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the 1979 to 1986 water years was 19.76 inches, while the average <br />for water years 1987 to 1991 was 16.02 inches. At the Seneca II <br />Mine, average precipitation for the 1983 to 1986 water years was <br />22.66 inches, while the mean annual precipitation for the 1987 to <br />1991 water years was 16.97 inches. <br />It should be noted, however, that the departure-from-normal data <br />for Hayden show that above-average precipitation was received in <br />the area during the 1979 to 1986 water years; the average <br />departure-from-normal value for this period of time is 3.75 <br />inches, while during the 1987 to 1991 water years, the average <br />departure-from-normal value was 0.12 inches. This indicates that <br />the results of previous analyses of surface water flow conditions <br />at the Seneca II Mine (i.e., analyses conducted on data collected <br />during the 1986 water year and earlier) reflect conditions of <br />above-average precipitation. <br />Grassy Creek Drainage. Instantaneous flow measurement data <br />. collected at surface water sites located in the Grassy Creek <br />drainage are presented in Appendix 7-4A; plots of historic flow <br />data can be found in Appendix 7-4B. <br />Discharges in the Grassy Creek basin are seasonally variable, <br />with the higher streamflows resulting from snowmelt runoff which <br />generally occurs as early as February during mild winters, or as <br />la*_e as the tat*_er part of April during years with more severe <br />winters. Table 7-91 contains a summary of mean, minimum, and <br />maximum instantaneous flow values measured at each surface water <br />site at the Seneca II Mine. No-flow conditions have been <br />observed at all of the Grassy Creek surface water sites, <br />indicating the ephemeral nature of the Grassy Creek drainage. As <br />the precipitation discussion above would suggest, the highest <br />flows measured in the Grassy Creek basin occurred prior to the <br />1987 water year during periods of above-average precipitation. <br />Downstream Site SW-52-2 had the highest recorded flow of all of <br />• the Grassy Creek sites, 24.757 cfs (cubic feet per second), <br />recorded during the 1982 water year. Flows at SW-S2-2 (or any <br />7-317c Revised 02/05/97 <br />