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. ",.... ..... CJ1 ",,'l. .1 . SPECIAL STUDIES (continued) Green and Colorado Rivers. This large region was divided into nine sub- regions or basins. Data were collected on streamflow, conductivity, and water Quality through chemical analyses. Measurements were taken in December and January when streamflow was at a low level, made up primarily by ground water discharge. Cold temperatures at this time of year would have made surface runoff from melting snow very unlikely, and there was no observed rainfall. The volume of irrigation returnflow should also have been at a minimum. Measurements were not taken as a part of this study in areas where adequate data existed as a result of previous studies, as, for example, the Piceance Basin oil shale investigations. The area covered by this investigation yields 74 percent of the water and 77 percent of the salt contributed by the Upper Basin. The measured ground water contribution to the total annual salt load of this part of the Upper Basin was 55 percent. However, the ground contributed by each of the nine subbasins varied considerably, from a low of 30 percent in the Upper Green River Basin, to a high of 93 percent in the Lower Colorado River headwaters. In general, the percent of total salt load contributed by ground water was higher for subbasins in the Colorado River Region, an average of 69 percent, than for subbasins in the Green River Region, an average of only 38 percent. The Upper Colorado Headwaters subbasin is an area of high mountain country on the west slopes of the Cootinental Divide. The geologic formations are classed as slightly saline to nonsaline. The soils are Inceptisols, Alfisols, and Mollisols which are normally low in salts or have been leached by virture of their location at higher elevations. Vegetation is alpine tundra, conifers, mountain scrub, and meadow- lands. Salt concentration in runoff would be very low. This subbasin produces 28 percent of the total water for the study region and only 16 percent of the total salt load. However, the subbasin accounts for 26 percent of the total salt contributed from ground water. Of the salt produced by the subbasin, 87 percent ComeS from the ground water yielded by the Eagle Valley Evaporate Formation and from Glenwood and Dotsero Hot Springs. Point source salt contributions can be very significant. The Water and Power Resources Service estimates that 500,000 tons (450 000 t) of salt, or nearly one half of the total subbasin salt load, are produced annually by the Glenwood and Dotsero Springs. The effect of irrigation returnflow on the salt load from ground water during the base flow period (winter) is very small for most sub- basins, especially those at high elevations. Soila being irrigated are the alluvial, relatively well-drained deposits along streams. The distance ground water (irrigation returnflow) must travel before entering a stream channel are in most cases, relatively short. Therefore, water quality data collected in December or January should reflect a minimal influence from irrigated agriculture. However, the large blocks of farm- land located on fine-textured, poorly drained soils, and/or at long distances from streams can have a great influence on ground water. Exam- ples of these would be the Uinta and Price River Basins in Utah, the Lower Gunnison and Grand Valley Projects in Colorado, and the Eden-Farson Project in Wyoming. 117