University of California Water Resources Center

Scouring winter floods strongly influence the structure and dynamics of food webs in rivers with winter flood, summer drought hydrographs. Reduction or elimination of scouring floods, in addtition to altering physical conditions within rivers, may negatively affect salmonid populations by reducing energy flow to them from lower trophic levels. We compared food webs of two northern California rivers with drastically different flow regimes to assess the effects of differences in food web structure on the distribution and growth of juvenile steelhead trout. The upper Mad River has a highly regulated flow regime and rarely experiences scouring winter floods, while the upper Van Duzen River is free-flowing and experiences frequent scouring floods. Thrroughout spring and summer 1994 densities of the large, grazing caddisfly Dicosmoecus gilvipes exceeded 801m2 in the Mad River, but were < 21m2 in the Van Duzen. Consequently, filamentous green algae was nearly absent in the Mad from June through September, but was relatively abundant in the Van Duzen. Densities of other stream-dwelling insects (primarily Chironomids and mayflies, which are the preferred prey of juvenile salmonids) and juvenile steelhead were consistently mlich lower in the Mad than the Van Duzen. At the end of the summer, Dicosmoecus pupated, and thus became inactive, which resulted in a large bloom of filamentous green algae (primarily Oedogonium and Cladophora) and a several-fold increase in the densities of Chironomids and mayflies in the Mad but not the Van Duzen. River flows in 1995 were much higher than in 1994 and both rivers experienced a number of scouring floods. As a consequence, Dicosmoecus densities were reduced to < 21m2 in both rivers throughout the spring and summer. The Mad experienced a large Cladophora bloom, and densities of Chironomids and mayflies were several times higher in 1995 than 1994. These observations support our hypothesis that eliminating scouring floods favors large, slow-growing benthic insect taxa over smaller, faster-growing taxa whose populations build up rapidly after floods. Since the former are invulnerable to predation by juvenile salmonids, energy flow is reduced and juvenile salmonid populations decline.

Experimental manipulation of juvenile steelhead in artificial channels with and without Dicosmoecus allowed us to test this hypothesis more directly and without potential influences from factors other than food web structure that may have varied between regulated and unregulated rivers. Experimental results revealed that Dicosmoecus significantly reduced the availability of small prey, which resulted in negative juvenile steelhead growth. Together, our surveys and experimental results show that elimination of scouring floods alters energy pathways in river food webs resulting in reduced biomass available to fish populations. Modifying flow regimes regulated by dams, so they more closely resemble natural hydrographs, may be an important step in restoring salmonid populations in some rivers.