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.. "">"/ . 't o U &1 t.....:.- "r~<~ DESCRIPTION OF STUDY AREA AND DRAINAGE BASINS USED IN STREAMFLOW SIMULATION f~l : :":;;.~ .. '....;' . '::;", f$i;t: "t&f ~~~ji G1~ ~~r i ",,:";": ~:::::.:: I ," ")/ ;:i~ ~~o); i :'~~ ;;}....- t~:1 ~;>:t :~~, 'i:~(~ :~,(~ ~.;.~ ~~ l~ .~'W. rir;.~~, ~"* flt{f.: i:~~~i '~1;t r.t:;?~ ..... ~{.:~~.~ ~~:~>.:;~ t~;t~ :.-:"-::;,,: The study area is in Moffatt, Routt, and Rio Blanco Counties and includes the Yampa River vaHey south of the Yampa River main stem (fig. l). The area is bounded on ~he east by the Oak Creek drainage and its confluence with the Yampa River. Upstream from this confluence, the drainage basin primarily is bounded by high mountains composed of igneous and metamorphic rock; therefore, minimal coal mining occurs in this upland area. The study area is bounded on the southwest by the Danforth Hills, a topographic high on the southwestern side of the Axial basin (fig. 1). Coal has been mined in the Danforth Hills for many years, and speculation of additional coal mining occurring in the area persist.s. Between these boundaries, the southern valley of the Yampa River has quite diverse hydrologic environments. Unfortunately, the number of streamflow-gaging stations that could .be installed was limited by economic considerations. Therefore, the study area was qualitatively divided into three hydrologic regions (fig. 1). In each region, streamflow-gaging stations were established to monitor streamflow in three small drainage basins. The three hydrologic regions were differentiated primarily by annual precipitation totals. However, such factors as vegetative type and density indicate the change in precipitation and provide a visual index of the change in the quantity of moisture. A general decrease in precipitation occurs from east to west in the study area. Thus, the three hydrologic regions generally are oriented east to west (fig. 1). The dominant form of precipitation is snow, and the quantity of annual precipitation is affected by elevation. Few long-term precipitation stations exist in the southern Yampa River basins; available stations generally are located in the valley at lower elevations. The general east-west trend of precipitation change is indicated by long-term precipitation data collected at Steamboat Springs and at Hayden (National Climatic Data Center, 1983, p. Sand 7). The long-term average precipitation between 1951 and 1980 was 23.30 in. at Steamboat Springs (elevation 6,770 ft) and l5. 90 in. at Hayden (elevation 6,375 ft), about 20 mi west of Steamboat Springs. Average precipitation for the same 30-year period was l7.59 in. at Hamilton (elevation 6,230 ft), 15 mi south of Craig. Other precipitation stations in the study area did not have sufficient records to calculate a long-term average; however, a map of normal annual precipitation for Colorado has been developed using all data from 1951 through 1980 (U.S. Weather Bureau, 1985). This map indicates about 12 in. of annual precipi- tation at Maybell, at the western edge of the study area. From these data, an approximate II-in. change in precipitation has been calculated along the Yampa River main stem, from the eastern to the western edge of the study area. Much of the area of the southern valley of the Yampa River basin is not classified in a hydrologic region (fig. 1); this area includes the Williams Fork drainage basin and the upper reaches of Milk Creek. Data collection from small drainage basins in these areas was not part of the scope of this-study. ".'." -..,.,.;;: 3 t.t::) ~;;:~~ V>:','i ~: ;-::. .X\~ ~{.~\:: ....:....: . ~;g~ V~"~5 "