Diversion of the upper Bear River: Glacial diffluence and Quaternary erosion, Sierra Nevada, California

Deep canyon erosion and diversion of more than 300 km(2) of the former upper Bear River is documented with stratigraphic and morphologic evidence. Stratigraphic relationships constrain canyon incision to no older than late Miocene in age. A hypothesis is advanced that channel diversion was caused by ice spilling over a divide between the Bear and South Yuba drainages leading to development of glacial diffluence and deep incision. The local glacial stratigraphy is mapped based on lateral moraines, erratics, and striae which indicate that at least two and probably three glacial advances occupied both South Yuba and Bear valleys. Stratigraphic relationships constrain most valley incision to before the last major glacial stage and probably by the end of an earlier, larger glacial advance. Morphologic evidence supports a hypothesis of dominantly sub-glacial erosion at an outlet glacier through the Yuba gorge where steep valley gradients, high shear stresses, and large meltwater discharges led to rapid erosion and formation of a deep V-shaped valley.