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a <br />11 <br />t <br />• study area. The Allen Mine is located on the Middle Fork, about 1.5 miles <br />upstream of the confluence of the North and Middle Forks of the river. The <br />l confluence of the South Fork is at Weston, some two miles downstream of the <br />Maxwell Mine. <br />The United States Geological Survey (USGS) operates surface water <br />gaging stations at Madrid, Colorado on the main stem of the Purgatoire P,iver <br />and at Stonewall, Colorado on the Middle Fork. The Madrid station is well <br />downstream of the study area and gages the runoff from approximately 550 <br />square miles of watershed, while the Stonewall station is approximately <br />four miles upstream of the Allen Mine and gages runoff from a much smaller <br />catchment area. Between these two stations, numerous ephemeral streams join <br />the river in addition to the North and South Forks mentioned above. Records <br />of runoff as measured at Madrid are available from 1972 to the present and <br />at Stonewall from May, 1978. <br />The headwaters of the Purgatoire River system are in excess of 9000 <br />feet elevation and receive substantial snowfall. For example, the mean <br />• annual precipitation at North Lake at the head of the North Fork (elev. <br />8800 ft.) is 20.55 in. of which 7.48 in. is in the form of snow. On the <br />other hand, the mean annual precipitation at Trinidad (elev. 6030 ft.) is <br />13.22 in. and only 2.19 in. of this total is in the form of snow. The <br />study area elevation ranges from 7000 to 7500 feet and is between the Trini- <br />dad and North Lake stations. As shown in some detail subsequently, the <br />mean annual precipitation at the Allen Mine is estimated to be 16.92 in. with <br />snowfall contributing 5.00 in. to this total. <br />The annual runoff is dominated by snowmelt, but peak discharges occur <br />in response to intense rainfall during the summer. Figures 1 and 2 are <br />example hydrographs at Stonewall and Madrid, respectively. (Note the <br />different discharge scales). Weekly mean discharges were used for ease in <br />presentation. The smoothing effect of calculating weekly averages does <br />not mask the August, 1979 flood, however. Also, the seasonal response to <br />snbwmelt beginning in late April and early I•tay is clearly shown by these <br />hydrographs. Of particular interest are the minimum flows that occur in <br />the late fall through the winter months. The data in Figures 1 and 2 indi- <br />• - Cate a minimum flow of 4.0 cfs at Stonewall and about 7 cfs at t•ladrid. <br />