Flow class analyses of suspended sediment concentration and particle size in a mixed-land-use watershed

Knowledge gaps remain concerning fundamental suspended sediment physical processes/relationships, such as particle size class dynamics and hydroclimatic variability. Streamwater grab samples were collected four times per week (Oct. 2009-Feb. 2014) at nested-scale gauging sites (n = 5), representing contrasting dominant land use practices. Streamflow was monitored in situ. Grab samples were analyzed for total suspended sediment concentration and mean particle size using laser particle diffraction. Comparisons were performed of suspended sediment parameters corresponding to different streamflow classes (i.e. 20th, 40th, 60th, 80th, and 99th percentile flows). Average suspended sediment concentrations displayed a decreasing trend from the predominately agricultural headwaters to the urbanized mid-watershed, and a subsequent increase to the suburban lower watershed. Results indicated significant (p < 0.05) differences in concentrations corresponding to different flow classes, with concentrations at more urban sites displaying greater "sensitivity" to streamflow variability. Significant (p < 0.05) differences between concentrations at different sites were found, but concentrations became progressively more similar (p > 0.05) at higher flows. Mean particle size results displayed significant differences (p < 0.05) between flow classes at every site. Notably, results showed a decrease in particle size during progressively higher flows, despite expectations based on stream velocity/competence relationships. Significant (p < 0.05) spatial differences in particle size were found between sites, specifically for flows within the 20th and 40th percentile flow class. However, the spatial pattern was weakened at higher flows (60th, 80th, and 99th percentile flow classes) as sites displayed greater statistical similarity. Collectively, results highlight the compounding influences of streamflow variability and land use practices on suspended sediment regimes; and considering unexpected results regarding relationships between particle size and flow, emphasize the need for continued research concerning particle size dynamics.