The Lower American River Area, California: A Model of Pleistocene Landscape Evolution

The Lower American River Area, California: A Model of Pleistocene Landscape Evolution Roy J. Shlemon* Geomorphologists and stratigraphers long have been concerned with the form and magnitude of valley development and landscape evolution during the Pleistocene Epoch. In middle latitudes especially , many workers believe that degradation probably occurred during glacial periods; conversely, others believe that aggradation dominated during these times with streams cutting primarily during interglacial stages. The differences of opinion generally concern phases of cutting and filling; whether each occurred in glacial, interglacial , or some transitional period. Almost all agree that landscapes more than several thousand years old were subject to Pleistocene climatic change and thus experienced cycles of aggradation and degradation.1 Of increasing significance is the growing realization that certain epochs of landscape evolution were characterized by relative stability in which great areas were neither significantly eroded nor aggraded. * Dr. Shlemon is an associate professor in the Center for Wetland Resources , Louisiana State University, Baton Rouge, 70803. This interpretation of the American River Quaternary stratigraphy to a great degree is based on unpublished subsurface engineering and geologic data made available over the last several years by the U. S. Bureau of Reclamation, U. S. Geological Survey, California Department of Water Resources, and California Division of Highways . A. Teichert and Son, operators of several gravel quarries in the Sacramento area, kindly authorized collecting of the vertebrate bones and wood used for radiometric dating. David S. McArthur read the manuscript and made valuable suggestions for improvement. 1 The literature abounds with hypotheses concerning Pleistocene climatic or man-induced cycles of landscape evolution. For a general treatment of regional cutting and filling in varying climates see Karl W. Butzer, Environment and Archeology, (Chicago: Aldine Publishing Company, 1966), 524 pp. A 61 62ASSOCIATION OF PACIFIC COAST GEOGRAPHERS Many models have been proposed attempting to simplify the complexities of landscape evolution in different morphogenetic regions . Some models have evaluated areas only where the major drainages originated in either glacial or in non-glacial terrain; others have assessed the relative effects of sea-level change.2 Only a few models, however, especially of North American landscapes, have considered areas in which trunk streams passed through all these environments, having headed in glacial terrain, traversed non-glacial topography, and finally debouched into a glacio-eustatically fluctuating sea. In part this is due simply to scale. As a classic example, continental glacial deposits in the upper Mississippi River Valley are at least 500 miles from the downstream eustatically-affected alluvial terrace and deltaic section. Thus whereas alluviation in the lower Mississippi River Valley is attributed to rising sea level during interglaciation , deposition upstream is believed to have occurred in glacial time.3 Typically the stratigraphy of the intervening area is extremely complex and not well understood. Quite unique, therefore, is the landscape in which the major fluvial system affected both by sealevel fluctuation and headwater glaciation records in its sediments an almost complete Pleistocene history of cutting, filling, and stability . Such a landscape occurs along the lower American River, California . model and review of arroyo development in the southwestern United States is given by Yi-Fu Tuan, "New Mexican Gullies: A Critical Review and Some Recent Observations," Annals, Association of American Geographers, Vol. 56 (1966), pp. 573-597. Examples of periodic landscape change based primarily on environmental interpretation of paleosols are discussed by B. E. Butler, "Soil Periodicity in Relationship to Landform Development in Southeastern Australia ," in J. N. Jennings, and J. A. Mabbutt (eds.), Landform Studies From Australia and New Guinea, (London, Cambridge University Press, 1967), pp. 231255 ; and by Robert B. Ruhe, Quaternary Landscapes in Iowa, (Ames: Iowa State University Press, 1969), 255 pp. 2 The numerous European landscape models are frequently based on paleoenvironmental interpretations of climatic terraces. For a synthesis see F. E. Zeuner, The Pleistocene Period, (London: Hutchinson and Company, 1959), 447 pp. Many North American models are reviewed by S. A. Schumm, "Quaternary Paleohydrology," in H. E. Wright and D. G. Frey (eds.), The Quaternary of the United States (Princeton: Princeton University Press, 1965), pp. 783-794. 3 Glacio-eustatic control of alluviation in the lower Mississippi River has been expounded primarily by H. N. Fisk, "Loess and Quaternary...