Relations between reduced streamflow variability and fish assemblages in eastern USA streams

We used predictive models to quantify deviation from expected natural streamflow variability for streams in the eastern U.S.  Sites with > 25% reduction in mean annual streamflow variability were defined as having reduced streamflow variability based on the 10^th percentile of the distribution of streamflow variability at hydrologic reference sites.  Sites with < 25% reduction in streamflow variability were defined as being at baseline streamflow variability.   We used predictive models for the eastern U.S. to assess fish assemblage condition based on the proportion of expected native fish species that were observed.  Fish assemblages with a loss of > 25% of expected fish species were defined as being in poor condition.  Sites with reduced streamflow variability had a significantly greater loss of fish species compared to sites with baseline streamflow variability.  Conditional probability analysis indicated that a 25% reduction in streamflow variability was associated with a 70% probability that a fish assemblage would be in poor condition.  Streamflow variability decreased significantly with an increasing amount of reservoir storage in a basin.  These results suggest that hydrologic modifications such as dams have contributed to reduced streamflow variability and that reduced streamflow variability is a major factor affecting fish assemblages in eastern streams.