Incision and morphologic evolution of an alluvial channel recovering from hydraulic mining sediment

Abstract

Hydraulic gold mining in the Sierra Nevada, California, produced such a large volume of sediment from 1853 to 1884 that channels could not carry a substantial proportion of the delivered sediment. The lower Bear River aggraded as much as 5 m and underwent a major avulsion during the 1870s. Channel morphology continues to respond to these changes more than 100 years after the cessation of most hydraulic mining. Textures of mine tailings and older alluvium are contrasted, and channel incision and channel-shape changes from 1930 to 1985 are documented. Unlike the Yuba and Sacramento Rivers, the lower Bear River had not returned to pre-mining base levels by 1950 as sediment loads decreased. Nor were incision rates or morphological change governed by reduced sediment loads below dams built in 1928 and in the mid-1960s. Channel stability was due to resistance of a cohesive stratum on the channel bed. An episode of channel down-cutting was instigated by a moderately large flood in December 1955 that pierced the cohesive layer and was sustained through the 1970s by moderate-magnitude floods. The 1955 flood did not directly erode large volumes of sediment, but it destabilized the channel, allowing smaller floods to erode the channel at an accelerated rate. Episodic channel-bed incision is contrasted to progressive morphologic changes. Cross-section shape narrowed and deepened steadily from 1930 through the mid-1970s as the channel was superimposed onto cohesive older alluvium. Progressive deepening set up a positive feedback that ensured the ultimate penetration of the paved bed. Channel morphologic readjustment from the avulsion thus was both progressive and episodic.