Pebble counts were used to characterize the sediment grain size distribution of the bed surface for two reaches of the lower Merced River in California's Central Valley to support research intended to evaluate the extent to which large-scale restoration projects provided improved salmon spawning habitat. A related goal of this study was to improve our understanding of the geomorphic factors influencing spawning site selection by salmon. The pebble counts were performed at the Merced River Ranch and Robinson Reach field sites using the method of Wolman [1954]. Grain diameters (n>100) were measured using a gravelometer by recording the largest opening on the gravelometer that an individual particle did NOT pass through, which is similar to recording the percent retained in a standard sieve-based analysis of a sediment sample. We used pre-restoration pebble count data collected on the Merced River Ranch in 2004 [Stillwater, 2004] and on the Robinson Reach in 2001 [CADWR, 2001]. Additional pebble count data were collected during the winter of 2014 at the Merced River Ranch and summer of 2016 on the Robinson Reach.
Reference:
Wolman, M. G. (1954), A method of sampling coarse river‐bed material, Eos Trans. AGU, 35(6), 951–956, doi: 10.1029/TR035i006p00951.