Abstract
During the Pleistocene Epoch, rivers flowed across the landscape as glaciers advanced and retreated and sea levels fell and rose numerous times. As the American River flowed from the Sierra Nevada west to the Sacramento River, steep river channels eroded across the Central Valley and were filled with gravel. After channels filled with gravel, deposition transitioned to finer sands and the river incised new channels to the north. This study investigates whether glacial and interglacial climatic changes influenced landscape evolution by varying sediment loads along the Lower American River. Dating fluvial sandstones with infrared stimulated luminescence (IRSL) on feldspars and reconstructing the depositional environment with grain size and mineralogical analyses reveals glacial influence on sedimentation. Shlemon (1972) proposed that early glaciation created a steep river gradient which incised into older alluvium, then high quantities of large sediments were deposited during maximum glaciation, and finally paleosols developed over a stable interglacial period. IRSL data supports this hypothesis as finer deposition occurred during an interglacial to glacial transition from 196.0 ± 57.4 ka to 173.4 ± 29.7 ka followed by coarser deposition during the following glacial conditions from 166.2 ± 25.6 ka to 163.5 ± 29.2 ka. Reconstructing the development of the Lower American River fluvial deposits increases understanding of the sedimentation in mid-latitude rivers draining glaciers and provides dates for an aquifer-bearing Quaternary formation in California’s Central Valley.