Abstract
The lower American River of California's Central Valley represents a possible archive of changes in sea level, climate, and tectonics. Pleistocene deposition of the American River was a function of structural shifts and glacial cycles that episodically modified eustasy, local glacier size, as well as regional precipitation patterns. Here we present a topographic investigation, age dating, and stratigraphic observations to determine controls on sedimentation of the lower American River. Topographic investigation utilizing publicly available light imaging detection and ranging data reveal at least three ancient river channels to the south-southeast of the modern channel. Previous sedimentological and dating work indicates that these channels decrease in age toward the north-northwest and are filled with normally graded fluvial deposits capped by paleosols. The preservation of ancient channel deposits on terraces to the south-southeast of the modern river and incision along the northwest channel margin form an asymmetric river valley, likely shaped by regional tectonics. An approximately 40-meter cutbank on the northwest side of the American River channel known as Fair Oaks Bluff represents the incised side of the asymmetric river valley. Grain size and sedimentary structure analyses of the siliciclastic rocks at Fair Oaks Bluff suggest floodplain deposition. Sediments dated by infrared stimulated luminesence (IRSL) to 173 to 196 thousand years ago (kya), are finer-grained, contain rooting horizons and blocky peds. These sedimentary structures indicate low energy flow conditions during the start of glaciation. Coarser sediments and rip-up clasts within a cut channel dated by IRSL to 163 to 166 kya indicate higher energy flow conditions during the core glaciation. A compilation of glaciation records from the Sierra Nevada suggests wetter conditions during glaciation and dryer conditions during interglacial periods. The glacial to interglacial variability in precipitation supports the sedimentary observations of variations in flow speed of the lower American River. We propose that tectonic mechanisms are responsible for the large-scale features of the lower American River, while climate controlled the smaller-scale deposition of sediments.