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i� <br /> i 11 <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 /> 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 River <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 /> d4d 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 /> j 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 May is clearly shown by these <br /> l ; 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 Madrid. <br />