MORPHOGENESIS OF THE YUBA RIVER FAN, CALIFORNIA

The lower Yuba River (LYR) is one of the master drainages flowing west out of the northern Sierra Nevada in northern California. A recent map of the buried valley and soil geomorphic maps published in the 1970s and 1980s constrain early evolution of the fan. Tilting of the Sierra block during the Cenozoic caused subsidence and deep burial of the ancestral LYR valley. Quaternary terraces reflect alluviation during glacial advances in the Sierra. This paper reviews the Cenozoic history then turns to human impacts on the system made famous by Gilbert’s classic monograph on hydraulic mining sediment (HMS). Georectification of early maps and historical aerial photographs document channel and historical terrace evolution over centennial time following storage of 250 x 10⁶ m³ of HMS. Pre-mining LYR channels are delineated and compared with subsequent changes and a modern high-water channel system. Floodplain aggradation of the LYR in response to episodic delivery of HMS followed by rapid channel incision mimicked glacial cycles, but geomorphic recovery has differed due to human interventions including gold dredging, agriculture, and engineering structures for which the history is poorly understood. Recent breaching of Shanghai Shoals downstream on the Feather River is lowering the LYR base level and has the potential to rejuvenate the system.