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• Steamboat Springs, Colorado, during the 1954 water year. This hydrograph illustrates that <br />spring runoff is the principal contributor to streamflow (April, May, and June). <br />Runoff generated by infrequent summer thunderstorms has only a minimal influence. The <br />relatively constant basef low depicted in Figure 7-11 (July through March) is maintained by <br />groundwater discharge to the Yampa River. <br />Figure 7-12 depicts a histogram of annual discharges measured for the Yampa River at <br />Steamboat Springs, Colorado from 1914 through 1957. Compared to other gaged streams <br />within the Yampa River Basin that course through more arid watersheds, the annual <br />variability in discharge at this site is less pronounced, largely due to less variation in <br />annual precipitation (dominated by snowfall) in the drainage basin above Steamboat <br />Springs. <br />lorns et. al. (19656) contains flow duration tables including data for the Yampa River at <br />Steamboat Springs, Colorado. The tables indicate that 90 percent of the time, discharge <br />equals or exceeds 73 cfs, and that flows equal to or greater than 140 cfs occur 50 percent <br />of the time. The mean discharge for the Yampa River at this location is 472 cfs. The <br />• flow duration curve constructed from these tables for the Yampa River at Steamboat Springs <br />is typical of snowmel t-type streams. <br />Regional Surface Water Ouali ty. The geologic units and soils over which runoff water <br />flows greatly influences the chemical quality of the surface water. In the Green Division <br />of the Upper Colorado River Basin, and more proximate, within the Yampa River Basin, this <br />statement has been supported by lorns et. al. (19656). <br />The Yampa River Basin, into which Dry and Sage Creeks flow, originates in the Park Range <br />Mountains which are principally composed of igneous (Tertiary volcanics) and metamorphic <br />(Precambrian) rocks. The lower altitude, interior portions of this basin are chiefly <br />underlain by marine and continental rocks (Williams Fork Formation, Mancos Shale) and <br />predominantly continental rocks (Dakota Sandstone, Morrison Formation). The marine and <br />continental rocks and the predominantly continental rocks have the greatest potential for <br />influencing water chemistry through solution of readily available minerals. <br />According to data presented in lorns et. al. (19656), at times of high flow generated <br />. largely by snowmelt runoff from the upland areas (underlain by igneous and metamorphic <br />rocks), the water is of the calcium bicarbonate type. During low flow, the water features <br />87 <br />