Within-basin variation in the short-term effects of a major flood on stream fishes and invertebrates

We measured changes in invertebrate and fish abundance before and after a major flood event in the northeastern US, in three stream reaches that differed with respect to flood intensity. Fish and invertebrate abundance was lower at all three sites following the flood. The smallest change in fish and invertebrate abundance occurred in the site experiencing the lowest-magnitude flood (similar to bankfull). The two remaining sites experienced overbank flooding and major changes in species abundances. Changes in abundance were greatest at the site experiencing greater geomorphic change (bedload movement and sedimentation), even though hydrologic intensity (velocity, shear stress, unit stream power) was greater at the other site. Aquatic invertebrate and underyearling fish abundances were substantially reduced at these sites, while overyearling salmonids exhibited normal or greater-than-normal abundance. Among invertebrates, abundances of baetid mayflies, which are multivoltine and disperse rapidly via drift, recovered more rapidly than other mayfly families. Our results reinforce the contention that geologic setting can strongly influence the short-term impact of floods. In addition, we provide direct evidence that particular species and age classes are resistant to even the most extreme floods expected in a given region.