Water quality (chemistry), habitat structure, flow regime (water volume, timing of flow), energy (food) source, and biological interactions (competition, predation, disease) are all variables that affect the structure of the biological communities in a river. Therefore, alteration of any or all of these variables can result in a decline in the biological diversity and ecological integrity of a stream. Constructing a list of water-quality characteristics that incorporates these variables can identify which variables have been affected by human activity. Identifying the cumulative effects of human activity on these resource variables can be a valuable tool in determining the feasibility of managing a water resource for biological diversity.
Since 1992, the U.S. Geological Survey National Water-Quality Assessment (NAWQA) program has been using an integrated approach to examine water quality in the South Platte River Basin. Surveys of algae, invertebrate, and fish communities; physical (habitat, flow, temperature) and chemical variables; and contaminants in fish tissue and bed sediment have been used to assess water quality at sites located in different land uses in the basin. These data can be used to assess the biological diversity to be expected under different land-use and water-quality settings. A list of water-quality characteristics that integrates the results from these surveys has been developed for the different land-use settings and indicates that water quality is related to land use. Furthermore, the data studied have indicated that physical, chemical, and biological variables are interrelated and interdependent. For example, more invertebrate and fish species were present in the South Platte River at sites characterized by wide, braided channels and lower nitrogen and phosphorus concentrations than at sites characterized by a single channel and higher nitrogen and phosphorus concentrations. Measurement or collection of just one of these variables is insufficient to assess the status of the system; moreover, recognizing the holistic nature of the system (South Platte River Basin) is essential.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225