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<br />1~2572 <br /> <br />streams. However, a number of major reservoirs have been proposed in the <br />basin having storage capacities totaling nearly 2.3 billion cubic metres, <br />whIch is 21 percent more than the long-term average annual streamflow dis- <br />charge from the basin. Reservoir-storage capacities have been identified <br />to provide sustained flows during thelow,-flow part of the year (commonly <br />July through September) primarily for agricultural and energy-conversion <br />uses. The impacts of specific reservoir projects will-be evaluated as . <br />part of the basin assessment. These studies would include analyses of <br />changing streamflow characteristics and subsequent effects on downstream <br />uses, fish, and stream aquatic I ife; and estimated water losses through <br />reservoir evaporation or recharge to aquifers. . <br /> <br />The geologic formations in the Yampa River basin associated with the <br />major coal seams also serve as major regional' aquifers. Stripping coal at <br />or near the outcrops of these formations tends to disrupt the recharge <br />areas. Conjunctive surface-water and ground-water use would appear to <br />have considerable potential in the basin, particularly as future demands <br />for surface-water suppl ies become more competitive. In a regional analy- <br />sis'of ground-water resources, consideration will be given to legal con~ <br />straints on use, and to quantity and quality implications of coal mining <br />and disposal of solid residuals. <br /> <br />Water-Quality Aspects <br /> <br />A considerable part of the initial basin-assessment analysis has in~ <br />volved evaluating ambient water-quality conditions in the basin. Where <br />possible, available historical data have been used for this effort. Areal <br />data deficiencies have been overcome by conducting a basin-wide stream- <br />flow-qual ity reconnaissance during seasonal low flows in August and Sep- <br />tember 1975 (Steele and others, 1976a). Analysis of the trace-metal and <br />biological data from this survey has revealed certain anomalous conditions, <br />only some of which reflect the impact of man's activities. Both a reglon- <br />al analysis of stream temperatures and a time-trend analysis of long-term <br />chemical-quality records for the two mainstem rivers (fig. 1) recently <br />have been completed. <br /> <br />Estimated sediment discharge from the Yampa River basin is nearly 1 <br />tonne per hectare per year. However, on a subareal basis, the sediment <br />discharge per unit area of the Little Snake River is more than three times <br />that of the Yampa River. This distinction is due primarily to the lower <br />areal precipitation in the Little Snake River subbasin. With the advent <br />of expanded strip mining of coal 'in the basin, site-specific estimates of <br />sediment discharge have been made as part of the Northwest Colorado Coal <br />EIS, using certain assumptions regarding overburden characteristics, slope <br />angles, and methods of sediment retention. These estimates wi 11 ,be evalu~ <br />ated and perhaps further refined using additional data and modeling based <br />on emp i rica I techn i ques . <br /> <br />Because of the projected population growth in the Yampa River basin, <br />due to both coal-resource development and increased recreational use, the <br />waste-assimilative capacity of the mainstem streams is being evaluated <br />with regard to treatment of I iquid residuals from waste-water treatment <br /> <br />12 <br />