Debris flows are mass movements of sediment, wood and water down stream channels that profoundly impact streams and adjacent riparian areas and are a major erosion process in many steep mountainous terrains. Their impact on aquatic ecosystems, however, is poorly understood. In this report, we describe the ecological effects of a debris flow on a headwater stream in old-growth temperate forest that occurred in December 1999. This geomorphic disturbance scoured our study reach to bedrock and removed live trees 5 to 15 m into the adjacent riparian forest. Summer water temperature was higher in the affected reach after the debris flow than before; these changes have persisted through 2002. Algal biomass after the debris flow was similar to before; however, we speculate that algal growth was constrained by herbivory, as the abundance of two insect herbivores Chironomidae and Baetis spp. increased 380 and 9300%, respectively, within the first year after the debris flow. Overall, these data show that a rare disturbance event can mediate the population dynamics of stream herbivores in small, headwater streams such as those found in Olympic National Park likely via increased primary production and water temperature.
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Vol. 152 • No. 2