DIVERSION OF THE UPPER BEAR-RIVER - GLACIAL DIFFLUENCE AND QUATERNARYEROSION, 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 evi dence. Stratigraphic relationships constrain canyon incision to no old er than late Miocene in age. A hypothesis is advanced that channel div ersion was caused by ice spilling over a divide between the Bear and S outh Yuba drainages leading to development of glacial diffluence and d eep incision. The local glacial stratigraphy is mapped based on latera l moraines, erratics, and striae which indicate that at least two and probably three glacial advances occupied both South Yuba and Bear vall eys. Stratigraphic relationships constrain most valley incision to bef ore the last major glacial stage and probably by the end of an earlier , larger glacial advance. Morphologic evidence supports a hypothesis o f dominantly sub-glacial erosion at an outlet glacier through the Yuba gorge where steep valley gradients, high shear stresses, and large me ltwater discharges led to rapid erosion and formation of a deep V-shap ed valley.