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Coal Creek - 2 - July 13, 1988 3. The plan states that 6 inches of topsoil will be salvaged from the upland area. All topsoil encountered in the area should be salvaged. As no stockpile locations are noted on Exhibit C, a report should be submitted with the annual report which indicates the stockpile location and measured volume of topsoil salvaged and respread in Section 11. 4. The plan states that topsoil in the lowland area will be stockpiled out of the mining area in the creek bottom. No locations are indicated on Exhibit C. What measures will be taken to protect these stockpiles from potential removal by flood flows? 5. INining is proposed to occur directly within the channel of Coal Creek and in adjacent floodplain areas. The proposed depth of mining is to remove 'the channel alluvial deposit down to bedrock, which varies between 10 and ~5 feet. Fine-grained reject material from processing would be returned to the channel in a layer about 3 feet thick. 'ihe lateral boundaries of mining have been chosen based on restrictions Niue to property boundary conditions and, where they exist, cottonwood stands. The alluvium beneath these tree groups would be left in place as peninsulas connected to adjacent hillslopes. ~4ining of the channel alluvium will affect the streamflow regime, ground water levels and sediment transport. Each of these three components need .:o be considered when evaluating impacts to the hydrologic balance. streamflow volume and rates during flood events will generally not be affected by the mining as proposed. streamflow volume and rates will be significantly reduced during baseflow conditions because of lost storage capacity and gradual release from removing the alluvium mined. No quantitative estimates are provided on the amount of storativity lost, or Flow this change in ground water level translates into stream flow depletions on a monthly or annual basis. Sediment transport will be affected by changes in channel slope, sediment supply from the area mined, location of the channel within the floodplain, a.nd channel control structures. The average longitudinal slope before and after mining will not change. However, local changes rn slope at points along the profile will change significantly. An increased slope will occur at the upstream end of mining. Increased erosion or channel scour will occur here. A 1000 foot segment is proposed at the downstream end of mining as a transition zone between the pit and the natural channel at Quincy Avenue Bridge. This area will have a positive slope to drain but will be at a lower slope than pre~nining baselevel. Aggradation is expected to occur here and immediately upstream.