- Email: [email protected]
Properties of a 5500-year-old flood-plain in the Loup River Basin, Nebraska
Geomorphology 56 (2003) 243 – 254 www.elsevier.com/locate/geomorph
Properties of a 5500-year-old flood-plain in the Loup River Basin, Nebraska David W. May * Department of Geography, University of Northern Iowa, Cedar Falls, IA 50614-0406, USA Received 15 June 2002; received in revised form 3 February 2003; accepted 21 March 2003
Abstract Flood-plain aggradation within the Loup River Basin of central Nebraska was episodic and alternated with incision throughout much of the Holocene. A widespread episode of flood-plain stability, however, occurred about 5700 – 5100 cal. year BP. The purpose of this paper is to describe the properties of this buried flood-plain at six sites in the basin, to consider why the properties of the buried flood-plain vary from site to site, and to evaluate possible reasons why the Loup River flood-plains stabilized 5500 years ago. Episodic valley-bottom aggradation was common during flood-plain formation at five of the six sites. The radiocarbon ages, particle-size data, and organic-carbon data for the buried flood-plain reveal that valley-bottom aggradation generally slowed between about 5700 and 5100 cal. year BP. Erratic down-profile changes in percentages of sand, clay, and organic matter indicate flood-plain sedimentation and soil formation were often episodic. Sand and clay rarely show a steady fining-upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. At two localities, the buried flood-plain is evident as a clay-rich stratum that must have been deposited in a paleochannel. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the effects of external climate forcing on vegetation and hydrologic changes. flood-plains of other rivers in the central Great Plains also stabilized at this time, further supporting a climatic explanation for slowing of valley aggradation and formation of a flood-plain at this time. Recognition of buried flood-plains is important to both soil mapping in valleys and to the discovery of cultural resources in valleys. D 2003 Elsevier Science B.V. All rights reserved. Keywords: flood-plain; Holocene; River; Buried soil; Great Plains
1. Introduction Flood-plains adjust to sediment inputs and discharge regimes on various timescales (Schumm, 1977; Knox, 1977; Schumm and Brakenridge, 1987; Walling et al., 1998). Over the timescale of the Holocene, such adjustments have been shown to be * Fax: +1-319-273-7103. E-mail address: [email protected] (D.W. May).
responses to internal adjustments within the fluvial system (Patton and Schumm, 1981), climate changes (Knox, 1983; Bull, 1991), and crustal movements (Dumont et al., 1996; Fraser et al., 1997). Fluvial adjustments can be largely by lateral movements of the river channel (Wolman and Leopold, 1957; Schumm and Brakenridge, 1987) or by vertical changes in the elevation of the flood-plain (Schumm and Lichty, 1963; Ritter et al., 1973; Stene, 1980; Nanson, 1986; Brakenridge, 1988).
0169-555X/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(03)00154-5
244
D.W. May / Geomorphology 56 (2003) 243–254
In the semi-arid Great Plains of North America, the Holocene record of fluvial changes has been one of primarily vertical adjustments of the flood-plain (Johnson and Logan, 1990; Martin, 1992; Arbogast and Johnson, 1994; May et al., 1995; Mandel and Bettis, 2001) as opposed to lateral migration of channels and concomitant flood-plain reworking that was common during the Holocene in the eastern United States (Knox, 1983; Jacobson and Coleman, 1986; Schumm and Brakenridge, 1987). The alternating episodes of aggradation and entrenchment of river valleys in the central Great Plains have been punctuated by brief episodes of flood-plain stability. The focus of this paper is the flood-plain that formed 5700– 5100 cal. year BP across valley floors in the Loup River basin of central Nebraska (Fig. 1) on or very near the surface of early-middle Holocene alluvium (Fill IV). The purposes of this paper are to describe the properties of the alluvium in the upper part of Fill IV that comprises this buried flood-plain, to identify variability in the properties of the buried flood-plain throughout the basin, and to evaluate causes, especially climatic change, for why this buried flood-plain formed. This paper addresses the importance of distinguishing alluvial strata from buried soil horizons, evaluates synchronous regional flood-plain
stability in the Great Plains, and demonstrates the relevance of understanding spatial variability in properties of buried flood-plains to soil mapping and archaeological investigations in river valleys.
2. Study area The major northwest-to-southeast tributaries to the Loup River drain the Sand Hills and Dissected Plains topographic regions of central Nebraska (Dreeszen, 1973). This area is underlain by Early Pleistocene and Pliocene alluvial sand, pebble gravel, and lacustrine sand and silt (Swinehart et al., 1994). Surficial deposits include Late Quaternary eolian sand, especially in the upper half of the Loup River basin, and Peoria Loess throughout the Dissected Plains region of the lower half of the basin (Swinehart et al., 1994; Mason, 2001). Groundwater discharge dominates the upper reaches of the major rivers in the Loup River basin (Bentall, 1989), while surface runoff comprises a greater proportion of the discharge downstream. The modern climate of the Loup River basin varies from east to west. The eastern portion of the basin receives an average of as much as 68.6 cm (27 in.) of precipitation annually, while the northwestern portion
Fig. 1. Map of Loup River basin and study localities: TI = Tibbets; CO = Coopers Canyon; MO = Moffet Creek; HO = Horn; HI = Hickenbottom; CH = Chesley. Base map drawn from Hydrologic Unit Map (US Geological Survey, 1974).
D.W. May / Geomorphology 56 (2003) 243–254
of the basin receives an average of as little as 45.7 cm (18 in.) (Lawson et al., 1977). The climate of the eastern portion of the basin is classified as Dfa (Moist Continental), while the climate of the western portion of the basin is classified as BS (Steppe) (Koppen, 1918). Pre-settlement vegetation in the Loup River basin consisted of Mixed Prairie (bluestem grasses, grama grasses, and buffalo grass) in the southern half of the basin and Sandhills Prairie (bluestem grasses, sandreed grass, needlegrass, and yucca) in mostly the northern half of the basin (Kaul, 1975). Holocene alluvial stratigraphy in the Loup River basin includes five valley fills (May et al., 1995). However, only two fills (IV and I), are germane to this study. Fill IV accumulated in valleys by vertical floodplain accretion during the more arid early-middle Holocene. Although valley aggradation was episodic, Fill IV contains no buried soils that date between about 8000 and 5700 cal. year BP (May, 1989). However, valley aggradation slowed abruptly beginning about 5700 cal. year BP and vertical flood-plain sedimentation remained slow until about 5100 cal. year BP. This 5500-year-old flood-plain is the focus of this paper. An unconformity at or slightly above the surface of the 5500-year-old flood-plain separates truncated Fill IV from overlying late Holocene alluvium. Where the late Holocene alluvium contains a 1200-year-old buried soil it is recognized as Fill I (May, 1989, 1992; May et al., 1995). Today, rivers in the Loup River basin are deeply incised to the base of Holocene fills and the 5500-year-old flood-plain is exposed in cutbanks 1– 4 m below the prominent 8- to 15-m-high Elba Terrace (Brice, 1964; May et al., 1995).
245
3. Methods The six sites used in this study are along the lower North Loup River, in the middle and upper reaches of the South Loup River, and along the main stem of the Loup River (Fig. 1). At all localities cleaned, vertical, cut-bank exposures were described, often from extension ladders. The uppermost 2 – 5 m of alluvium and soils were described using the terminology of Birkeland (1999). Soil/sediment samples were collected from each field-designated soil horizon or alluvial stratum for later laboratory analyses. Additionally, bulk samples of buried soil horizons and organic-rich alluvial strata were collected for conventional radiocarbon assays. In addition, at one locality (Chesley site) the spatial variability in the depth of the 5500year-old buried flood-plain was determined using an automatic level and stadia rod. Soil/sediment samples were analyzed for particlesize by the pipette method and oxidizable organiccarbon contents were determined using the Walkley – Black method (Singer and Janitzky, 1986). Organic-carbon amounts were converted to organicmatter contents by multiplying organic carbon by 1.7. Calcium carbonate contents were determined using the Chittick method (Dreimanis, 1962). All radiocarbon ages were determined on total humates. Nearly all the samples were dated by the University of Texas Radiocarbon Lab; one sample was dated by Beta Analytic. All but one of the radiocarbon ages were corrected for the effects of isotopic fractionation to yield conventional radiocarbon ages.All ages were calibrated using the online version (4.3) of the CALIB program
Table 1 Radiocarbon ages and calibrated ages of soil/sediment humates Site name
Depth (cm)
Laboratory number
Tibbets Tibbetts Coopers Canyon Moffet Creek Moffet Creek Horn Hickenbottom Chesley (No. 138.35) Chesley (No. 138.35)
140 – 150 239 – 245 202 – 212 128 – 140 162 – 178 197 – 202 420 – 426 115 – 120 185 – 190
Tx-6647 Tx-6645 Tx-6643 Tx-6646 Tx-6644 Beta-11631 Tx-6368 Tx-6369 Tx-6732
a
d13C value not determined. Value assumed to be
d13C 18.3 17.3 19.8 17.6 17.7 25.0a 16.4 15.4 19.2 25.0.
Conventional 14C age (year BP)
Range of calibrated ages (year BP at 1r)
Mean calendar age (year BP)
4560 F 260 4830 F 190 4670 F 730 4210 F 80 4450 F 290 4780 F 100 4950 F 120 1300 F 60 4960 F 100
5574 – 4872 5909 – 5149 6192 – 4416 4846 – 4593 5569 – 4653 5603 – 5328 5887 – 5590 1286 – 1176 5882 – 5596
5220 5530 5300 4720 5110 5470 5740 1230 5740
246
D.W. May / Geomorphology 56 (2003) 243–254
Fig. 2. Generalized stratigraphic sections at five of the six study sites showing relationship of late Holocene alluvium (including Fill I) to Fill IV and depth intervals of dated alluvium. The modern surface at all locations is the Elba Terrace.
At the Tibbets site along the main stem of the Loup River it is not possible to unequivocally delineate the surface of Fill IV. It appears to be the modern surface,
but there might be an increment of late Holocene alluvium within about the upper 1.5 m of alluvium at the site (Fig. 2). A cumulic soil is developed in the upper 286 cm of alluvium. The variable percentage of sand as a function of depth at this site reveals the influence of varying flood magnitudes on vertical construction of the 5500-year-old flood-plain and concomitant cumulative soil-forming processes (Birkeland, 1999). High percentages of sand at depths of
Fig. 3. Percent sand as a function of depth at the Tibbets site along the Loup River.
Fig. 4. Percent clay as a function of depth at the Tibbets site along the Loup River.
(Table 1) (Stuiver and Braziunas, 1993; Stuiver et al., 1998a,b).
4. Results
D.W. May / Geomorphology 56 (2003) 243–254
Fig. 5. Percent organic matter as a function of depth at the Tibbets site along the Loup River.
227 and 119 cm indicate that large floods carried sand onto the evolving flood-plain (Fig. 3). Radiocarbon ages from the section reveal that these floods occurred sometime after about 5530 cal. year BP and again about 5220 cal. year BP (Figs. 2 and 3). The episodic nature of vertical accretion on the 5500-year-old Loup River flood-plain is also revealed by the variable amounts of clay in the upper 286 cm of Fill IV. As expected in aggrading flood-plain settings, there is no single, smoothly fining-upward trend revealed by clay contents (Fig. 4). Organic-matter contents as a function of depth are traditionally used to recognize cumulic soils (Soil Survey Staff, 1975; Birkeland, 1999, p. 166). The percent organic matter in alluvium at the Tibbets cutbank site reveals both the cumulic nature of the floodplain soil and the relative stability of the surface of the terrace (Fig. 5). Here, organic-matter content in the alluvium first increases slightly toward the surface, decreases between depths of 269 and 180 cm, and then increases steadily toward the surface from a depth of 180 cm as would be expected in a soil forming on a very slowly aggrading surface (e.g., Cumulic Haplustoll) (Soil Survey Staff, 1975; Birkeland, 1999). The Coopers Canyon locality is a deeply incised tributary through late Quaternary fills in the lower North Loup River valley (Miller and Scott, 1955). The buried Turtle Creek soil recognized by Miller and Scott (1955) that is exposed here delineates a flood-plain dating to 5300 cal. year BP (Fig. 2) (May et al., 1995). Although no particle-size or organicmatter data are available for this section, the Turtle Creek soil is both finer-textured and darker than other
247
strata and soil horizons in the upper 280 cm of alluvium, based on the field description, except the A horizons of the modern soil. Slightly more than 2 m of late-Holocene alluvium overlie the 5500-yearold buried flood-plain. The Moffet Creek site is on the north edge of the North Loup River valley where the creek debouches into the North Loup River valley and a small, low-angle alluvial fan is graded to the Elba Terrace. Here one buried A horizon (162 – 178 cm deep) occurs near the surface of Fill-IV alluvium and a second occurs at the surface of this fill (128 – 140 cm deep) (Fig. 2). Although no particle-size or organicmatter data are available for this section, the description of this section reveals that these buried A
Fig. 6. Alluvium comprising the upper part of Fill IV (depth interval, 170 – 287 cm) at the Horn site along the South Loup River. The base of the thick, dark buried A horizon at top of photo has been dated at 5470 cal. year BP. Three other dark, organic- and clayenriched horizons in the cumulic soil are also evident.
248
D.W. May / Geomorphology 56 (2003) 243–254
horizons are finer-textured and have lower Munsell color chromas than any other horizons or strata within the upper 264 cm of alluvium at this site, except the modern soil horizons (upper 42 cm). Humates in the lower buried A horizon (162 –178 cm) have been dated at 5110 cal. year BP, while those in the upper buried A horizon (128 –140 cm) have been dated at 4720 cal. year BP (Table 1 and Fig. 2). May (1986) described and dated a 10-m section at the Horn site in the middle reaches of the South Loup River valley. Here a cumulic soil formed on the 5500year-old flood-plain (Fig. 6). Locally Fill IV is unconformably overlain by 156 cm of late-Holocene alluvium (Fig. 2). The sand-content data reveal Fill IV generally coarsens upward from a depth of at least 268 cm to the surface of the fill at 156 cm (Fig. 7). Clay content in much of the upper part of Fill IV averages 19.7% (average of 11 strata, 375– 259 cm), but it decreases to an average of only 6.7% for eight stratigraphic units and soil horizons in the uppermost 103 cm of Fill IV (156 – 259 cm depths). However, there is not a simple coarsening-upward sequence revealed in the clay-content data for the upper part of Fill IV (Fig. 8). Rather, the variability in clay content with depth in the upper 103 cm of Fill IV reveals the variable rate of vertical accretion on an episodically aggrading floodplain (Soil Survey Staff, 1975, pp.72 –74). The four darker horizons in the upper 103 cm of Fill IV contain greater amounts of clay than other horizons in the upper portion of Fill IV (Fig. 6). The increased clay content of these horizons probably reflects slower
sedimentation rates, but some clay illuviation cannot be ruled out without microscopic examination of thin sections of these sediments (Mandel and Bettis, 2000, pp. 180 –181). At the Horn site organic matter is highest (1.1%), in the thickest and darkest of the four dark-colored and clay-enriched horizons (175 –202 cm), which is near, but not at, the surface of Fill IV (Figs. 6 and 9). However, organic-matter content is also generally higher in the other three, dark-colored horizons than in the intervening lighter-colored horizons (Fig. 9). The four dark-colored, clay- and organic-carbonenriched horizons of the cumulic soil are enriched in calcium carbonate (Fig. 6 and 10). Radiocarbon dating of the lowermost 5 cm (197 –202 cm deep) of the darkest, thickest, and most organic-enriched horizon
Fig. 7. Percent sand as a function of depth at the Horn site along the South Loup River.
Fig. 9. Percent organic matter as a function of depth at the Horn site along the South Loup River.
Fig. 8. Percent clay as a function of depth at the Horn site along the South Loup River.
D.W. May / Geomorphology 56 (2003) 243–254
249
Fig. 10. Percent calcium carbonate as a function of depth at the Horn site along the South Loup River.
Fig. 12. Percent clay as a function of depth in profile 138.52 at the Chesley site along the South Loup River.
in the buried cumulic soil revealed that this soil began forming before 5470 cal. year BP (Fig. 2). The 5500-year-old flood-plain is most deeply buried (420 cm) at the Hickenbottom site along the middle reaches of South Loup River. Here the buried flood-plain is evident as a thin (6 cm thick) organicand clay-rich alluvial stratum with angular– blocky structure and abrupt upper and lower boundaries; it is not a buried soil horizon (Mandel and Bettis, 2000). Humates in this stratum have been dated at 5740 cal. year BP (Table 1 and Fig. 2). The structure, texture, and thickness of this clay-rich stratum, as well as its depth, indicate that it likely
was deposited in an abandoned channel of the South Loup River. May (1992) illustrated the general stratigraphy below the Elba Terrace at the Chesley site in the upper South Loup River valley, but did not discuss the alluvium pertinent to this study. Three profiles along a 425-m-long gently arcing section of the Chesley cut bank were first studied by May (1986) and were revisited for this study (Fig. 11). The surficial alluvium of Fill IV is clay-rich and is the 5500-year-old buried flood-plain (Figs. 11 and 12). The thickness of the surficial clay-rich deposit of the 5500-year-old flood-plain is clearly related to topography across the buried flood-plain. The clayrich deposit is thinnest (21 cm) in the highest paleolandscape setting (profile 138.25) and thickest (57 cm) in the lowest (profile 138.35) (Fig. 11). The
Fig. 11. Stratigraphic sections at the Chesley site showing spatial variability in the surface elevation of the buried flood-plain and the thickness of the clay-rich paleochannel fill comprising the upper portion of Fill IV.
Fig. 13. Percent sand as a function of depth in profile 138.52 at the Chesley site along the South Loup River.
250
D.W. May / Geomorphology 56 (2003) 243–254
paleochannel on the buried flood-plain is only subtly expressed at the modern surface because it has been buried by the 1200-year-old late-Holocene Fill I (Fig. 13).
5. Discussion
Loess mantling the landscape must have increased sediment concentrations and enhanced flood-plain aggradation. However, the upper Middle Loup and upper North Loup river valleys in the Nebraska Sand Hills have not been sufficiently studied to evaluate the relative importance of silty sediment versus sandy sediment load in the formation of thick overbank strata that bury developing A horizons.
5.1. Variability in flood-plain properties throughout the basin
5.2. Sediment versus soil at the Chesley site
The properties of the 5500-year-old flood-plain vary spatially throughout the Loup river basin. This flood-plain aggraded perhaps most rapidly at the site with the largest drainage area (Tibbets). Here, there is a thick cumulic surface soil on the Elba Terrace that began forming about 5530 cal. year BP and that has not been buried by a discrete increment of lateHolocene alluvium. The likely reason why the 5500year-old flood-plain along the main-stem Loup River was not rapidly buried by a thick increment of late Holocene alluvium is that the Loup River channel probably was proportionately more deeply incised than the channels in tributary valleys. Perennial streamflow along the main stem of the Loup River even during the dry early and middle Holocene would have helped maintain an incised channel. Thus, the main stem Loup River could more readily accommodate the large, late-Holocene floods that were responsible for aggradation of the valley bottoms elsewhere in the basin. Furthermore, the absence of discrete, late Holocene alluvium on the Elba Terrace at the Tibbets site indicates that late-Holocene increases in channelbed elevation were minimized along the Loup River. Conversely, the depth functions for particle-size and organic-matter contents for the upper part of FillIV alluvium at sites in the upper part of the basin, especially the Horn site in the South Loup River valley, attest to much more episodic deposition of thicker increments of alluvium than downstream. These thicker flood deposits isolated developing A horizons on the evolving flood-plain 5500 years ago. Possible explanations for why flood-plain aggradation 5500 years ago was more episodic in the South Loup River valley are that the discharge regime was more variable in the loess-dominated landscape in the South Loup River valley, and erosion of silt by first- and second-order streams (gullies) incised into the Peoria
At the Chesley site, the surface of the 5500-yearold flood-plain is a thick, clay-rich alluvial unit. Although this stratum displays some properties of a Bt horizon (high clay content and strong angular block structure), it is not the B horizon of a soil. The clay-rich stratum is a slackwater deposit in a paleochannel of the South Loup River because it has an abrupt, wavy lower boundary and dark yellowish brown (10YR 4/4) mottling throughout (Mandel and Bettis, 2000). The fine texture of this deposit is a product of very slow aggradation, but insufficient radiocarbon ages are available to specify sedimentation rates. A low variability of discharges probably prevailed during this period of slow sedimentation, as no sand laminae are present within the thick clay-rich alluvium at the surface of Fill IV (Fig. 13). Such a discharge regime occurs today in the upper Loup River basin where stream discharges are dominated by groundwater discharge (Bentall, 1989). The 5500-year-old clayey alluvium at the Chesley site is unconformably overlain by 1200-year-old sandy alluvium (Fig. 13). The basal 18– 31 cm of Fill I in the described sections is organically enriched and is immediately overlain by sediments containing low amounts of organic matter (Fig. 14). Although at first appearance the organic-enriched lower part of Fill I looks like an A horizon and the clay-rich upper increment of Fill IV appears to be a Bt horizon of the same soil, this is clearly not the case. The sandcontent data indicate that two discrete lithologic units are present and Fill I unconformably overlies Fill IV (Fig. 13). Also, the lower part of organic-enriched stratum of Fill I has an abrupt lower boundary. Although a radiocarbon age is not available for the surficial portion of the clay-rich stratum to provide a maximum-limiting age for the episode of erosion that created the unconformity, the radiocarbon age of the
D.W. May / Geomorphology 56 (2003) 243–254
Fig. 14. Percent organic matter as a function of depth in profile 138.52 at the Chesley site along the South Loup River.
base of the organic-enriched stratum at the base of Fill I indicates that the erosion occurred before 1200 cal. year BP. From the available data, I infer that a large flood scoured then rapidly filled the South Loup River channel sometime before 5740 cal. year BP. Shortly before 5740 years ago the active meandering channel was abandoned. Very fine-grained, suspended-load sediment accumulated slowly in the resulting oxbow lake. A sudden increase in the magnitude and frequency of flooding probably was responsible for deposition of the 18- to 31-cm-thick increment of sandier alluvium (basal Fill I) in the oxbow lake at the Chesley site. 5.3. Causes of flood-plain stabilization 5700 – 5100 cal. year BP Two possible causes may have played a role in the stabilization of flood-plains in the Loup River basin between about 5700 and 5100 cal. year BP. First, flood-plain stabilization may have been part of a long-term complex response to Late Wisconsin loess accumulation similar to Bettis and Autin’s (1997) suggestion of causes of Holocene fluvial changes in a small river basin in eastern Iowa. Secondly, and more likely, a change to a more mesic climate at about 5700 cal. year BP may have played a very important role in slowing valleybottom aggradation. With increased precipitation, the density of prairie grasses across the landscape would have increased. This increased density of grass cover likely increased critical shear stress for
251
erosion, thus preventing surface wash erosion and initiation of rills and gullies (Prosser et al., 1995). Reduced soil erosion in low-order tributaries and from hillslopes throughout the entire basin would have greatly reduced sediment supply to the major rivers within the Loup River basin, starving the rivers of high sediment concentrations during overbank flows. Increased precipitation not only probably triggered substantial reductions in sediment yield, but also led to elevated water tables in the valley fills. Together, these changes likely would have produced more perennial flow, reduced the variability of discharge, enhanced riparian vegetation growth, stabilized stream banks, and led to finer-textured overbank deposition. Thus, climate and concomitant vegetation changes could explain cessation of or reduced valley-floor aggradation about 5700 cal. year BP. One of the best dated and documented sets of proxy data for climatic and environmental changes coinciding with the slowing of flood-plain aggradation within the Loup River system has been gathered from well-dated alluvium within the South Fork of the Big Nemaha River in extreme southeastern Nebraska. Baker et al. (2000) report that between 6600 and 3270 cal year BP (5800 – 3100 14C year BP) riparian forests were well established after being limited in composition and areal extent during the preceding few thousand years. They also report that more wetland plants were present after 6600 years ago than during the preceding interval. Especially germane to this study is their evidence of a peak in arboreal pollen just before 5740 cal. year BP (5000 14C year BP). A second line of evidence that the climate of central Nebraska underwent changes toward a more mesic climate as the flood-plains of the Loup River stabilized is the record of dune migration in the Nebraska Sand Hills. Loope and Swinehart (2000) have used dates on peats and soils under dunes, optically stimulated luminescence (OSL) dating of dune sands, and the timing of formation of wetlands and lakes in dune-blocked drainages in the Nebraska Sand Hills to construct a chronology of dune migration for the Late Wisconsin through the Holocene. Especially important to this study is their identification of dune activity from about 9440 to 5160 cal. year BP (8500 –4500 14C year BP). The cessation of dune activity, presumably because of more mesic conditions across the Sand Hills and upper Loup
252
D.W. May / Geomorphology 56 (2003) 243–254
River drainage basin, occurred when valley bottoms throughout the Loup River basin stabilized. A third line of evidence that climate was a strong controlling factor is the regional evidence from the central Great Plains that flood-plains of other rivers stabilized at about 5500 cal. year BP. For instance, Martin (1992) has documented stabilization of a flood-plain beneath a 12-m-high terrace along the Republican River in south-central Nebraska at 5220 and 4810 cal. year BP (4550 and 4270 14C year BP). Johnson and Logan (1990) have reported an age of 4870 cal. year BP (4290 14C year BP) for a buried soil beneath the Holiday terrace complex along the lower Kansas River in eastern Kansas, and an age of 4760 cal. year BP (4260 14C year BP) along Stranger Creek, a tributary to the lower Kansas River. In northcentral Kansas along the Solomon River humates from the upper part of a buried A horizon in the upper part of alluvium (2.25 –2.35 m depth) below the Kirwin Terrace have been dated at 5720 cal. year BP (4960 14C year BP), while humates from the upper 10 cm of a truncated A horizon in the section (148 – 158 cm deep) have been dated at 5330 cal. year BP (4630 14C year BP) (May, 1993). Mandel and Bettis (2001, p. 13) have reported an age of 5470 cal. year BP (4780 14C year BP) for the upper 10 cm of a buried soil in the upper part of the Gunder Member along the South Fork of the Big Nemaha River in southeastern Nebraska. Mandel (1994) has reported an age of 5740 cal. year BP (4970 14C year BP) for humates in the fourth soil below the surface of Terrace 2 at archaeological site 14HO316 in the Pawnee River basin of southwestern Kansas. Thus, flood-plain stabilization occurred regionally at the same time as in the Loup River basin, and climatic change is likely the reason for such synchronous fluvial responses. 5.4. Significance for soil mapping and archaeologic studies The finding that the depth of burial of the 5500year-old flood-plain varies throughout the Loup River basin is significant for mapping of modern soils across flood-plains and the discovery of cultural materials in valleys. Soil mappers readily appreciate how modern soils vary across flood-plains. However, this study has shown that across valleys in the Loup
River basin the soil developed at the surface of the Elba Terrace may have formed in a single lithologic unit in some parts of the drainage basin and multiple lithologic units elsewhere in the basin. Buried soils may be present at shallow depths in some parts of the basin, while they may not exist or may be deeply buried at other locations with a basin. Furthermore, paleotopography on buried flood-plains and its control on the texture of accumulating sediment in the past may influence the formation, and thus, properties, of near-surface soil horizons. In short, buried floodplains provide challenges to mappers of soils across valley bottoms. The search for cultural remains on valley floors is often hampered because they are often buried by alluvium. The episodes of aggradation responsible for burial may have occurred historically, within the past few hundred or thousand years, or in the Late Glacial. Archaeologists now readily acknowledge the importance of working with geomorphologists to understand the depths at which artifacts are likely to be found beneath various land surfaces in valleys (Gardner and Donahue, 1985). In the Loup River basin, it is clear that except for perhaps along the main stem of the Loup River, artifacts older than about 1000 years will be buried beneath a mantle of late-Holocene alluvium across the Elba Terrace below the plow zone. Only artifacts younger than about 1000 years are likely to occur within the plow zone on the Elba Terrace in all parts of the basin.
6. Conclusions The particle-size and organic-matter data for the 5500-year-old buried flood-plain at most sites in the Loup River basin reveal that valley-bottom aggradation from about 5700– 5100 cal. year BP slowed but still was episodic, as was soil formation on the evolving flood-plain. Sporadic flood-plain aggradation is evident at most sites in the percentages of sand, clay, and organic matter in the upper part of Fill IV. Sand and clay rarely show a steady fining upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the
D.W. May / Geomorphology 56 (2003) 243–254
effects of external climate forcing on vegetation and surface and groundwater hydrologic changes. The climate was becoming more mesic and the flood-plain probably became wetter and supported more vegetation. Other studies of rivers in the central Great Plains indicate that this period is a time of flood-plain stability across the region.
Acknowledgements Andres Aslan, Sue Boley-May, Joe Mason, and an anonymous reviewer provided many helpful suggestions that improved an earlier draft of the manuscript. I am grateful for the time and energy that Paul Hudson dedicated to organizing a special session at the Association of American Geographer’s Meeting in Los Angeles, and to his perseverance as editor of this special volume.
References Arbogast, A.F., Johnson, W.C., 1994. Climatic implications of the Late Quaternary alluvial record of a small drainage basin in the Central Great Plains. Quaternary Research 41, 298 – 305. Baker, R.G., Fredlund, G.G., Mandel, R.D., Bettis III, E.A., 2000. Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska. Quaternary International 67, 75 – 88. Bentall, R., 1989. Streams. In: Bleed, A., Flowerday, C. (Eds.), An Atlas of the Sand Hills. Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska – Lincoln, Lincoln, NE, pp. 93 – 114. Bettis III, E.A., Autin, W.J., 1997. Complex response of a midcontinent North America drainage system to late Wisconsinan sedimentation. Journal of Sedimentary Research B: Stratigraphy and Global Studies 67 (4), 740 – 748. Birkeland, P.W., 1999. Soils and Geomorphology. Oxford Univ. Press, New York, NY. 430 pp. Brakenridge, C.R., 1988. River flood regime and flood-plain stratigraphy. In: Baker, V.R., Kochel, R.C., Patton, R.C. (Eds.), Flood Geomorphology. Wiley, New York, NY, pp. 139 – 156. Brice, J.C., 1964. Channel patterns and terraces of the Loup Rivers in Nebraska. United States Geological Survey Professional Paper 422-D, D1-D41. Bull, W.B., 1991. Geomorphic Responses to Climatic Change. Oxford Univ. Press, New York, NY. 326 pp. Dreeszen, V.H., 1973. Topographic Regions Map, State of Nebraska (Scale 1:3,000,000). Conservation and Survey Division, University of Nebraska – Lincoln, Lincoln, NE. Dreimanis, A., 1962. Quantitative gasometric determination of cal-
253
cite and dolomite by using Chittick method apparatus. Journal of Sedimentary Petrology 32, 520 – 529. Dumont, J.F., Mering, C., Parrot, J.F., Taud, H., 1996. Morphological and mathematical analysis of asymmetrical fluvial pattern: a study case from the Ucayali River (Peru). Zietschrift fur Geomorphologie, Supplementband 103, 269 – 282. Fraser, G.S., Bennett, S.W., Thompson, T.A., Olyphant, G.A., Furer, L., 1997. Geomorphic response to tectonically-induced ground deformation in the Wabash Valley. Seismological Research Letters 68 (4), 662 – 674. Gardner, G.D., Donahue, J., 1985. The little platte drainage, Missouri: a model for locating temporal surfaces in a fluvial environment. In: Stein, J.K., Farrand, W.R. (Eds.), Archaeological Sediments in Context. Center for the Study of Early Man Institute for Quaternary Studies, University of Maine, Orono, ME, pp. 69 – 89. Jacobson, R.B., Coleman, D.J., 1986. Stratigraphy and recent evolution of Maryland Piedmont flood-plains. American Journal of Science 286, 617 – 637. Johnson, W.C., Logan, B., 1990. Geoarchaeology of the Kansas River basin, central Great Plains. In: Lasca, N.P., Donahue, J. (Eds.), Archaeology of North America. Geological Society of America, Boulder, CO, pp. 267 – 299. Kaul, R.B., 1975. Vegetation of Nebraska (circa 1850) (Scale 1:1,000,000). Conservation and Survey Division, University of Nebraska – Lincoln, Lincoln, NE. Knox, J.C., 1977. Human impacts on Wisconsin stream channels. Annals of the Association of American Geographers 67 (3), 323 – 342. Knox, J.C., 1983. Responses of river systems to Holocene climates. In: Wright Jr., H.E. (Ed.), Late-Quaternary Environments of the United States. The Holocene, vol. 2. University of Minnesota Press, Minneapolis, MN, pp. 26 – 41. Koppen, W., 1918. Klassifikation der klimate nach temperatur, niederschlag und jareslauf (Climate classification according to annual and monthly temperature and precipitation). Petermanns Geographische Mitteilungen 64, 193 – 203. Lawson, M.P., Dewey, K.F., Neild, R.E., 1977. Climatic atlas of Nebraska. Univ. of Nebraska Press, Lincoln, NE. 88 pp. Loope, D.B., Swinehart, J.B., 2000. Thinking like a dune field: geologic history in the Nebraska Sand Hills. Great Plains Research 10, 5 – 35. Mandel, R.D., 1994. Holocene landscape evolution in the Pawnee River valley, southwestern Kansas. Kansas Geological Survey Bulletin 236. Kansas Geological Survey, Lawrence, KS, and Kansas State Historical Society, Topeka, KS. 117 pp. Mandel, R.D., Bettis III, E.A., 2000. Use and analysis of soils by archaeologists and geoscientists. In: Goldberg, P., Holliday, V.T., Ferring, C.R. (Eds.), Earth Sciences and Archaeology. Kluwer Academic Publishing/Plenum, New York, NY, pp. 173 – 204. Mandel, R.D., Bettis III, E.A., 2001. Late Quaternary Landscape Evolution in the South Fork of the Big Nemaha River Valley, Southeastern Nebraska and Northeastern Kansas, Guidebook No. 11. Conservation and Survey Division, University of Nebraska – Lincoln, Lincoln, NE. Martin, C.W., 1992. Late Holocene alluvial chronology and climate
254
D.W. May / Geomorphology 56 (2003) 243–254
change in the Central Great Plains. Quaternary Research 37, 315 – 322. Mason, J.A., 2001. Transport direction of Peoria Loess in Nebraska and implications for loess sources on the central Great Plains. Quaternary Research 56, 79 – 86. May, D.W., 1986. Holocene alluviation, soil genesis, and erosion in the South Loup Valley, Nebraska. Unpublished PhD thesis. University of Wisconsin – Madison. 217 pp. May, D.W., 1989. Holocene alluvial fills in the South Loup Valley, Nebraska. Quaternary Research 32, 117 – 120. May, D.W., 1992. Late Holocene valley-bottom aggradation and erosion in the South Loup River Valley, Nebraska. Physical Geography 13, 115 – 132. May, D.W., 1993. Soils in middle and late Holocene alluvium. In: Johnson, W.C. (Ed.), INQUA Commission 6, Second International Paleopedology Symposium, Field Excursion, August 13 – 20, 1993. Kansas Geological Survey Open-File Report 93-30, Lawrence, KS, pp. 15-1 – 15-11. May, D.W., Swinehart, J.B., Loope, D., Souders, V., 1995. Late Quaternary fluvial and eolian sediments: Loup River Basin and the Sand Hills of Nebraska. In: Flowerday, C.A. (Ed.), Geologic Field Trips in Nebraska and Adjacent Parts of Kansas and South Dakota. Parts of the 29th Annual Meetings of the NorthCentral and South-Central Sections, Guidebook No. 10. Geological Society of America, Conservation and Survey Division, University of Nebraska – Lincoln, NE, pp. 13 – 31. Miller, R.D., Scott, G.R., 1955. Sequence of alluviation along the Loup Rivers, Valley County area, Nebraska. Bulletin of the Geological Society of America 66, 1431 – 1448. Nanson, G.C., 1986. Episodes of vertical accretion and catastrophic stripping: a model of disequilibrium flood-plain development. Geological Society of America Bulletin 97, 1467 – 1475. Patton, P.C., Schumm, S.A., 1981. Ephemeral-stream processes: implications for studies of Quaternary valley fills. Quaternary Research 15, 24 – 43. Prosser, I.P., Dietrich, W.E., Stevenson, J., 1995. Flow resistance and sediment transport by concentrated overland flow in a grassland valley. Geomorphology 13, 71 – 86. Ritter, D.F., Kinsey III, W.F., Kauffman, M.E., 1973. Overbank sedimentation in the Delaware River Valley during the last 6000 years. Science 179, 374 – 375. Schumm, S.A., 1977. The Fluvial System. Wiley-Interscience, New York, NY. 338 pp.
Schumm, S.A., Brakenridge, G.R., 1987. River responses. In: Ruddiman, W.F., Wright Jr., H.E. (Eds.), North America and Adjacent Oceans During the Last Deglaciation. The Geology of North America, vol. K-3. Geol. Soc. of America, pp. 221 – 240. Schumm, S.A., Lichty, R.W., 1963. Channel widening and floodplain construction along Cimarron River in southwestern Kansas. United States Geological Survey Professional Paper, 352-D, 71 – 88. Singer, M.J., Janitzky, P., 1986. Field and laboratory procedures used in a soil chronosequence study. United States Geological Survey, 1648. Soil Survey Staff, 1975. Soil Taxonomy: a basic system of soil classification for making and interpreting soil surveys. Agriculture Handbook No. 436. Soil Conservation Service, US Department of Agriculture, Madison, WI. 754 pp. Stene, L.P., 1980. Observations on lateral and overbank deposition—evidence from Holocene terraces, southwestern Alberta. Geology 8, 314 – 317. Stuiver, M., Braziunas, T.F., 1993. Sun, ocean, climate and atmospheric 14CO2: an evaluation of causal and spectral relationships. The Holocene 3, 289 – 305. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, F.G., v.d. Plicht, J., Spurk, M., 1998a. INTCAL98 radiocarbon age calibration, 24,000 – 0 cal B.P. Radiocarbon 40, 1041 – 1083. Stuiver, M., Reimer, P.J., Braziunas, T.F., 1998b. High-precision radiocarbon age calibration for terrestrial and marine samples. Radiocarbon 40, 1127 – 1151. Swinehart, J.B., Dreeszen, V.H., Richmond, G.M., Tipton, M.R., Bretz, R., Steece, F.V., Hallberg, G.R., Goebel, J.E., 1994. Quaternary Geologic Map of the Platte River 40 60 Quadrangle, United States (Scale 1:1,000,000). U.S. Geological Survey, Miscellaneous Investigations Series. US Geological Survey, 1974. Hydrologic Unit Map. State of Nebraska. 1:500,000. Walling, D.E., Owens, P.N., Leeks, G.J.L., 1998. The role of channel and flood-plain storage in the suspended sediment budget of the River Ouse, Yorkshire, UK. Geomorphology 22, 225 – 242. Wolman, M.G., Leopold, L.B., 1957. River flood-plains: some observations on their formation. United States Geological Survey Professional Paper 282-C, 87 – 109.
Properties of a 5500-year-old flood-plain in the Loup River Basin, Nebraska David W. May * Department of Geography, University of Northern Iowa, Cedar Falls, IA 50614-0406, USA Received 15 June 2002; received in revised form 3 February 2003; accepted 21 March 2003
Abstract Flood-plain aggradation within the Loup River Basin of central Nebraska was episodic and alternated with incision throughout much of the Holocene. A widespread episode of flood-plain stability, however, occurred about 5700 – 5100 cal. year BP. The purpose of this paper is to describe the properties of this buried flood-plain at six sites in the basin, to consider why the properties of the buried flood-plain vary from site to site, and to evaluate possible reasons why the Loup River flood-plains stabilized 5500 years ago. Episodic valley-bottom aggradation was common during flood-plain formation at five of the six sites. The radiocarbon ages, particle-size data, and organic-carbon data for the buried flood-plain reveal that valley-bottom aggradation generally slowed between about 5700 and 5100 cal. year BP. Erratic down-profile changes in percentages of sand, clay, and organic matter indicate flood-plain sedimentation and soil formation were often episodic. Sand and clay rarely show a steady fining-upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. At two localities, the buried flood-plain is evident as a clay-rich stratum that must have been deposited in a paleochannel. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the effects of external climate forcing on vegetation and hydrologic changes. flood-plains of other rivers in the central Great Plains also stabilized at this time, further supporting a climatic explanation for slowing of valley aggradation and formation of a flood-plain at this time. Recognition of buried flood-plains is important to both soil mapping in valleys and to the discovery of cultural resources in valleys. D 2003 Elsevier Science B.V. All rights reserved. Keywords: flood-plain; Holocene; River; Buried soil; Great Plains
1. Introduction Flood-plains adjust to sediment inputs and discharge regimes on various timescales (Schumm, 1977; Knox, 1977; Schumm and Brakenridge, 1987; Walling et al., 1998). Over the timescale of the Holocene, such adjustments have been shown to be * Fax: +1-319-273-7103. E-mail address: [email protected] (D.W. May).
responses to internal adjustments within the fluvial system (Patton and Schumm, 1981), climate changes (Knox, 1983; Bull, 1991), and crustal movements (Dumont et al., 1996; Fraser et al., 1997). Fluvial adjustments can be largely by lateral movements of the river channel (Wolman and Leopold, 1957; Schumm and Brakenridge, 1987) or by vertical changes in the elevation of the flood-plain (Schumm and Lichty, 1963; Ritter et al., 1973; Stene, 1980; Nanson, 1986; Brakenridge, 1988).
0169-555X/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(03)00154-5
244
D.W. May / Geomorphology 56 (2003) 243–254
In the semi-arid Great Plains of North America, the Holocene record of fluvial changes has been one of primarily vertical adjustments of the flood-plain (Johnson and Logan, 1990; Martin, 1992; Arbogast and Johnson, 1994; May et al., 1995; Mandel and Bettis, 2001) as opposed to lateral migration of channels and concomitant flood-plain reworking that was common during the Holocene in the eastern United States (Knox, 1983; Jacobson and Coleman, 1986; Schumm and Brakenridge, 1987). The alternating episodes of aggradation and entrenchment of river valleys in the central Great Plains have been punctuated by brief episodes of flood-plain stability. The focus of this paper is the flood-plain that formed 5700– 5100 cal. year BP across valley floors in the Loup River basin of central Nebraska (Fig. 1) on or very near the surface of early-middle Holocene alluvium (Fill IV). The purposes of this paper are to describe the properties of the alluvium in the upper part of Fill IV that comprises this buried flood-plain, to identify variability in the properties of the buried flood-plain throughout the basin, and to evaluate causes, especially climatic change, for why this buried flood-plain formed. This paper addresses the importance of distinguishing alluvial strata from buried soil horizons, evaluates synchronous regional flood-plain
stability in the Great Plains, and demonstrates the relevance of understanding spatial variability in properties of buried flood-plains to soil mapping and archaeological investigations in river valleys.
2. Study area The major northwest-to-southeast tributaries to the Loup River drain the Sand Hills and Dissected Plains topographic regions of central Nebraska (Dreeszen, 1973). This area is underlain by Early Pleistocene and Pliocene alluvial sand, pebble gravel, and lacustrine sand and silt (Swinehart et al., 1994). Surficial deposits include Late Quaternary eolian sand, especially in the upper half of the Loup River basin, and Peoria Loess throughout the Dissected Plains region of the lower half of the basin (Swinehart et al., 1994; Mason, 2001). Groundwater discharge dominates the upper reaches of the major rivers in the Loup River basin (Bentall, 1989), while surface runoff comprises a greater proportion of the discharge downstream. The modern climate of the Loup River basin varies from east to west. The eastern portion of the basin receives an average of as much as 68.6 cm (27 in.) of precipitation annually, while the northwestern portion
Fig. 1. Map of Loup River basin and study localities: TI = Tibbets; CO = Coopers Canyon; MO = Moffet Creek; HO = Horn; HI = Hickenbottom; CH = Chesley. Base map drawn from Hydrologic Unit Map (US Geological Survey, 1974).
D.W. May / Geomorphology 56 (2003) 243–254
of the basin receives an average of as little as 45.7 cm (18 in.) (Lawson et al., 1977). The climate of the eastern portion of the basin is classified as Dfa (Moist Continental), while the climate of the western portion of the basin is classified as BS (Steppe) (Koppen, 1918). Pre-settlement vegetation in the Loup River basin consisted of Mixed Prairie (bluestem grasses, grama grasses, and buffalo grass) in the southern half of the basin and Sandhills Prairie (bluestem grasses, sandreed grass, needlegrass, and yucca) in mostly the northern half of the basin (Kaul, 1975). Holocene alluvial stratigraphy in the Loup River basin includes five valley fills (May et al., 1995). However, only two fills (IV and I), are germane to this study. Fill IV accumulated in valleys by vertical floodplain accretion during the more arid early-middle Holocene. Although valley aggradation was episodic, Fill IV contains no buried soils that date between about 8000 and 5700 cal. year BP (May, 1989). However, valley aggradation slowed abruptly beginning about 5700 cal. year BP and vertical flood-plain sedimentation remained slow until about 5100 cal. year BP. This 5500-year-old flood-plain is the focus of this paper. An unconformity at or slightly above the surface of the 5500-year-old flood-plain separates truncated Fill IV from overlying late Holocene alluvium. Where the late Holocene alluvium contains a 1200-year-old buried soil it is recognized as Fill I (May, 1989, 1992; May et al., 1995). Today, rivers in the Loup River basin are deeply incised to the base of Holocene fills and the 5500-year-old flood-plain is exposed in cutbanks 1– 4 m below the prominent 8- to 15-m-high Elba Terrace (Brice, 1964; May et al., 1995).
245
3. Methods The six sites used in this study are along the lower North Loup River, in the middle and upper reaches of the South Loup River, and along the main stem of the Loup River (Fig. 1). At all localities cleaned, vertical, cut-bank exposures were described, often from extension ladders. The uppermost 2 – 5 m of alluvium and soils were described using the terminology of Birkeland (1999). Soil/sediment samples were collected from each field-designated soil horizon or alluvial stratum for later laboratory analyses. Additionally, bulk samples of buried soil horizons and organic-rich alluvial strata were collected for conventional radiocarbon assays. In addition, at one locality (Chesley site) the spatial variability in the depth of the 5500year-old buried flood-plain was determined using an automatic level and stadia rod. Soil/sediment samples were analyzed for particlesize by the pipette method and oxidizable organiccarbon contents were determined using the Walkley – Black method (Singer and Janitzky, 1986). Organic-carbon amounts were converted to organicmatter contents by multiplying organic carbon by 1.7. Calcium carbonate contents were determined using the Chittick method (Dreimanis, 1962). All radiocarbon ages were determined on total humates. Nearly all the samples were dated by the University of Texas Radiocarbon Lab; one sample was dated by Beta Analytic. All but one of the radiocarbon ages were corrected for the effects of isotopic fractionation to yield conventional radiocarbon ages.All ages were calibrated using the online version (4.3) of the CALIB program
Table 1 Radiocarbon ages and calibrated ages of soil/sediment humates Site name
Depth (cm)
Laboratory number
Tibbets Tibbetts Coopers Canyon Moffet Creek Moffet Creek Horn Hickenbottom Chesley (No. 138.35) Chesley (No. 138.35)
140 – 150 239 – 245 202 – 212 128 – 140 162 – 178 197 – 202 420 – 426 115 – 120 185 – 190
Tx-6647 Tx-6645 Tx-6643 Tx-6646 Tx-6644 Beta-11631 Tx-6368 Tx-6369 Tx-6732
a
d13C value not determined. Value assumed to be
d13C 18.3 17.3 19.8 17.6 17.7 25.0a 16.4 15.4 19.2 25.0.
Conventional 14C age (year BP)
Range of calibrated ages (year BP at 1r)
Mean calendar age (year BP)
4560 F 260 4830 F 190 4670 F 730 4210 F 80 4450 F 290 4780 F 100 4950 F 120 1300 F 60 4960 F 100
5574 – 4872 5909 – 5149 6192 – 4416 4846 – 4593 5569 – 4653 5603 – 5328 5887 – 5590 1286 – 1176 5882 – 5596
5220 5530 5300 4720 5110 5470 5740 1230 5740
246
D.W. May / Geomorphology 56 (2003) 243–254
Fig. 2. Generalized stratigraphic sections at five of the six study sites showing relationship of late Holocene alluvium (including Fill I) to Fill IV and depth intervals of dated alluvium. The modern surface at all locations is the Elba Terrace.
At the Tibbets site along the main stem of the Loup River it is not possible to unequivocally delineate the surface of Fill IV. It appears to be the modern surface,
but there might be an increment of late Holocene alluvium within about the upper 1.5 m of alluvium at the site (Fig. 2). A cumulic soil is developed in the upper 286 cm of alluvium. The variable percentage of sand as a function of depth at this site reveals the influence of varying flood magnitudes on vertical construction of the 5500-year-old flood-plain and concomitant cumulative soil-forming processes (Birkeland, 1999). High percentages of sand at depths of
Fig. 3. Percent sand as a function of depth at the Tibbets site along the Loup River.
Fig. 4. Percent clay as a function of depth at the Tibbets site along the Loup River.
(Table 1) (Stuiver and Braziunas, 1993; Stuiver et al., 1998a,b).
4. Results
D.W. May / Geomorphology 56 (2003) 243–254
Fig. 5. Percent organic matter as a function of depth at the Tibbets site along the Loup River.
227 and 119 cm indicate that large floods carried sand onto the evolving flood-plain (Fig. 3). Radiocarbon ages from the section reveal that these floods occurred sometime after about 5530 cal. year BP and again about 5220 cal. year BP (Figs. 2 and 3). The episodic nature of vertical accretion on the 5500-year-old Loup River flood-plain is also revealed by the variable amounts of clay in the upper 286 cm of Fill IV. As expected in aggrading flood-plain settings, there is no single, smoothly fining-upward trend revealed by clay contents (Fig. 4). Organic-matter contents as a function of depth are traditionally used to recognize cumulic soils (Soil Survey Staff, 1975; Birkeland, 1999, p. 166). The percent organic matter in alluvium at the Tibbets cutbank site reveals both the cumulic nature of the floodplain soil and the relative stability of the surface of the terrace (Fig. 5). Here, organic-matter content in the alluvium first increases slightly toward the surface, decreases between depths of 269 and 180 cm, and then increases steadily toward the surface from a depth of 180 cm as would be expected in a soil forming on a very slowly aggrading surface (e.g., Cumulic Haplustoll) (Soil Survey Staff, 1975; Birkeland, 1999). The Coopers Canyon locality is a deeply incised tributary through late Quaternary fills in the lower North Loup River valley (Miller and Scott, 1955). The buried Turtle Creek soil recognized by Miller and Scott (1955) that is exposed here delineates a flood-plain dating to 5300 cal. year BP (Fig. 2) (May et al., 1995). Although no particle-size or organicmatter data are available for this section, the Turtle Creek soil is both finer-textured and darker than other
247
strata and soil horizons in the upper 280 cm of alluvium, based on the field description, except the A horizons of the modern soil. Slightly more than 2 m of late-Holocene alluvium overlie the 5500-yearold buried flood-plain. The Moffet Creek site is on the north edge of the North Loup River valley where the creek debouches into the North Loup River valley and a small, low-angle alluvial fan is graded to the Elba Terrace. Here one buried A horizon (162 – 178 cm deep) occurs near the surface of Fill-IV alluvium and a second occurs at the surface of this fill (128 – 140 cm deep) (Fig. 2). Although no particle-size or organicmatter data are available for this section, the description of this section reveals that these buried A
Fig. 6. Alluvium comprising the upper part of Fill IV (depth interval, 170 – 287 cm) at the Horn site along the South Loup River. The base of the thick, dark buried A horizon at top of photo has been dated at 5470 cal. year BP. Three other dark, organic- and clayenriched horizons in the cumulic soil are also evident.
248
D.W. May / Geomorphology 56 (2003) 243–254
horizons are finer-textured and have lower Munsell color chromas than any other horizons or strata within the upper 264 cm of alluvium at this site, except the modern soil horizons (upper 42 cm). Humates in the lower buried A horizon (162 –178 cm) have been dated at 5110 cal. year BP, while those in the upper buried A horizon (128 –140 cm) have been dated at 4720 cal. year BP (Table 1 and Fig. 2). May (1986) described and dated a 10-m section at the Horn site in the middle reaches of the South Loup River valley. Here a cumulic soil formed on the 5500year-old flood-plain (Fig. 6). Locally Fill IV is unconformably overlain by 156 cm of late-Holocene alluvium (Fig. 2). The sand-content data reveal Fill IV generally coarsens upward from a depth of at least 268 cm to the surface of the fill at 156 cm (Fig. 7). Clay content in much of the upper part of Fill IV averages 19.7% (average of 11 strata, 375– 259 cm), but it decreases to an average of only 6.7% for eight stratigraphic units and soil horizons in the uppermost 103 cm of Fill IV (156 – 259 cm depths). However, there is not a simple coarsening-upward sequence revealed in the clay-content data for the upper part of Fill IV (Fig. 8). Rather, the variability in clay content with depth in the upper 103 cm of Fill IV reveals the variable rate of vertical accretion on an episodically aggrading floodplain (Soil Survey Staff, 1975, pp.72 –74). The four darker horizons in the upper 103 cm of Fill IV contain greater amounts of clay than other horizons in the upper portion of Fill IV (Fig. 6). The increased clay content of these horizons probably reflects slower
sedimentation rates, but some clay illuviation cannot be ruled out without microscopic examination of thin sections of these sediments (Mandel and Bettis, 2000, pp. 180 –181). At the Horn site organic matter is highest (1.1%), in the thickest and darkest of the four dark-colored and clay-enriched horizons (175 –202 cm), which is near, but not at, the surface of Fill IV (Figs. 6 and 9). However, organic-matter content is also generally higher in the other three, dark-colored horizons than in the intervening lighter-colored horizons (Fig. 9). The four dark-colored, clay- and organic-carbonenriched horizons of the cumulic soil are enriched in calcium carbonate (Fig. 6 and 10). Radiocarbon dating of the lowermost 5 cm (197 –202 cm deep) of the darkest, thickest, and most organic-enriched horizon
Fig. 7. Percent sand as a function of depth at the Horn site along the South Loup River.
Fig. 9. Percent organic matter as a function of depth at the Horn site along the South Loup River.
Fig. 8. Percent clay as a function of depth at the Horn site along the South Loup River.
D.W. May / Geomorphology 56 (2003) 243–254
249
Fig. 10. Percent calcium carbonate as a function of depth at the Horn site along the South Loup River.
Fig. 12. Percent clay as a function of depth in profile 138.52 at the Chesley site along the South Loup River.
in the buried cumulic soil revealed that this soil began forming before 5470 cal. year BP (Fig. 2). The 5500-year-old flood-plain is most deeply buried (420 cm) at the Hickenbottom site along the middle reaches of South Loup River. Here the buried flood-plain is evident as a thin (6 cm thick) organicand clay-rich alluvial stratum with angular– blocky structure and abrupt upper and lower boundaries; it is not a buried soil horizon (Mandel and Bettis, 2000). Humates in this stratum have been dated at 5740 cal. year BP (Table 1 and Fig. 2). The structure, texture, and thickness of this clay-rich stratum, as well as its depth, indicate that it likely
was deposited in an abandoned channel of the South Loup River. May (1992) illustrated the general stratigraphy below the Elba Terrace at the Chesley site in the upper South Loup River valley, but did not discuss the alluvium pertinent to this study. Three profiles along a 425-m-long gently arcing section of the Chesley cut bank were first studied by May (1986) and were revisited for this study (Fig. 11). The surficial alluvium of Fill IV is clay-rich and is the 5500-year-old buried flood-plain (Figs. 11 and 12). The thickness of the surficial clay-rich deposit of the 5500-year-old flood-plain is clearly related to topography across the buried flood-plain. The clayrich deposit is thinnest (21 cm) in the highest paleolandscape setting (profile 138.25) and thickest (57 cm) in the lowest (profile 138.35) (Fig. 11). The
Fig. 11. Stratigraphic sections at the Chesley site showing spatial variability in the surface elevation of the buried flood-plain and the thickness of the clay-rich paleochannel fill comprising the upper portion of Fill IV.
Fig. 13. Percent sand as a function of depth in profile 138.52 at the Chesley site along the South Loup River.
250
D.W. May / Geomorphology 56 (2003) 243–254
paleochannel on the buried flood-plain is only subtly expressed at the modern surface because it has been buried by the 1200-year-old late-Holocene Fill I (Fig. 13).
5. Discussion
Loess mantling the landscape must have increased sediment concentrations and enhanced flood-plain aggradation. However, the upper Middle Loup and upper North Loup river valleys in the Nebraska Sand Hills have not been sufficiently studied to evaluate the relative importance of silty sediment versus sandy sediment load in the formation of thick overbank strata that bury developing A horizons.
5.1. Variability in flood-plain properties throughout the basin
5.2. Sediment versus soil at the Chesley site
The properties of the 5500-year-old flood-plain vary spatially throughout the Loup river basin. This flood-plain aggraded perhaps most rapidly at the site with the largest drainage area (Tibbets). Here, there is a thick cumulic surface soil on the Elba Terrace that began forming about 5530 cal. year BP and that has not been buried by a discrete increment of lateHolocene alluvium. The likely reason why the 5500year-old flood-plain along the main-stem Loup River was not rapidly buried by a thick increment of late Holocene alluvium is that the Loup River channel probably was proportionately more deeply incised than the channels in tributary valleys. Perennial streamflow along the main stem of the Loup River even during the dry early and middle Holocene would have helped maintain an incised channel. Thus, the main stem Loup River could more readily accommodate the large, late-Holocene floods that were responsible for aggradation of the valley bottoms elsewhere in the basin. Furthermore, the absence of discrete, late Holocene alluvium on the Elba Terrace at the Tibbets site indicates that late-Holocene increases in channelbed elevation were minimized along the Loup River. Conversely, the depth functions for particle-size and organic-matter contents for the upper part of FillIV alluvium at sites in the upper part of the basin, especially the Horn site in the South Loup River valley, attest to much more episodic deposition of thicker increments of alluvium than downstream. These thicker flood deposits isolated developing A horizons on the evolving flood-plain 5500 years ago. Possible explanations for why flood-plain aggradation 5500 years ago was more episodic in the South Loup River valley are that the discharge regime was more variable in the loess-dominated landscape in the South Loup River valley, and erosion of silt by first- and second-order streams (gullies) incised into the Peoria
At the Chesley site, the surface of the 5500-yearold flood-plain is a thick, clay-rich alluvial unit. Although this stratum displays some properties of a Bt horizon (high clay content and strong angular block structure), it is not the B horizon of a soil. The clay-rich stratum is a slackwater deposit in a paleochannel of the South Loup River because it has an abrupt, wavy lower boundary and dark yellowish brown (10YR 4/4) mottling throughout (Mandel and Bettis, 2000). The fine texture of this deposit is a product of very slow aggradation, but insufficient radiocarbon ages are available to specify sedimentation rates. A low variability of discharges probably prevailed during this period of slow sedimentation, as no sand laminae are present within the thick clay-rich alluvium at the surface of Fill IV (Fig. 13). Such a discharge regime occurs today in the upper Loup River basin where stream discharges are dominated by groundwater discharge (Bentall, 1989). The 5500-year-old clayey alluvium at the Chesley site is unconformably overlain by 1200-year-old sandy alluvium (Fig. 13). The basal 18– 31 cm of Fill I in the described sections is organically enriched and is immediately overlain by sediments containing low amounts of organic matter (Fig. 14). Although at first appearance the organic-enriched lower part of Fill I looks like an A horizon and the clay-rich upper increment of Fill IV appears to be a Bt horizon of the same soil, this is clearly not the case. The sandcontent data indicate that two discrete lithologic units are present and Fill I unconformably overlies Fill IV (Fig. 13). Also, the lower part of organic-enriched stratum of Fill I has an abrupt lower boundary. Although a radiocarbon age is not available for the surficial portion of the clay-rich stratum to provide a maximum-limiting age for the episode of erosion that created the unconformity, the radiocarbon age of the
D.W. May / Geomorphology 56 (2003) 243–254
Fig. 14. Percent organic matter as a function of depth in profile 138.52 at the Chesley site along the South Loup River.
base of the organic-enriched stratum at the base of Fill I indicates that the erosion occurred before 1200 cal. year BP. From the available data, I infer that a large flood scoured then rapidly filled the South Loup River channel sometime before 5740 cal. year BP. Shortly before 5740 years ago the active meandering channel was abandoned. Very fine-grained, suspended-load sediment accumulated slowly in the resulting oxbow lake. A sudden increase in the magnitude and frequency of flooding probably was responsible for deposition of the 18- to 31-cm-thick increment of sandier alluvium (basal Fill I) in the oxbow lake at the Chesley site. 5.3. Causes of flood-plain stabilization 5700 – 5100 cal. year BP Two possible causes may have played a role in the stabilization of flood-plains in the Loup River basin between about 5700 and 5100 cal. year BP. First, flood-plain stabilization may have been part of a long-term complex response to Late Wisconsin loess accumulation similar to Bettis and Autin’s (1997) suggestion of causes of Holocene fluvial changes in a small river basin in eastern Iowa. Secondly, and more likely, a change to a more mesic climate at about 5700 cal. year BP may have played a very important role in slowing valleybottom aggradation. With increased precipitation, the density of prairie grasses across the landscape would have increased. This increased density of grass cover likely increased critical shear stress for
251
erosion, thus preventing surface wash erosion and initiation of rills and gullies (Prosser et al., 1995). Reduced soil erosion in low-order tributaries and from hillslopes throughout the entire basin would have greatly reduced sediment supply to the major rivers within the Loup River basin, starving the rivers of high sediment concentrations during overbank flows. Increased precipitation not only probably triggered substantial reductions in sediment yield, but also led to elevated water tables in the valley fills. Together, these changes likely would have produced more perennial flow, reduced the variability of discharge, enhanced riparian vegetation growth, stabilized stream banks, and led to finer-textured overbank deposition. Thus, climate and concomitant vegetation changes could explain cessation of or reduced valley-floor aggradation about 5700 cal. year BP. One of the best dated and documented sets of proxy data for climatic and environmental changes coinciding with the slowing of flood-plain aggradation within the Loup River system has been gathered from well-dated alluvium within the South Fork of the Big Nemaha River in extreme southeastern Nebraska. Baker et al. (2000) report that between 6600 and 3270 cal year BP (5800 – 3100 14C year BP) riparian forests were well established after being limited in composition and areal extent during the preceding few thousand years. They also report that more wetland plants were present after 6600 years ago than during the preceding interval. Especially germane to this study is their evidence of a peak in arboreal pollen just before 5740 cal. year BP (5000 14C year BP). A second line of evidence that the climate of central Nebraska underwent changes toward a more mesic climate as the flood-plains of the Loup River stabilized is the record of dune migration in the Nebraska Sand Hills. Loope and Swinehart (2000) have used dates on peats and soils under dunes, optically stimulated luminescence (OSL) dating of dune sands, and the timing of formation of wetlands and lakes in dune-blocked drainages in the Nebraska Sand Hills to construct a chronology of dune migration for the Late Wisconsin through the Holocene. Especially important to this study is their identification of dune activity from about 9440 to 5160 cal. year BP (8500 –4500 14C year BP). The cessation of dune activity, presumably because of more mesic conditions across the Sand Hills and upper Loup
252
D.W. May / Geomorphology 56 (2003) 243–254
River drainage basin, occurred when valley bottoms throughout the Loup River basin stabilized. A third line of evidence that climate was a strong controlling factor is the regional evidence from the central Great Plains that flood-plains of other rivers stabilized at about 5500 cal. year BP. For instance, Martin (1992) has documented stabilization of a flood-plain beneath a 12-m-high terrace along the Republican River in south-central Nebraska at 5220 and 4810 cal. year BP (4550 and 4270 14C year BP). Johnson and Logan (1990) have reported an age of 4870 cal. year BP (4290 14C year BP) for a buried soil beneath the Holiday terrace complex along the lower Kansas River in eastern Kansas, and an age of 4760 cal. year BP (4260 14C year BP) along Stranger Creek, a tributary to the lower Kansas River. In northcentral Kansas along the Solomon River humates from the upper part of a buried A horizon in the upper part of alluvium (2.25 –2.35 m depth) below the Kirwin Terrace have been dated at 5720 cal. year BP (4960 14C year BP), while humates from the upper 10 cm of a truncated A horizon in the section (148 – 158 cm deep) have been dated at 5330 cal. year BP (4630 14C year BP) (May, 1993). Mandel and Bettis (2001, p. 13) have reported an age of 5470 cal. year BP (4780 14C year BP) for the upper 10 cm of a buried soil in the upper part of the Gunder Member along the South Fork of the Big Nemaha River in southeastern Nebraska. Mandel (1994) has reported an age of 5740 cal. year BP (4970 14C year BP) for humates in the fourth soil below the surface of Terrace 2 at archaeological site 14HO316 in the Pawnee River basin of southwestern Kansas. Thus, flood-plain stabilization occurred regionally at the same time as in the Loup River basin, and climatic change is likely the reason for such synchronous fluvial responses. 5.4. Significance for soil mapping and archaeologic studies The finding that the depth of burial of the 5500year-old flood-plain varies throughout the Loup River basin is significant for mapping of modern soils across flood-plains and the discovery of cultural materials in valleys. Soil mappers readily appreciate how modern soils vary across flood-plains. However, this study has shown that across valleys in the Loup
River basin the soil developed at the surface of the Elba Terrace may have formed in a single lithologic unit in some parts of the drainage basin and multiple lithologic units elsewhere in the basin. Buried soils may be present at shallow depths in some parts of the basin, while they may not exist or may be deeply buried at other locations with a basin. Furthermore, paleotopography on buried flood-plains and its control on the texture of accumulating sediment in the past may influence the formation, and thus, properties, of near-surface soil horizons. In short, buried floodplains provide challenges to mappers of soils across valley bottoms. The search for cultural remains on valley floors is often hampered because they are often buried by alluvium. The episodes of aggradation responsible for burial may have occurred historically, within the past few hundred or thousand years, or in the Late Glacial. Archaeologists now readily acknowledge the importance of working with geomorphologists to understand the depths at which artifacts are likely to be found beneath various land surfaces in valleys (Gardner and Donahue, 1985). In the Loup River basin, it is clear that except for perhaps along the main stem of the Loup River, artifacts older than about 1000 years will be buried beneath a mantle of late-Holocene alluvium across the Elba Terrace below the plow zone. Only artifacts younger than about 1000 years are likely to occur within the plow zone on the Elba Terrace in all parts of the basin.
6. Conclusions The particle-size and organic-matter data for the 5500-year-old buried flood-plain at most sites in the Loup River basin reveal that valley-bottom aggradation from about 5700– 5100 cal. year BP slowed but still was episodic, as was soil formation on the evolving flood-plain. Sporadic flood-plain aggradation is evident at most sites in the percentages of sand, clay, and organic matter in the upper part of Fill IV. Sand and clay rarely show a steady fining upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the
D.W. May / Geomorphology 56 (2003) 243–254
effects of external climate forcing on vegetation and surface and groundwater hydrologic changes. The climate was becoming more mesic and the flood-plain probably became wetter and supported more vegetation. Other studies of rivers in the central Great Plains indicate that this period is a time of flood-plain stability across the region.
Acknowledgements Andres Aslan, Sue Boley-May, Joe Mason, and an anonymous reviewer provided many helpful suggestions that improved an earlier draft of the manuscript. I am grateful for the time and energy that Paul Hudson dedicated to organizing a special session at the Association of American Geographer’s Meeting in Los Angeles, and to his perseverance as editor of this special volume.
References Arbogast, A.F., Johnson, W.C., 1994. Climatic implications of the Late Quaternary alluvial record of a small drainage basin in the Central Great Plains. Quaternary Research 41, 298 – 305. Baker, R.G., Fredlund, G.G., Mandel, R.D., Bettis III, E.A., 2000. Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska. Quaternary International 67, 75 – 88. Bentall, R., 1989. Streams. In: Bleed, A., Flowerday, C. (Eds.), An Atlas of the Sand Hills. Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska – Lincoln, Lincoln, NE, pp. 93 – 114. Bettis III, E.A., Autin, W.J., 1997. Complex response of a midcontinent North America drainage system to late Wisconsinan sedimentation. Journal of Sedimentary Research B: Stratigraphy and Global Studies 67 (4), 740 – 748. Birkeland, P.W., 1999. Soils and Geomorphology. Oxford Univ. Press, New York, NY. 430 pp. Brakenridge, C.R., 1988. River flood regime and flood-plain stratigraphy. In: Baker, V.R., Kochel, R.C., Patton, R.C. (Eds.), Flood Geomorphology. Wiley, New York, NY, pp. 139 – 156. Brice, J.C., 1964. Channel patterns and terraces of the Loup Rivers in Nebraska. United States Geological Survey Professional Paper 422-D, D1-D41. Bull, W.B., 1991. Geomorphic Responses to Climatic Change. Oxford Univ. Press, New York, NY. 326 pp. Dreeszen, V.H., 1973. Topographic Regions Map, State of Nebraska (Scale 1:3,000,000). Conservation and Survey Division, University of Nebraska – Lincoln, Lincoln, NE. Dreimanis, A., 1962. Quantitative gasometric determination of cal-
253
cite and dolomite by using Chittick method apparatus. Journal of Sedimentary Petrology 32, 520 – 529. Dumont, J.F., Mering, C., Parrot, J.F., Taud, H., 1996. Morphological and mathematical analysis of asymmetrical fluvial pattern: a study case from the Ucayali River (Peru). Zietschrift fur Geomorphologie, Supplementband 103, 269 – 282. Fraser, G.S., Bennett, S.W., Thompson, T.A., Olyphant, G.A., Furer, L., 1997. Geomorphic response to tectonically-induced ground deformation in the Wabash Valley. Seismological Research Letters 68 (4), 662 – 674. Gardner, G.D., Donahue, J., 1985. The little platte drainage, Missouri: a model for locating temporal surfaces in a fluvial environment. In: Stein, J.K., Farrand, W.R. (Eds.), Archaeological Sediments in Context. Center for the Study of Early Man Institute for Quaternary Studies, University of Maine, Orono, ME, pp. 69 – 89. Jacobson, R.B., Coleman, D.J., 1986. Stratigraphy and recent evolution of Maryland Piedmont flood-plains. American Journal of Science 286, 617 – 637. Johnson, W.C., Logan, B., 1990. Geoarchaeology of the Kansas River basin, central Great Plains. In: Lasca, N.P., Donahue, J. (Eds.), Archaeology of North America. Geological Society of America, Boulder, CO, pp. 267 – 299. Kaul, R.B., 1975. Vegetation of Nebraska (circa 1850) (Scale 1:1,000,000). Conservation and Survey Division, University of Nebraska – Lincoln, Lincoln, NE. Knox, J.C., 1977. Human impacts on Wisconsin stream channels. Annals of the Association of American Geographers 67 (3), 323 – 342. Knox, J.C., 1983. Responses of river systems to Holocene climates. In: Wright Jr., H.E. (Ed.), Late-Quaternary Environments of the United States. The Holocene, vol. 2. University of Minnesota Press, Minneapolis, MN, pp. 26 – 41. Koppen, W., 1918. Klassifikation der klimate nach temperatur, niederschlag und jareslauf (Climate classification according to annual and monthly temperature and precipitation). Petermanns Geographische Mitteilungen 64, 193 – 203. Lawson, M.P., Dewey, K.F., Neild, R.E., 1977. Climatic atlas of Nebraska. Univ. of Nebraska Press, Lincoln, NE. 88 pp. Loope, D.B., Swinehart, J.B., 2000. Thinking like a dune field: geologic history in the Nebraska Sand Hills. Great Plains Research 10, 5 – 35. Mandel, R.D., 1994. Holocene landscape evolution in the Pawnee River valley, southwestern Kansas. Kansas Geological Survey Bulletin 236. Kansas Geological Survey, Lawrence, KS, and Kansas State Historical Society, Topeka, KS. 117 pp. Mandel, R.D., Bettis III, E.A., 2000. Use and analysis of soils by archaeologists and geoscientists. In: Goldberg, P., Holliday, V.T., Ferring, C.R. (Eds.), Earth Sciences and Archaeology. Kluwer Academic Publishing/Plenum, New York, NY, pp. 173 – 204. Mandel, R.D., Bettis III, E.A., 2001. Late Quaternary Landscape Evolution in the South Fork of the Big Nemaha River Valley, Southeastern Nebraska and Northeastern Kansas, Guidebook No. 11. Conservation and Survey Division, University of Nebraska – Lincoln, Lincoln, NE. Martin, C.W., 1992. Late Holocene alluvial chronology and climate
254
D.W. May / Geomorphology 56 (2003) 243–254
change in the Central Great Plains. Quaternary Research 37, 315 – 322. Mason, J.A., 2001. Transport direction of Peoria Loess in Nebraska and implications for loess sources on the central Great Plains. Quaternary Research 56, 79 – 86. May, D.W., 1986. Holocene alluviation, soil genesis, and erosion in the South Loup Valley, Nebraska. Unpublished PhD thesis. University of Wisconsin – Madison. 217 pp. May, D.W., 1989. Holocene alluvial fills in the South Loup Valley, Nebraska. Quaternary Research 32, 117 – 120. May, D.W., 1992. Late Holocene valley-bottom aggradation and erosion in the South Loup River Valley, Nebraska. Physical Geography 13, 115 – 132. May, D.W., 1993. Soils in middle and late Holocene alluvium. In: Johnson, W.C. (Ed.), INQUA Commission 6, Second International Paleopedology Symposium, Field Excursion, August 13 – 20, 1993. Kansas Geological Survey Open-File Report 93-30, Lawrence, KS, pp. 15-1 – 15-11. May, D.W., Swinehart, J.B., Loope, D., Souders, V., 1995. Late Quaternary fluvial and eolian sediments: Loup River Basin and the Sand Hills of Nebraska. In: Flowerday, C.A. (Ed.), Geologic Field Trips in Nebraska and Adjacent Parts of Kansas and South Dakota. Parts of the 29th Annual Meetings of the NorthCentral and South-Central Sections, Guidebook No. 10. Geological Society of America, Conservation and Survey Division, University of Nebraska – Lincoln, NE, pp. 13 – 31. Miller, R.D., Scott, G.R., 1955. Sequence of alluviation along the Loup Rivers, Valley County area, Nebraska. Bulletin of the Geological Society of America 66, 1431 – 1448. Nanson, G.C., 1986. Episodes of vertical accretion and catastrophic stripping: a model of disequilibrium flood-plain development. Geological Society of America Bulletin 97, 1467 – 1475. Patton, P.C., Schumm, S.A., 1981. Ephemeral-stream processes: implications for studies of Quaternary valley fills. Quaternary Research 15, 24 – 43. Prosser, I.P., Dietrich, W.E., Stevenson, J., 1995. Flow resistance and sediment transport by concentrated overland flow in a grassland valley. Geomorphology 13, 71 – 86. Ritter, D.F., Kinsey III, W.F., Kauffman, M.E., 1973. Overbank sedimentation in the Delaware River Valley during the last 6000 years. Science 179, 374 – 375. Schumm, S.A., 1977. The Fluvial System. Wiley-Interscience, New York, NY. 338 pp.
Schumm, S.A., Brakenridge, G.R., 1987. River responses. In: Ruddiman, W.F., Wright Jr., H.E. (Eds.), North America and Adjacent Oceans During the Last Deglaciation. The Geology of North America, vol. K-3. Geol. Soc. of America, pp. 221 – 240. Schumm, S.A., Lichty, R.W., 1963. Channel widening and floodplain construction along Cimarron River in southwestern Kansas. United States Geological Survey Professional Paper, 352-D, 71 – 88. Singer, M.J., Janitzky, P., 1986. Field and laboratory procedures used in a soil chronosequence study. United States Geological Survey, 1648. Soil Survey Staff, 1975. Soil Taxonomy: a basic system of soil classification for making and interpreting soil surveys. Agriculture Handbook No. 436. Soil Conservation Service, US Department of Agriculture, Madison, WI. 754 pp. Stene, L.P., 1980. Observations on lateral and overbank deposition—evidence from Holocene terraces, southwestern Alberta. Geology 8, 314 – 317. Stuiver, M., Braziunas, T.F., 1993. Sun, ocean, climate and atmospheric 14CO2: an evaluation of causal and spectral relationships. The Holocene 3, 289 – 305. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, F.G., v.d. Plicht, J., Spurk, M., 1998a. INTCAL98 radiocarbon age calibration, 24,000 – 0 cal B.P. Radiocarbon 40, 1041 – 1083. Stuiver, M., Reimer, P.J., Braziunas, T.F., 1998b. High-precision radiocarbon age calibration for terrestrial and marine samples. Radiocarbon 40, 1127 – 1151. Swinehart, J.B., Dreeszen, V.H., Richmond, G.M., Tipton, M.R., Bretz, R., Steece, F.V., Hallberg, G.R., Goebel, J.E., 1994. Quaternary Geologic Map of the Platte River 40 60 Quadrangle, United States (Scale 1:1,000,000). U.S. Geological Survey, Miscellaneous Investigations Series. US Geological Survey, 1974. Hydrologic Unit Map. State of Nebraska. 1:500,000. Walling, D.E., Owens, P.N., Leeks, G.J.L., 1998. The role of channel and flood-plain storage in the suspended sediment budget of the River Ouse, Yorkshire, UK. Geomorphology 22, 225 – 242. Wolman, M.G., Leopold, L.B., 1957. River flood-plains: some observations on their formation. United States Geological Survey Professional Paper 282-C, 87 – 109.