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-25- <br />The stormflow that produces erosion is spotty and normally does not <br />cover large areas. Accordingly, sediment yields from the small watersheds that <br />are tributary to Curtis Creek vary greatly in time and space. In the vicinity <br />of the Rienau Mine, several years might pass without significant erosion activity. <br />However, one large storm can account for virtually a]1 of the long-term sediment <br />production. Fortunately, such storms have a relatively long recurrence interval. <br />The lack of long-term data on the effects of mining on erosion and <br />sediment yield in the Meeker area prevents an accurate assessment of impacts. <br />However, Frickel et al. (1975) estimated that source-area sediment yields <br />range from 0.3 to 0.7 acre-foot per square mile per year for watersheds having <br />characteristics somewhat similar to Curtis Creek. They further estimated that <br />reclaimed spoils would produce sediment at rates of 0.5 to 1 a-f/mil/yr initially, <br />and 0.5 a-f/mil/yr after vegetation is established. <br />The added sediment loads produced by disturbance from mining activities <br />justifies consideration of mitigating measures in addition to engineered <br />controls, revegetation, etc. in these steep watersheds. Such measures will <br />consist of settling ponds immediately downstream from disturbed areas designed <br />to control both sediment and natural runoff, and perhaps mine drainage. Design <br />criteria will be based on recent EPA regulations that require such ponds <br />(U. S. EPA Federal Register, 1977). These effluent limitation provisions require <br />that retention structures for sediment control be constructed downstream from <br />all disturbed areas. They must be designed with a capacity at least sufficient <br />to contain a 10-year, 24-hour storm event in addition to dead storage capacity <br />for sediment accumulation. A 10-year, 24-hour event will produce direct runoff <br />of approximately 0.04 acre-foot/acre from disturbed areas and 0.01 acre-foot/acre <br />from undisturbed areas. These values are based on antecedent conditions that <br />might reasonably be expected during the summer when a 10-year event is most <br />likely to occur. <br />