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_27_ Middle Fork, about 1.5 miles upstream of the confluence of the North and Middle forks of the river. The Maxwell Mine is located downstream of the confluence of the North and Middle forks of the Purgatoire River, and approximately two miles upstream of the confluence of the South Fork and the main stem of the Purgatoire River at Weston. The United States Geological Survey (USES) operates surf ace water gaging stations at Madrid, Colorado on the main stem of the Purgatoire River and at Stonewall, Colorado on the Middle Fork. The Madrid station is Located at the downstream limit of the study area and gages the runoff from approximately 550 square miles of watershed, while the Stonewall station is approximately four miles upstream of the Allen Mine and gages runoff from a 52.1 square mile drainage area. Between these two stations, numerous ephemeral and intermittent streams, in addition to the North and South forks mentioned above, join the river. Records of runoff as measured at Madrid are available from 1972 to the present and at Stonewall from May, 1978 to the present. The headwaters of the Purgatoire River system are in excess of 9,000 ft. elevation and receive substantial snowfall. For example, the mean annual precipitation at North Lake at the head of the North Fork (elevation 8,800 ft.) is 20.55 in. of which 7.48 in. is in the form of snow. On the other hand, the mean annual precipitation at Trinidad (elevation 6,030 ft.) is 13.22 in. and only 2.i9 in. of this total is in the form of snow. The disturbed areas of the mine sites are between y,000 and 7,500 ft. in elevation and are located between the Trinidad and North Lake stations. The mean annual precipitation at the Allen Mine is calculated to be 16.9 in. with snowfall contributing 5.00 in. to this total. Although runoff is dominated by snowmelt, the peak discharges occur in response to intense rainfall during the summer. At the Stonewall gaging station during water year 1979, the average discharge was 18.1 cfs with flow ranging from 2.8 cfs to 172 cfs. The minimum average weekly flow at Stonewall was approximately 4 cfs. During water year 1979, flows ranged from 7.6 cfs to 6,620 cfs at the Madrid station. The minimum weekly average flow during the water year reported in the permit application was about 7 cfs at the Madrid station. At the Madrid gaging station, the average flow for the 9 year period of record is 45.7 cfs, and the average annual yield is 33,100 acre-feet. Over the period of record, flow ranged from a low flow of 3.0 cfs to a high flow of 14,300 cfs. Water quality data on the Middle Fork of the Purgatoire at Stonewall and the Purgatoire at Madrid are available from the USGS. Data above and below both the Allen and the Maxwell mines have been included in the permit application. The water in the Purgatoire and the Middle Fork of the Purgatoire is a calcium-bicarbonate type water. Total dissolved solids concentrations in the mine area normally run below 300 mg/1 throughout the year.