The Impact of Summer Cattle Grazing on Surface Water Quality in High Elevation Mountain Meadows

Surface waters were tested before and after the arrival of cattle to summer grazing allotments for pathogenic bacteria indicators E. coli, fecal coliform bacteria, and total coliform bacteria in the high elevation mountains of the Stanislaus National Forest, California, USA. Water samples were collected from one control/ungrazed stream site and at four grazed stream sites before cattle grazing began and during the time when livestock were present. All sample sites were higher than 1900 meters in elevation. Samples were transported within 6 hours to a water analysis laboratory, where they were analyzed following standardized lab methods. Results showed that individual and average concentrations of fecal coliform bacteria in surface waters were below regulatory thresholds at the ungrazed site and at the grazed sites before cattle arrived. Mean fecal coliform concentration increased from 9 to 350 CFU/100 mL, and mean E. coli increased from 8 to 240 CFU/100 mL, respectively, after grazing. The increase in mean concentration of fecal coliform at each grazed site was significant (p < 0.05). Total coliform bacteria and E. coli concentrations showed the same pattern. Cattle grazing in the sampled high elevation Sierra Nevada meadows resulted in a significant increase in indicator bacteria.