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We examined the short-term effects of the natural catastrophic collapse of a beaver dam on downstream benthic insect density and fish community structure in a headwater tributary of the Mississippi River. The catastrophic collapse of the dam and ensuing flash flood resulted in a dramatic (> 90%) decrease in benthic insect density in riffle and pool habitats. Sixty days after collapse of the dam, insect densities in riffles were 62% of pre-collapse densities. Insect recolonization of pools was slower than for riffles; 60 days after collapse of the dam insect densities in pools were 8% of pre-collapse levels. Collapse of the beaver dam altered the structure of the downstream fish community by causing a short-term (2-4 days) influx of pond species, resulting in a brief increase in species richness and abundance. Fish species richness and abundance then decreased for 4-60 days to levels below those prior to the collapse.