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.

Ac-nowledgements I would like to acknowledge the contributions of the following: B i l l Young, Duncan Foley, Larry Dee, and Allen Merritt for their support of and help with this project. I would also like to acknowledge Paul Castelin for all of his help and patience.

The results of the monitoring indicate that while water temperatures have remained constant, the system shows a gradual overall decline in artesian pressure superimposed on fluctuations

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A field research site has been developed to explore the combined use of physical experiments and mathematical modeling to analyze large‐scale infiltration and chemical transport through the unsaturated media overlying the Snake River Plain Aquifer in southeastern Idaho. This site offers opportunities to observe water and contaminant migration influenced by fluid dynamics and microbial activity through heterogeneous‐porous and fractured media.At many waste disposal facilities, the presence of toxic or radioactive wastes between the land surface and underlying aquifers poses a serious and ongoing threat to public health and safety.To reduce the risk associated with these industrial and Cold War by‐products, a combination of remediation and long‐term monitoring will be required.