Investigating the anthropogenic effects on sedimentation rates and provenance of Lake Vera, California

Lake Vera is a man-made reservoir located on Rock Creek in the Sierra Nevada foothills in Nevada County, CA. This reservoir provides an opportunity to investigate the effects of natural and anthropogenic processes on reservoir sedimentation rates and provenance through a combined stratigraphic analysis, radionuclide Cesium- 137 (137Cs) dating study, hydrologic analysis, and provenance study. Three bank-cut sites from Rock Creek’s stream channel in Lake Vera’s lake bed were used to stratigraphically map and date unique layers in the lake bed; this was complemented with 137Cs radionuclide dating. Direct analysis of Lake Vera’s inflows, Rock Creek and Brush Creek, spanning the 2010–2011 and 2011–2012 rainy seasons measured each stream’s discharge and turbidity at times of sediment deposition in the lake. A petrographic study determined provenance by comparing lithics from three stratigraphically mapped correlated layers at each site. Sand-grain compositional data from the correlated layers was compared with that of upstream samples and known rocks in the watershed to establish different sediment sources over the life of the reservoir. The study outlined three time periods of deposition in the reservoir’s life: 1926–1954, 1954–1963, and 1963–2012. The calculated sedimentation rates for each time period were 0.76, 1.26, and 0.24 inches per year, respectively. The significant decrease in sedimentation rate from 1.26 to 0.24 inches per year is attributed mostly to the change in outflow structures of the dam in 1966 and better forest management practices to deal with erosion. The most recent rate of 0.24 inches per year is less than the 0.50 inches per year originally hypothesized by the lake’s owners, which extends Lake Vera’s life past the estimated 150 years. Brush Creek was found not to be a significant source of sediment during times of sediment deposition. Provenance of the most recently deposited sediment was found to be Eoceneaged gravel deposits. However, non-uniform results from samples in the lower sections in the stratigraphy prevented any provenance determination. These results are attributed to a large logging operation on the North Branch of Rock Creek in the early 1940s, and reworking of sediment from a previous dam at the location. Flushing was found to be the most effective and affordable technique to prevent sediment build-up in Lake Vera.