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.