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• <br />centrations are low, ranging from about 1 mg/1 at high flow to 15 mg/1 at low <br />flow. Dissolved solids and hardness concentrations at base flow are about 175 <br />to 275 and 125 to 175 mg/1, respectively. Detailed water quality characteriza- <br />tion is presented in Section 2.04.7. <br />Comparison of samples collected in 1975 and 1976 on the Yampa River below <br />• <br />Trout Creek near Milner and below the diversion near Hayden indicate that the <br />dissolved solids increase about 25 percent in this reach of the river during <br />periods of low flow. This increase is due to inflow from several tributary <br />streams that are influenced by agriculture, mining, and drilling activities. <br />Additionally, the Yampa River flows across the outcrop of the Iles formation, <br />which contains more highly mineralized waters than other formations in the area. <br />This information is illustrated below and in Section 2.04.7. <br />Sun Coal Company <br />Yampa River Water Quality Characteristics <br />Routt County, Colorado <br />Discharge Specific Conductance <br />Cubic Feet Per Second Micro Ohms/Centimeter <br />Date Upstream Downstream pstream ownstream <br />9/3/75 209 208 <br />9/4/75 110* 320 <br />9/23/75 179 260 <br />9/29/75 125* 380 <br />3/2/76 180 300 <br />3/3/76 145* 400 <br />*Downstream discharges are lower than upstream flows due <br />to withdrawals to supply the Hayden Power Plant. <br />The Yampa River alluvium ranges in width from a few hundred yards up to two <br />to three miles above and below Steamboat Springs to one-quarter to one-half mile <br />in the reach between Milner to Hayden. The alluvium ranges in thickness up to <br />• about 100 feet (Covey and Tobin, 1981). Depth of wells in the Hayden to Milner <br />-58- <br />(Rev. 5/86) <br />