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Regional Surface Yater guality. The geologic units and soils over which runoff water • flows greatly influences the chemical quality of the surface water. In the Green D(vis(on of the Upper Colorado River Basin, end more proximate, within the Yampa River Basin, this statement has been supported by Iorns et. al. (1965b>. The Yampa River Basin, into which Dry and Sage Creeks flow, originates in the Park Range Mountains which are principally composed of igneous (Tertiary volcenits) end metamorphic (Precambrian) rocks. The tower altitude, interior portions of this basin are chiefly underlain by marine end continental rocks (Yiltiams Fark Formation, Mancos Shale) and predominantly continental rocks (Dakota Sandstone, Morrison Formation). The marine and continental rocks and the predominantly continental rocks have the greatest potential for influencing water chemistry through solution of readily available minerals. According to date presented in Iorns et. al. (1965b), at times of high flow generated largely by snowmelt runoff from the upland areas (underlain by igneous end metamorphic rocks), the water is of [he Calcium bicarbonate type. During low flow, the water features higher percentages of sodium, magnesium, and sulfate. As flow progresses downstream along the Yampa, [he water type generally remains dominated by calcium or sodium, end • bicarbonate. Total dissolved solids (TDS) conc'entretion varies inversely with discharge, end also increases in the downstream direction, Iorns et. al. (1965b) presents data that portrays the relation between water discharge and chemical quality of water for the Yampa River near Maybell, Colorado. For a weighted average discharge of 108 cf s, TDS averages 389 ppm, while for a weighted everege discharge of 9,750 cfs, TDS averages only 48 ppm. The weighted everege TDS concentration et this site is reported at 140 ppm, with en everege TDS discharge of 699 tons per day. Much further upstream et Steamboat Springs, Colorado, the Yampa River is reported to have a weighted average TDS concentration of only 74 ppm, with an everege TDS discharge of only 94 tons per day. Iorns et. el. (1965b) also includes water chemistry date for evaluating the suitability of water for irrigation collected at low, medium, end hi gh flows at three progressively downstream sites along the Yempe River: )) Steamboat Springs; 2) Crei g; and 3) near Maybell. For ell three sites over [he range of flows sampled, the water in the Yampa River is clessifled es either Ct-51 or C2-S1, which categorizes the Yampa River as • exhibiting low to medium salinity hazard, end low sodium (alkali) hazard. This categorize tion makes water in the Yempe River well suited for irrigation. Based on data 86