The objective of this study was to examine the spatial and temporal variability of fish communities in the South Platte River Basin in relation to environmental factors. This study was a part of the U.S. Geological Survey National Water-Quality Assessment program. Fish were collected by electrofishing at 11 sites during three summers from 1993 to 1995. Thirty-eight fish species distributed among 12 families were captured at the 11 sites. Fish communities varied spatially, and the number of fish species increased from upstream sites (2-5 species in mountains) to downstream sites (4-14 species in plains tributaries and 10-33 species in plains main stem). Fish abundance (number of fish per length of stream) at a site varied from year to year, whereas species composition (presence/absence of a species at a site) at most sites varied little among years. Natural and human factors affected fish communities. Although high flows of long duration occurred in 1995, major shifts in fish species at a site did not occur except for one site. Eight species were identified in 1995 that were not present in the previous 2 years at the most downstream site. At main-stem sites, the number of fish species increased downstream in response to natural changes in habitat, masking any human effects on fish communities. A decreased number of species occurred at a mountain site downstream of mining, compared to other mountain sites and no fish were captured at a plains tributary site affected by agriculture, as compared to the upstream site in rangeland.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225