varies from valley to valley in accordance with the lithologic characteristics of proximal source terrains, as channels were loaded with slope-derived materials. Where present, coarse-grained facies consist of horizontally bedded and imbricated gravels and sandy gravels, and rarely possess large high-angle foresets indicative of deep floods. Thin vertical accretion facies support the interpretation of infrequent deep overbank flood events, and

suggest floodplain construction occurred by slow lateral migration and channel aggradation.

Late Holocene fluvial systems The Late Holocene period was characterized by dry climatic conditions from ca. 5000 to 2500 yr B.P., then a shift to humid conditions that persisted from ca. 2500 to 1000 yr B.P. Midlatitude cyclones and tropical storms produced the majority of significant discharge events, but were perhaps infrequent during the early part of the Late Holocene, and more frequent during the latter part. The near complete removal of weathered mantles on upland surfaces promoted rapid concentration of runoff into stream channels, especially after ca. 2500 yr B.P.

FLUVIAI RESPONSE I O LATE Q U A T E R N A R Y CLIMATIC AN[) E N V I R O N M E N I A [ . C H A N G E , EDWARDS PLATEAU. FEXAS

I7

ca. 5000-2500 yrs BP - floodplain abandonmentand terrace formation bedload sediment supply exceeds transport capacity initial deposition of late Holocene fills D. ~ \ channel facies dominant \ \ Terrace Flood Channel Terrace ~[][[ITelrrla~e[[~t/~ -

-

-

ca. 2500-1000 yrs BP

- bedload sediment supply exceeds transport capacity continued deposition of late Holocene fills deep flooding with burial of low terraces and soils floodplain facies widespread and thick -

-

E.

-

~

Channel

Floodplain

\ .

.

.

.

/

Fl°°dplain ~ :t I : Te~Cle:t V / ~

.

ca. last 1000 yrs

- bedload sedimentsupply extremelylimited floodplainabandonmentand terrace formation developmentof modern incised and underfit ~channelsand narrow f l o o d p l a i n ~ -

F .

-

-

-

~

i

Channel

.... ""':!

Rood

i.:~'

Terrace

,',., ......

I II

Terrace .............

f ,

Fig. l 1. Schematiccross-sections illustrating evolution of late Pleistocene and Holocene alluvial sequences along major valley axes of the Edwards Plateau, Texas, in response to climatically-drivenchanges in discharge regimes and sediment supply. Late Holocene fluvial activity closely reflects these climatic and environmental changes. From ca. 5000 2500 yr B.P., major rivers of the Edwards Plateau abandoned early to middle Holocene floodplains hut continued to migrate laterally and store sediment (Fig. lid). Floodplain abandonment with soil formation resulted from decreased flood magnitudes associated with a shift to dry climatic conditions. By contrast, from ca. 2500 to 1000 yr B.P., major rivers deposited thick vertical accretion facies, cut and filled chute channels on floodplain surfaces, and buried soils developed on early to middle Holocene surfaces with terrace veneer facies (Fig. l le). Such activity reflects frequent moderate- to high-magnitude floods associated with a change to moist climatic conditions,

coupled with thin to non-existent weathered manties in the uplands. Vertical accretion of floodplain and terrace veneer facies continued until ca. 1000 yr B.P. or shortly thereafter, when these surfaces were abandoned as frequently active depositional environments following a shift to drier climatic conditions (Fig. l l f). Late Holocene allostratigraphic units represent a major component of valley fill sequences along large trunk streams of the Edwards Plateau, but a volumetrically minor component along lower order tributaries. Hence discharges produced by midlatitude cyclonic and tropical storms changed the manner in which sediments were cycled through the system. Rather than storage in proximal localities throughout the drainage network, as was the

[8

M.D. BLUM ET AL.

case during the early to middle Holocene, sediments were flushed from tributaries to principal valley axes. The texture and composition of alluvial fills supports this view, since bedload sediments were derived from distal sources within respective drainages.

Modern fluvial systems Climatic conditions of the last millennium have fluctuated somewhat, but on average were similar to those of today. Precipitation events resulted from midlatitude cyclonic, convectional, and tropical storms, and precipitation inputs were impacting on bedrock slopes. Storm runoff has been routed through channel networks that were sedimentstarved due to near complete removal of weathered materials from upland source regions and clearing of tributary networks. The geomorphic and stratigraphic result of these conditions has been abandonment of late Holocene floodplains, net sediment removal from valley axes, incised and in some cases underfit channels flanked by narrow floodplains, and a volumetrically minor allostratigraphic unit (Fig. l lf). Modern facies assemblages resemble late Holocene counterparts, with thick vertical accretion facies and numerous chute channels inset into floodplain surfaces, but are considerably smaller in lateral extent. Extreme high magnitude floods of historic age, perhaps due to changes in land use, have overtopped older Holocene terrace surfaces but left clearly distinguishable depositional records in selected localities only.

Summary and conclusions Valleys of the Edwards Plateau, west-central Texas, contain flights of alluvial terraces that record fluvial activity beginning in the early middle Pleistocene. Older deposits and landforms are fragmentary, undated, and difficult to interpret, but the more recent part of the stratigraphic record provides an opportunity to reconstruct the evolution of fluvial landscapes in some detail. Large valley axes were, for example, characterized by channel aggradation, floodplain construction, and sediment storage from ca. 20,000 14,000 yr B.P. Abandonment of late Pleistocene floodplains ca.

14,000 yr B.P. was followed by excavation of bedrock valleys to near present depths by ca. 11,000 yr B.P., then by two episodes of net channel aggradation and floodplain construction during the early to middle (ca. 11,000-5000 yr B.P.) and late Holocene (ca. 5000 1000 yr B.P.). The two fills are separated by erosional unconformities representing floodplain abandonment and soil formation. Early to middle Holocene fills are dominated by sediments delivered to major valley axes from relatively local sources within the respective drainage, whereas late Holocene fills include sediments delivered from distal portions of the system. The modern incised channels and associated floodplains represent the last millennium of activity when sediment supply to major valley axes has been limited. Discontinuities in the late Pleistocene through Holocene alluvial record correlate with independently-defined climatic changes, and represent a series of fluvial responses to climatically-driven changes in the relationship between discharge regimes and the concentration of sediments along valley axes. Allostratigraphic units define extended periods of sediment storage, whereas unconformities record widespread morphological and sedimentary adjustments. Unconformities that developed from ca. 14-11,000 yr B.P. reflect deep incision of bedrock valleys in response to decreases in sediment supply associated with slope stability in the uplands. By contrast, unconformities that developed ca. 5000 and 1000 yr B.P. represent floodplain abandonment and soil formation, accompanied by continued channel migration and sediment storage, but little additional bedrock valley cutting. These episodes of floodplain abandonment occurred as a result of decreased flood magnitudes following shifts to drier climatic conditions. Different episodes of fluvial activity were roughly time parallel in major valley axes throughout the Edwards Plateau, at least within the limits of resolution of radiocarbon dating. Fluvial responses to climatic change were conditioned by progressive degradation of soils on upland surfaces of the Edwards Plateau. This degradation caused increases through time in the rates at which runoff was transferred to valley axes, corresponding increases in the peakedness or

I:LL;~I,\[ RESPON~;E T O LAI I: Q U A T E R N A R Y C L I M A T I C A N D [ - N V I R O N M E N T A L C H A N G E , E D W A R D S P L A T E A U . T E X A S

flashiness of flood hydrographs, and changes in the processes of floodplain construction. Flood events on late Pleistocene through middle Holocene rivers were, for the most part, contained within channel perimeters, and floodplains were constructed by channel aggradation and lateral migration without significant vertical accretion. Increased flood peaks during the late Holocene promoted the increasing importance of floodplain construction by vertical accretion, thus late Holocene and modern allostratigraphic units con-. rain thick vertical accretion facies. High magnitude floods were most significant from ca. 2500 to 1000 yr B.P., when moist climatic conditions prevailed, and relatively frequent basinwide precipitation events were impacting on exposed bedrock landscapes. At this time large chute channels were cut and filled on floodplain surfaces, and soils developed on previously stable terrace surfaces were buried by up to 2 m of fine sands and muds. Correlations of fluvial response to climatic change within and between physiographic regions remains complicated due to the operation of internal complex response mechanisms, and the differential sensitivities of fluvial systems that arise from differences in geological controls and antecedent conditions. Alluvial sequences along major valley axes of the Edwards Plateau developed independently of each other but in response to the same sequence of climatic and environmental changes. Since each major valley axis drains physiographically and hydrologically similar terrains it is not surprising that each has a similar history of changes in the relationship between discharge regimes and concentration of sediments. Significant differences exist between alluvial sequences along lower-order tributaries and higher-order trunk streams, which probably reflects internal cycling of materials, but these effects were filtered out and did not upscale into larger valley axes. This suggests that models for regional fluvial response to climatic change should be developed from larger drainages which filter out internal complex response mechanisms and other smallscale localized effects. On the Edwards Plateau, erosional and depositional processes during the last 20,000 years were

]9

strongly influenced by the initial presence of deep weathering profiles on upland landscapes, their subsequent gradual degradation to present-day exposed bedrock surfaces, and the cycling of eroded sediments through drainage networks. Hence responses of Edwards Plateau fluvial systems to climatic change may not be representative of fluvial systems within other physiographic regions where geologic controls and the history of sediment supply may have been significantly different. This interregional variability, dependent as it is on geologic controls and antecedent conditions, must be addressed before general models for fluvial response to climatic change can be fully developed.

Acknowledgements We would like to thank a number of individuals who read or commented on various stages of this manuscript. These include Karl Butzer, Steve Hall, Kees Kasse, and Whitney Autin, as well as Palaeo 3 reviewers William B. Bull and Richard M. Forester. M.D. Blum's work on the upper Colorado River was supported by Prewitt Associates Inc. and Mariah Associate Inc., both of Austin, Texas. Additional parts of this research were supported by U.S. National Science Foundation G r a n t SES-9001243 to M.D. Blum and K.W. Butzer, and Geological Society of America grants to M.D. Blum.

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