Processes and rates of formation of Holocene alluvial terraces in Harris Wash, Escalante River basin, south-central Utah PETER C. PATTON Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06457 PAUL J. BOISON* Earth Sciences Board, University of California, Santa Cruz, California 95064 ABSTRACT were aggrading. Gregory (1916, 1917), Bryan mation. The results of this study indicate that (1925), Gregory and Moore (1931), and Bailey compound alluvial terraces are formed by sev- Harris Wash, a tributary of the Escalante (1935) later described the entrenchment of this eral processes, which include (1) general aggra- River in south-central Utah, has two well- pre-1880 alluvium, which indicated the begin- dation of the valley, (2) deposition from large preserved alluvial terraces of compound ning of a new "epicycle" of erosion. Other floods that mark periods of alluvial-fill incision, origin in its lower reaches. The alluvial stra- workers recognized that this recent period of and (3) aggradation of flood-plain surfaces dur- tigraphy of the deposits that compose these valley incision was only the latest phase of nu- ing periods of base-level stability. In general, ag- terraces reflects the complex processes of merous cycles of degradation and subsequent gradational periods span time intervals of canyon filling and erosion. The compound aggradation found in a complex Quaternary al- thousands of years, whereas periods of alluvial- terraces were formed by general aggradation luvial record (Hack, 1942; Bryan, 1954; Hunt, fill incision are significantly shorter. of the valley, by deposition from large floods 1955; Cooley, 1962; Lance, 1963). Some inves- Recognition and interpretation of the com- that also resulted in alluvial-fill incision, and tigators have made regional stratigraphic corre- plex terrace stratigraphy has provided insight by aggradation of flood-plain surfaces during lations of these alluvial sequences (Euler and into the processes, timing, and relative magni- periods of base-level stability. The alluvial others, 1979), whereas others have noted dissim- tude of sediment production, as well as the posi- chronology indicates that Harris Wash was ilarities in the numbers and ages of alluvial de- tion and residence time of sediment storage, aggrading for most of the Holocene and that posits not only from basin to basin but even within the Escalante River drainage basin. Data only two late Holocene periods of rapid inci- within a single drainage basin (Bailey, 1935; on the magnitude and residence time of stored sion, between 2500 and 1900 yr B.P. and be- Cooley, 1962; Kottlowski and others, 1965). sediment are necessary to create accurate sedi- tween 1000 and 300 yr B.P., are evident from Cyclic aggradation and degradation of semi- ment budgets, yet these data are seldom availa- the terrace stratigraphy. Rapid aggradation arid stream valleys has been attributed to ble from historical records alone (Graf, 1983b; of flood plains during the past 150 yr pro- several mechanisms, which include: varying de- Meade, 1982; Trimble, 1981, 1983). An under- duced historic flood-plain deposits 5 m above grees of change in climate (Bryan, 1940; Leo- standing of past sediment budgets can also be the elevation of the stream. The historic pold, 1951; Euler and others, 1979); adverse used to evaluate the degree to which land-use flood-plain deposits and other similar depos- land-use practices, especially overgrazing practices alter the natural processes of sediment its in other Escalante River tributaries are at- (Dodge, 1902; Swift, 1926; Bailey and others, production, storage, and transport. tributed to increased runoff and erosion 1934; Hastings, 1959); and the inherent geo- The detailed history of Harris Wash and adja- related to land-use changes in the basin dur- morphic processes by which sediment is pro- cent streams on the Colorado Plateau is also ing this time interval. Older alluvial deposits duced and transported by streams in semiarid important in determining the effect that climatic in Harris Wash cannot be correlated with al- watersheds (Dellenbaugh, 1912; Thornthwaite shifts, deduced from other evidence, may have luvial sequences in other western tributaries and others, 1942; Schumm and Hadley, 1957; had on these semiarid stream systems (Van De- of the Escalante River basin. In these drain- Patton and Schumm, 1981). Different causes vender and Spaulding, 1979; Cole, 1982; Wil- ages, valley aggradation and incision are ap- can therefore account for the particular alluvial liams and Wigley, 1983). Finally, a better parently more sensitive to intrabasin proc- stratigraphy in separate watersheds. As a result, understanding of the alluvial history of streams esses of sediment production and storage and a universal cause for the aggradation and degra- on the Colorado Plateau will aid in fitting the less dependent on regional factors such as dation of these stream valleys may not exist extensive archaeological record (Jennings, climate change. (Cooke and Reeves, 1976; Graf, 1983a). In- 1966) to a paleo-environmental framework. stead, complex assemblages of alluvial deposits INTRODUCTION and landforms can be produced, depending on SETTING AND METHOD OF STUDY factors that control the processes of runoff, sed- The Quaternary alluvial history of the Colo- iment production, and sediment storage Harris Wash has a drainage area of 707 km2. rado Plateau has been of interest to geologists (Schumm and Parker, 1973; Patton and The wash descends from an elevation of 2,800 since Dutton (1882) observed that its stream Schumm, 1975; Womack and Schumm, 1977; m on the Kaiparowits Plateau to an elevation of valleys were being choked with sediment and Bull, 1979). 1,400 m at its confluence with the Escalante The purpose of this study is to describe the River (Fig. 1). •Present address: Leggette, Brashears and Graham alluvial history of lower Harris Wash and to A detailed study was made of the alluvial Inc., 72 Danbury Road, Wilton, Connecticut 06897. determine the processes and rates of terrace for- deposits in the lowermost 17 km of the wash Geological Society of America Bulletin, v. 97, p. 369-378, 8 figs., 1 table, March 1986. 369 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/369/3445173/i0016-7606-97-3-369.pdf by guest on 26 September 2021 370 PATTON AND BOISON (Fig. 2), which is cut into the Mesozoic Navajo disseminated in alluvium are considered maxi- group of terrace remnants of diverse lithclogy Sandstone, Kayenti, and Wingate Formations mum ages for the zone from which the charcoal and age and is compound in origin. Third, a high (Sargent and Hansen, 1982). Here, terrace de- was collected (Blong and Gillespie, 1978). The terrace is represented by only a few isolated posits in large alcoves on the outside of incised radiocarbon dates that are the most stratigraphi- remnants at heights >9 m above the modern meanders are well exposed and allow detailed cally consistent are those derived from fine- stream. These latter remnants are the oldest study of their internal alluvial stratigraphy. Such grained organic material. This organic material, deposits recognized in the canyon. exposures demonstrate that geomorphic terraces reworked from surface litter, occurs as dense The stratigraphy of the deposits that form the are constructed from deposits that differ in age mats capping sedimentation units. Even these terraces reflects complex canyon filling and ero- and sedimentology. The excellent terrace expo- organic mats, however, yield maximum ages. sion within Harris Wash. Within these terraces, sures in this reach of Harris Wash permit a de- The correlations that follow, therefore, rely six stratigraphically distinct alluvial deposits are tailed reconstruction of the number of erosional mainly on the stratigraphic position of units recognized and can be conveniently grouped by and depositional pe riods related to each terrace. within the alluvial sequence. the geomorphic surfaces that they form (Fig. 3). The location and height above the modern The high terrace consists of two units: the high stream of terrace remnants were measured at 46 TERRACE DESCRIPTION AND alluvial fill which forms the base of the terrace, sites (Fig. 2). Detailed stratigraphic sections ALLUVIAL STRATIGRAPHY and the old flood deposits which overlie it. The were made at 35 locations where the terrace low terrace comprises three distinct alluvial strat- deposits were not obscured by bank collapse or The geomorphic terrace surfaces can be di- igraphic units: the low alluvial fill which forms slope wash. The ages of some alluvial deposits vided into three groups based on elevation the base of the terrace, the historic flood-plain were determined by the radiocarbon dating of above stream level. First, the modern flood plain, deposits which are inset against the low alluvial wood, charcoal, a.nd organic-rich layers of which ranges from 1.5 to 3 m above the wash, is fill, and recent flood deposits which cap boti the leaves, seeds, and small fragments of wood the most laterally persistent and youngest depo- low alluvial fill and the historic flood-plain de- found within the alluvium. sitional feature. Second, a low terrace at heights posits. The youngest stratigraphic unit recognized Radiocarbon dates for transported charcoal from ~5 to 8 m above the wash is defined by a is the modern flood-plain alluvium. Low Terrace Stratigraphy Î8POO The low terrace is described first, as it has the best and most numerous exposures. In addition, an analysis of the low terrace stratigraphy pro- vides a model which can be used to interpret the stratigraphy of the limited exposures of the high terrace. Figure 1. Map of the Escalante River basin in southern Utah. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/369/3445173/i0016-7606-97-3-369.pdf by guest on 26 September 2021 HOLOCENE ALLUVIAL TERRACES, UTAH 371 Figure 2. Map of lower Harris Wash, showing sites of measured terraces. This reach of channel corresponds to that of the longitudinal profile in Figure 7.