2012-01-06_PERMIT FILE - C1982057A (4)

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because 1) the background water quality makes it marginal for stock and Srrlgati on use; 2) there are no flood irrigation practices downstream of the 009 tributary; and 3) at a • distance of approximately 2.5 miles downstream from the 009 tributary, dryland farming is practiced and saline seeps will degrade the water chemistry to such an extent that the spoil discharge TDS increases are unmeasurable. Surface Water Effects of Mining on the Local Geomorphology. Mining impacts to the local geomorphology in the vicinity of [he Seneca II-W Mine can be grouped under two categories: 1) impacts to drainage basins; and 2) impacts to stream channels. Otainage basin impacts may include changes in such basin parameters as watershed area, average watershed slope, and drainage density. Impacts to stream channels include the disruption of natural stream courses followed by the reestablishment of drainages that feature different lengths and gradients. Drainage Basins. Mining activities within the Seneca II-W permit boundary will result in disturbance to approximately 19 percent of the 005 Gulch basin, and 50 percent of the 006 Gulch basin. Table 17-Z1 presents select geomorphic parameters measured in both basins under both premining and postmininq conditions, respectively. The evaluation of changes • in these parameters that will result from surface mining activities allows for inferences about the significance of impacts from mining on basin geomorphology. Within each basin, minor changes in smaller subwatersheds will occur after the spoiled overburden has been regraded, and the proposed postmininq topography has been reconstructed. Tables 17-23 and 17-29 present areas for subwatersheds created in the upper portions of both the 005 and 006 basins used to model pre- and postmininq runoff and sediment loads. The tables show that increases and decreases in these subwatershed areas will result from the establishment of the postmininq landscape. However, Table 17-21 shows that the overall basin azeas and shapes (circularity ra tio~ of both the 005 and 006 watersheds will not change as a result of mining. Changes in mean basin elevations (six- foot decrease, 005 Gulch; six-foot increase, 006 Gulch) aftermining will be minor. Within the upper portions of each basin, the reestablished postmininq topography will result in changes in average watershed slopes measured in subwatersheds created for the purpose of modeling pre- and postmininq runoff and sediment loads using SEDIMOT II (see Tables 17-26 and 17-27). Some of these changes in subwatershed slopes ma y be as much as • 59