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-zz- <br />Excess water .is produced from October 1 - June 30. From July 1 - September 30, <br />evapotranspiration greatly exceeds precipitation. During this time, soil water <br />storage is greatly depleted. Minor amounts of streamflovr are generated during <br />the summer months from short-duration thunderstorm activity. Table 5 is an <br />estimated subsurface and surface runoff summary for Curtis Creek in the average <br />year. Of the 1.9-inch water excess, perhaps 0.4 inch results as near-surface <br />runoff during April and Ftay. The remaining 1.5 inches is generated as ground- <br />water and deep seepage flow. <br />E. REFUSE DISPOSAL <br />The quantities of spoils from the proposed mining operation are expected <br />to be negligible. However, an alternative that would cause the least environ- <br />mental damage from spoils, should disposal be required, would be to construct <br />cross-valley embankments. The essential components of a typical embankment <br />cross-section are shown in figure I1. These standards are in conformance vrith <br />recommendations set forth in MESA (1976), and general design criteria can be <br />found in this handbook. <br />F. IMPACTS ON WATER QUALITY <br />1. Sediment Yield <br />Source-area sediment yields from the Rienau Mine area are low. Channel <br />erosion rates also appear to be low. Under natural conditions, much of this <br />type of activity results from short-duration thunderstorms. Extensive dis- <br />turbarce of the vegetation and land surface vrill increase the runoff potential <br />from snov.~elt. It is estimated that with disturbance, the near-surface flow <br />component, which is of most significance to on-site and channel processes, would <br />account for perhaps 13 percent of the annual yield. In the undisturbed state, <br />perhaps no near-surface flow component exists in 8 of 10 years on the very <br />small watersheds that are tributary to Curtis Creek. <br />