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Bacterial Variations in Streams from a Southwest Idaho Rangeland Watershed¹

ABSTRACT

Sources and variations in bacterial indicators are reported from stream sites over a 3-year period on a 233-km² rangeland watershed in southwest Idaho. The occurrence of fecal coliforms was directly related to the presence of cattle on summer range and winter pastures. Fecal coliform counts in adjacent streams were found to increase soon after cattle were turned in and remained high for several months after cattle were removed.

Runoff from rainstorms increased both total and fecal coliform concentrations in streams on summer range with limited management and adjacent to winter pastures, but runoff from snowmelt had little effect. Total coliform counts varied more with change in streamflow than did fecal coliform counts. In fenced summer range allotments, under deferred grazing management, the effects were the same, except bacterial counts were not as high or persistent.

The decrease in bacterial concentrations at several downstream sampling sites indicated that certain stream segments were self-purifying. The presence or absence of livestock along the streams overshadow any effect variations in chemical concentration of the water might have on bacterial concentrations.

Key Words: water quality • rangeland pollution • management practices • nonpoint sources

¹ Contribution from the Northwest Watershed Res. Center, ARS-USDA; Bur. of Land Manage., USDI; and Idaho Agric. Exp. Stn., cooperating.

² Geologist and Biologic Technician, respectively, Northwest Watershed Res. Center, ARS-USDA, Boise, ID 83701.

Received for publication June 13, 1977.

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