We sampled fishes four times each year at 12 sites on the Solomon, North Fork Solomon, and South Fork Solomon rivers in north-central Kansas during 1996 and 1997. Kirwin and Webster reservoirs are located on the North Fork Solomon River and South Fork Solomon River, respectively, and Waconda Reservoir inundates the confluence of these two rivers at the head of the Solomon River. Multivariate analyses identified two fish assemblages that were related to stream discharge. One fish assemblage was associated with stream segments that had lower discharges, such as those located upstream from Kirwin and Webster reservoirs. This assemblage was characterized by equal numbers of extirpations of native species and introductions of nonnative species. The other fish assemblage was associated with the reservoirs and stream segments that had higher discharges. This assemblage was characterized by a large number of species that were introduced or had immigrated into these areas. For the basin as a whole, 32% of the native species of fishes have been extirpated, and 51% of the present assemblage was comprised of nonnative species. Most of the extirpations and introductions are associated with habitat changes caused by agricultural development and the construction of impoundments. The relatively large component of the fish assemblage comprised of nonnative species reflects the trend toward homogenization of fish assemblages throughout the United States.