Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 27, 2012
Publication Date: January 4, 2012
Citation: Clark, P., Moffet, C.A., Lewis, G.S., Seyfried, M.S., Hardegree, S.P., Pierson Jr, F.B. 2012. Water quality effects of herded stream crossings by domestic sheep bands. Journal of Environmental Quality. 41:1-11. Interpretive Summary: The water quality effects when domestic sheep flocks or bands are tightly-herded during crossings of rangeland streams are largely unknown. We evaluated suspended sediment and Escherichia coli bacteria levels before, during, and after crossings made by ewe-lamb bands (2,000 – 2,500 individuals) on high-elevation stream systems in eastern Idaho and southwestern Montana. Although crossings induced intense but brief elevations in both pollutants, these effects attenuated rapidly with downstream distance and were unlikely to cause permanent or long-lasting impacts to stream quality. Domestic sheep producers and natural resource managers charged with developing and implementing sheep grazing management plans for high-elevation rangelands will
Technical Abstract: On rangelands, free-ranging or loosely-herded domestic sheep tend not to linger in shrub-dominated riparian areas thus limiting their impacts on stream water quality. The water quality effects when sheep are tightly-herded during stream crossings, however, are largely unknown. In this study, downstream concentrations of suspended sediment and Escherichia coli bacteria were evaluated before, during and after herded, one-way stream crossings by sheep bands (about 2 000 to 2 500 individuals). Stream crossing trials were conducted on two reaches, one moderate-gradient and one low-gradient, of each of 3 stream systems located on high-elevation, summer rangelands in the Centennial Mountains of eastern Idaho and southwestern Montana. Water samples were collected at an upstream station and at multiple stations downstream from the crossing point on each reach. The timing, magnitude, and duration of downstream changes in water quality were evaluated relative to background levels upstream. Herded stream crossings briefly increased the concentration of both suspended sediment and E. coli in water downstream from the crossing point. Concentrations of both pollutants then rapidly declined over time and downstream distance. Post-peak concentrations of suspended sediment at all downstream stations tended to return to background levels within the time span of a crossing trial (= 78 min). Suspended sediment delivery differed among streams but generally declined in an exponential or curvi-linear fashion with downstream distance. Streams exhibited differing levels of E. coli delivery to a sampling station located 500 m downstream from the crossing point. Increases in E. coli in the study streams appeared to have come directly from the sheep and not from re-suspension of bacteria living in the stream bed sediment. Carry-over of bacteria deposited in the bed sediment of these streams probably was quite short-lived.