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-24- <br />Grassy Creek is a smaller tributary of the Yampa downstream of Trout Creek and <br />upstream of Hayden, Colorado. The Grassy Creek drainage has a total area of <br />approximately 36 square miles. The headwaters of Grassy Creek originate at an <br />elevation of 8,300 feet. The confluence of Grassy Creek with the Yampa River <br />is at approximately 6,560 feet in elevation. <br />No discharge or chemistry data is available for the mouth of Grassy Creek but <br />data is provided by the applicant in the permit application for a station <br />located at the bridge on County Road No. 27 and other upstream sites. The <br />chemistry data is given in Table 1 of the section of this document entitled <br />"Probable Hydrologic Consequences of the Grassy Gap Mine". <br />There is a Cipolletti weir located in Grassy Creek in the permit area. For <br />the period of record from 5/80 to 11/83 there were 45 sampling events, 24 of <br />which had no measurable flow over the weir. No flow occurred during summer <br />months of dry years and during winter months when the stream is frozen. For <br />21 sampling occasions with measurable flow the mean was 1.05 cfs with a <br />maximum flow of 6.52 cfs. <br />An element of baseflow is added to the headwaters of Grassy Creek by seeps <br />originating from landslide debris which store spring runoff and snowmelt <br />moisture. Numerous beaver dams and stockponds in the headwaters extend the <br />duration of flow during low runoff seasons by seepage through the embankment. <br />Probable Hydrologic Consequences of the Grassy Gap Mine <br />A, Mine Description <br />The Grassy Gap Mine is a small surface operation utilizing scrapers and front <br />end loaders. There are 6 pits located at the mine site, 4 of which have been <br />backfilled and regraded. Pit 6 is in the process of being backfilled. Pit <br />No. 4 is the only active pit. All of the regraded pits have been topsoiled <br />and seeded. <br />The total projected disturbed acreage in these pits is 242 acres with Pit No. <br />4 projected to have a total disturbance of 137 acres. The haul roads total <br />another 95 acres of disturbance. Runoff from the haul roads is not routed to <br />sediment ponds. Sediment production from road runoff is minimized by the use <br />of straw bale checks. <br />B. Ground Water <br />The Grassy Gap Mine is located on the Hayden syncline. This syncline is tied <br />both structurally and by ground water transmission into the larger Sand Wash <br />Basin lying to the northwest. <br />