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ori21()27 Sed iment Yields A sediment-yield map of the Yampa River basin (fig. 5) was prepared from the data summarized in table 2. The sediment-yield areas were drawn from the average unit-area sediment yields computed for each gaging station, Local areal irregularities then were smoothed, based on mean~annual precipitation, for reasons which will be discussed in detail later. Two areas of relatively large sediment yield, 300-500 (tons/mi2)/yr or 105-175 (t/km2)/yr, were iden- tified (fig. 5). The largest area lies In the northwestern one-third of the basin and Is drained by the Little Snake River. This area, as previously noted, contributes about 60 percent of the total-sediment load of the Yampa River at Deerlodge Park. A second smaller area lies near the southern boundary of the basin and is drained primarily by Milk Creek and the Williams Fork River (fig. 5), both tributaries to the Yampa River. This area contrib- utes approximately 20 percent of the annual total-sediment load of the Yampa River at Deerlodge Park. About 48 percent of the Yampa River basin has sediment yields from 100 to 300 (tons/mi2)/yr or 35 to 105 (t/km2)/yr (fig. 5). The smallest sediment yields, less than 100 (tons/mi2)/yr or 35 (t/km2)Jyr, occur along the eastern fringe of the drainage basin. .This area is about 1,300 mi2 (3,370 km2) or 17 percent of the basin. Factors Affecting Sediment Yields The quantity of sediment eroded from a watershed is influenced by sever- al factors. Bedrock geology, soil type, vegetation, climate (particularly precipitation and air temperature), topography, and land use are the most Important factors determining sediment yield. Many of these factors are interrelated. Soil type Is primarily a function of bedrock and cl imate; whereas, vegetation is determined largely by soil type and climate. As described above, sediment yields vary considerably throughout the Yampa River basin. A majority of the total-sediment load of the Yampa River at Deerlodge Park is contributed by only 35 percent of the basin. Converse- ly, more than 30 percent of the basin, primarily in the eastern upland areas, contributes less than 14 percent of the total-sediment load. It Is useful to consider which of the above-named factors are prlmari ly responsible. for the basinwide variability in distribution of sediment yields. Geology A generalized bedrock geologic map of the Yampa River basin (fig. 6, adapted from Steele and others, 1978) combines formations of similar lithol- ogy and age so that the units shown indicate their relative erodibility. For the most part, the bedrock of the Yampa River basin Is composed of interbed- ded sandstones, mudstones, and.shales of Tertiary and Cretaceous age. The induration of the fine-grained sediments generally Increases with age, so 16