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. . t.tJ 00 N Q') . . . . . . . . . I I erodable shale beds are still a major. source of river sediment. Geologv. The geology of the basin has a marked influence on the occurrence and quality of Colorado River wa ter. Since most of the to- tal discharge originates in the Upper ,Basin the geology of this region is critical. In the mountainous regions a close relation can be found between the groundwater in consolidated rocks and in the alluvium with the water in surface streams. Rainfall and snowmelt enters the groundwater reser- voir and reaches the stream by springs, seeps, and through alluvium in stream channels. As streams rise and fall a continuous interchange exists between the ground and surface water systems through the alluvium. The rocks in intimate contact with the groundwater impart a characteristic chemical quality to the water. ln the basins and interior valleys groundwater in consolidated rocks can be expected to contribute only a minor amount to the discharge and chemical quality in the streams except in situations such as thermal springs which discharge into streams. In the lower valleys, precipi- tation is insufficient to provide groundwater discharge. The water bear- ing aquifers originating in the mountains are buried deep beneath thicknes- es of impermeable material in the interior valleys. Consolidated rock material in the lower valleys, although they do not contribute to flow from groundwater in surface streams, do affect quality. Rocks and their residuum react with surface runoff when the infrequent 7 i:,~