Aquatic Resource Program Report

River ecosystems are influenced by processes operating and interacting on a variety of spatial scales. Quantifying the interplay of large and small-scale processes is central to both a basic ecological understanding of river ecosystems as well as the development of effective watershed management strategies. A key management goal for many western rivers restricted by large dams is to restore ecosytem function through restoration of the natural hydrograph. However, factors operating on small spatial scales may also compromise ecosystem function. In a watershed in the California Sierra Nevada without a major dam, we examined the influence of land use patterns, vegetation cover, water quality, native and introduced fishes and periphyton abundance on the distribution patterns of aquatic invertebrates. We found that patterns of invertebrate species richness, biomass and density were significantly influenced by water quality, surrounding land use and vegetation, and fish distributions, although the abundance of nonnative fishes had no additional influence. Position in the watershed also had little influence on variation in several food web properties. Our data indicate that in this watershed, the influence of processes operating on larger spatial scales were of approximately the same magnitude as processes acting more locally. In particular, declines in water quality and decreased summer flows had the most significant negative effects on aquatic invertebrates. Our study highlights that even if high flows are maintained on western rivers, these systems are significantly at risk to reduced summer flows, altered water quality and nonnative species.