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002,61'2. PRESENT AND POTENTIAL SEDIMENT YIELDS IN THE YAMPA RIVER BASIN, COLORADO AND WYOMING By Edmund D. Andrews ABSTRACT Average annua 1 suspended- and tota l-sed i ment loads in streamf I ow we,re determined by the flow-duration sediment-transport-curve method at 18 sites in the Yampa River basin, Colorado and, Wyoming. These computations indicate that about 2.0 mi 11 ion tons (1.8 mi 11 ion metric tons) of sediment are carried by the Yampa River at Deerlodge Park during an average year. Significant areal differences in the sediment yield from various parts of the basin also were determined, The lower Little Snake River subbasin contributes about 60 percent of the total basin sediment yield, although it represents less than 35 percent of the area and supplies less than 3 percent of the streamflow. In contrast, the upland (eastern) one-third of the basin contributes only about 14 percent of the sediment yield but 76 percent of the streamflow. Projected economic development of the basin, especially surface mining of coal, will impact the physical environment. Depending upon the amount ,of coal mined, as well as the extent and location of land disturbed, an estimated 10,000 to 30,000 tons per year (9,000 to 27,000 metric tons per year) of additional sediment wi 11 be contributed to the main-stem' Yampa River. The impact of this additional sediment load wi 11 depend upon where within the basin it enters the stream channel. Although the increased sediment load due to surface mining represents approximately 2 percent of the present total-sediment load, it could increase the sediment load of the upper reaches of the Yampa River by as much as 30 percent. INTRODUCTION Development of coal resources of the Yampa River basin in northwestern Colorado and south-central Wyoming will have significant effects upon the environment and natural resources of the basin. The Yampa River Basin Assessment (Steele and others, 1976a, 1976b) was designed to describe the availability and quality of the basin's water resources and to evaluate the potential environmental and selected socioeconomic impacts of alternative coal-resource development plans, The study of sediment yields is one of several investigations comprising the overall basin assessment.