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-12- Item: Snowmelt run-off characteristics should be discussed to the extent possible. Response: Homestake chose the 200-year, 24-hour recurrence interval precipitation event as the basis to calculate the design run-off. In general, peak flows related to snowmelt are less than peak flows for rainfall events of greater than 50 year recurrence interval for the mine catchment area. Examination of the run-off for a subbasin of approximately 250 acres for the mine area indicates a rainfall event peak run-off of 105 cfs or 270 csm (cfs/mi2). Using the method outlined in Colorado Water Conservation Board Technical Manual Ho. I for the estimation of peak discharges due to snowmelt, the following values for various recurrence intervals for a similar area may be calculated. Flowrecurrence Peak flow Peak Unit Flow Interval cfs) cfs/mil = csm Q-10 13.3 26.6 Q-50 20.8 41.6 Q-100 23.9 47.8 Q-500 32.7 65.4 Q-200 (interpolated) 27.5 55.0 These calculations are based on a 0.5 square mil drainage area and an annual average precipitation of 27.5 inches and thus may be expected to be somewhat conservative in comparison to the snowmeit event flows for Pitch. It can be seen that the snowmelt run-off event for the same recurrence interval used for design flows was about one quarter of that calculated for the rainfall event. Item: In order to assure that the channel leading to the South Pit is stable, the designs should address riprap gradation and the need for a filter blanket. Response: The final designs will address the sizing and gradation of riprap and requirements to utilize a filter blanket to segregate soil from the riprap. The energy dissipation requirements will determine the size and distribution of the riprap. As in the previously-described case of the spillway, cascade structure, and drainage designs, Homestake has included adequate surety to cover the site specific design of these channels in Exhibit L under the heading "Engineering Investigations and Design." Item: The amendment should address the problem of channel scouring during unvegetated conditions. Response: In order to reduce flows during the early stages of reclamation when revegetation will be ongoing, Homestake will maintain the Crown Ditch diversion. This, as described earlier and as can be seen by the relative size of the area involved, will reduce flows into these ditches in a significant manner. During the period of revegetation, the newly planted areas will be routinely inspected for signs of excessive erosion. Item: The amendment should address the need to install a trash rack for the v-notch weir spillway of the South Pit Lake.