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Stream Channel Parameters <br />• Exhibit SSA and Changes Due to Mining-Induced Subsidence Page 4 <br />were approximately 200 AF per square mile per year. This value compares favorably with the <br />yield estimate of 160 AF per square mile per yeaz based on the U.S. Geological Survey (USGS) <br />regional regression equations (USGS 1985). <br />The 1936 through 1947 and 1985 through 2002 periods of gaging flow on Minnesota Creek show <br />an average annual yield for 41.3 square miles of 385 AF per square mile <br />For the purpose of annual average sediment yields for the subject basins, WWE and Mr. <br />Pemberton have concluded that an appropriate (i.e., conservative) mean annual runoff for the <br />subject basins of 475 AF per year should be adopted, even though site-specific data for the basins <br />would likely indicate a mean annual runoff of less than 475 AF per year per square mile. The <br />adopted value represents a high mean annual runoff for use in conservative sediment and channel <br />stability studies. For water rights purposes, the average year yield estimate of 200 AF per yeaz is <br />suitable, as approved by the Colorado Water Court in 1986. <br />• <br />Peak Runoff Rates <br />Peak rates of storm runoff from rainfall and snowmelt events were defined for three return <br />frequencies: 2-, 10-, and 100-year events. <br />These dischazges aze based on statistical evaluation of peak daily flow data collected by MCC in <br />the permit azea since 1978, study of the long-term published records for the North Fork at <br />Somerset, and statistical evaluation of published streamflow data for basins in the vicinity of <br />West Elk Mine. Special evaluations were made of the Lick Creek, Sylvester Gulch, and Horse <br />Creek basins using the USGS peak rate of runoff computational procedure applicable to small <br />basins in the Colorado mountainous area (USGS 1985). <br />Figure 2provides asemi-logarithmic plot of the peak discharge rates. The semi-logarithmic <br />plotting technique was adopted to analyze the small tributary basins under consideration. It is <br />consistent with the regional analyses for the North Fork. <br /> <br />DRAFT <br />