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<br />Surface Water Hydrology <br />Grassy Creek passes through the mine area from south to north and is <br />the only drainage in the permit area (see Hydrology Map). Its total drain- <br />age area is approximately 5 square miles (above Routt County Road #27). <br />Downcutting by Grassy Creek has been responsible for breeching the anti- <br />clinal structure present at The Grassy Creek Mine. Grassy Creek is tribu- <br />tart' to the Yampa River and its confluence is at the old townsite of Mt. <br />Harris. In times of abundant rainfall Grassy Creek may flow year-round. <br />Normally it does not flow in the mine area in late summer, fall and winter. <br />There is seepage and minor flow year-round at surface monitor site #1 below <br />the mine area. Beaver ponds and stock ponds in the drainage prolong the <br />summer flow by seepage through and under the dams. Runoff begins in late <br />March-early April and continues until about July 1. A combination "V" notch- <br />Cipolloti weir has been installed near surface water monitor site #4 (SW-4) <br />• as shown on the Hydrology Map. Weir flow since installation date in May is <br />shown in Table 1. <br />The data shows that late April and early Ma~ is the peak flow period <br />(up to 6.52 cfs) and by the first of June the flow is down to less than 1 cfs <br />and by July down to 0.08 cfs. Thereafter Grassy Creek ceases to have measu- <br />rable flow until the following year's runoff period, except for an unusual <br />event. Water stored in the alluvium and in the stock ponds and beaver dams <br />maintains intermittent stream flow by seepage which is monitored at SW-1. <br />There was measureable flow at the weir (SW-4) only for four months in 1980 <br />and for one month in 1981. High flows would be too early for irrigation <br />downstream if there were any lands presently irrigable, which there are not. <br />Grassy Creek is tributary to Yampa River and the only known present use <br />for this water is for watering livestock. Usage of Grassy Creek water is <br />therefore limited to natural and artificial storage of runoff during April <br />and May for livestock use later on in the year. <br />• water quality in Grassy Creek is basically good and compares favorably <br />with neighboring streams, even better than those draining farmed lands. <br />8-81 II.F-2 <br />