Effects of Tributary Spatial Position, Urbanization, and Multiple Low-Head Dams on Warmwater Fish Community Structure in a Midwestern Stream

A survey of stream fish communities was conducted in the upper Des Plaines River watershed in Illinois and Wisconsin to determine the distribution of fish species and to evaluate the effects of tributary spatial position, urbanization, and multiple low-head dams on fish species diversity and species composition. Forty-eight sites upstream of Salt Creek in Illinois to the headwaters in Wisconsin were surveyed between 2002 and 2004. We found that fish species diversity decreased as agricultural land was replaced by urban land. In addition, tributary position within the drainage network caused significant differences in fish species diversity; specifically, We found significantly less fish species diversity in main-stem tributary streams located lower in the drainage network than in similarly sized streams located in the headwaters of the drainage area. Fish species composition. determined by multivariate principal components analysis, also showed significant differences among stream position within the drainage network. Increases in urbanization did not have a significant effect on fish species diversity within undammed tributary sites, but it did have a strong influence on fish species composition. Hence. as urbanization increased in these undammed tributaries, the fish species composition changed from coolwater-riverine specialist to warmwater-riverine generalist assemblages. In contrast, for tributary streams with low-head dams, the presence of dams had a greater effect on fish species composition than urbanization. In addition, the presence of multiple low-head dams on the main stem had a cumulative detrimental effect on fish species diversity, which decreased along an upstream-to-downstream gradient. Consequently, it is important to consider tributary spatial position within the drainage network, the amount of urbanization, and the number and locations of darns when characterizing the structure of warmwater fish communities. especially in relation to the development of restoration plans in highly urbanized or urbanizing watersheds.