Evaluation of Rangeland Stream Condition and Recovery using Physical and Biological Assessments of Nonpoint Source Pollution
Livestock grazing is the most common land use in the western United States, and has caused widespread degradation of water quality as a result of impacts to stream and riparian ecosystems. The problem is especially severe in the arid Great Basin, where stream channels and associated vegetation occupy small areas and carry low runoff, but are exposed to intense grazing pressure due to congregation of livestock in areas of shade and water supply. The federal Clean Water Act requires states to assess nonpoint source water pollution, including that resulting from livestock grazing.
Water quality has traditionally been evaluated using water chemistry criteria (e.g. for point source pollution), but these measures may not be effective in detecting nonpoint source water quality problems typically caused by livestock grazing. More responsive monitoring parameters for rangeland streams may include physical characteristics of stream channels, and biological measures of the characteristics of aquatic invertebrate communities and fishes. The paucity of relevant scientific information on how grazing impacts can best be detected and ameliorated, however, has hindered efforts to improve water quality and recover stream and riparian ecosystems damaged by livestock grazing.
Our research had two major objectives. These were (1) to compare the effectiveness of different monitoring techniques in detecting livestock impacts to stream ecosystems and water quality (short-term objective); and (2) to compare the recovery of damaged streams under different livestock grazing practices (long-term objective). We addressed both of these objectives through a two year research program designed to quantify stream channel characteristics and attributes of aquatic invertebrate and fish populations on rangeland streams flowing through pastures managed under different grazing practices, including corridor fencing, rest-rotation grazing, and season-long grazing. The results presented here from an eastern Sierra Nevada Great Basin watershed address our short-term goal of comparing physical and biological monitoring approaches, and provide a baseline for our long-term goal of evaluating stream recovery under varied grazing practices.
Physical habitat quality was calculated from composite indices of bank, channel, and substrate characteristics, and was highest in the un grazed stream reach. The Habitat Quality Index calculated for each study reach was highly correlated between different seasons and years, and was also highly correlated with trout biomass. Correlations between the Benthic Habitat Quality Index and a multiple metric Benthic Invertebrate Community Index were high for spring surveys (before the grazing season), but low for fall surveys (at the end of the grazing season), possibly reflecting short term grazing induced changes to the benthic invertebrate communities not detected by the stream channel monitoring. We conclude that measurements of stream channel characteristics, fish population structure, and invertebrate community composition generally provide similar information about the condition of particular stream reaches, although stream reach characterization based on fish populations and stream channel attributes were more similar to each other than those based on benthic invertebrate communities. However, while the monitoring of stream channel characteristics and fish populations will likely provide reliable information on long term trends in stream condition, benthic invertebrate community composition may provide an indication of short term grazing-related changes in stream condition and water quality that are not detectable by monitoring programs that focus solely on stream channel characteristics or fish populations. Separating natural variability in invertebrate community composition from that attributable to grazing will require long-term monitoring to develop reliable indicators. Both physical and biological measures further indicate that while impacts are evident on most grazed reaches, others appear to be unimpaired during some periods.