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Elevated loads of salt and selenium can impair the quality of water for both anthropogenic and natural uses. Understanding the environmental processes controlling how salt and selenium are introduced to streams is critical to managing and mitigating the effects of elevated loads. Dominant relations between salt and selenium loads and environmental characteristics can be established by using geospatial data. The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, investigated statistical relations between seasonal salt or selenium loads emanating from the Upper Colorado River Basin and geospatial data. Salt and selenium loads measured during the irrigation and nonirrigation seasons were related to geospatial variables for 168 subbasins within the Gunnison and Colorado River Basins. These geospatial variables represented subbasin characteristics of the physical environment, precipitation, geology, land use, and the irrigation network. All subbasin variables with units of area had statistically significant relations with load. The few variables that were not in units of area but were statistically significant helped to identify types of geospatial data that might influence salt and selenium loading. Following a stepwise approach, combinations of these statistically significant variables were used to develop multiple linear regression models. The models can be used to help prioritize areas where salt and selenium control projects might be most effective. gation water can vary considerably within a subbasin depending on the irrigation method and can be as much as 60 in. annually. Soils in near-stream areas and agricultural fields are dominantly derived from Mancos Shale, which is a Cretaceous marine shale occupying 1,090 mi 2 , or 18.5 percent of the study area (fig. 4) (Day and others, 1999).

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Map showing bed-material sampling locations. A, the Jensen mid-channel bar (spawning habitat), 5 miles downstream from the Green River near Jensen, Utah, streamflow-gaging station. B, 09261000, Green River near Jensen, Utah, streamflow-gaging station. C, 09152500, Gunnison River near Grand Junction, Colorado, streamflow-gaging station .