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West Elk Mrne approximately 300 feet upstream of the confluence with the North Fork where the drainage flattens out. Since MCC began pumping from the NW Panels sealed sump into Lone Pine Gulch, the water quality has met NPDES standards without further treatment. Based on existing stream channel conditions and the projected discharge rates and duration, no significant changes are anticipated to occur to the stream channel. At the outfall N013 location, the watershed size is approximately 945 acres. Using the same hydrologic calculations that were presented in the Apache Rocks Permit Revision, the 2-year return frequency streamflow is 21 cfs, the mean annual runoff is 700 AF and the mean annual sediment discharge is about 70 tons. A maximum discharge of 2,000 gpm (about 4.5 cfs) is far smaller than the 2-year peak discharge (21 cfs). A general principle of stream channel morphology is that the channel slope and character is determined by the dominant discharge, which is typically the bank-foil discharge. The bank full discharge, in turn, occurs in non-urbanized (natural) watersheds about once every 2 years. If the Lone Pine Gulch discharge occurs concurrently with the 2-year flood, the flow would increase from 21 cfs to 25.5 cfs. This increase would not cause incremental channel erosion because the increases in flow velocity, wetted perimeter of the channel, and tractive stress and tractive force would not be significant. Sediment loads would not measurably increase. Table 49 Actual and Projected Water Usage' Year Production (millions of tons per year) Amount of Water Pumped (acre feet per year) Actual Projected Actual Projected 1994 4.3 - 197 - 1995 5.3 -- 224 -- 1996 5.9 - 267 - 1997 6.] - 372 -- 1998 - 6.0 - 400 1999 -- 6.4 -- 410 2000 -- 6.6 -- 420 2001 - 6.8 -- 440 2002 - 8.0 -- Maximum -- 82 -- 500 1 AcWal Usase isbased onawareryear (Octotxrl-September30) A prolonged discharge of 4.5 cfs into Lone Pine Gulch (i.e., for a period of 1 to 2 weeks) under "dry weather" conditions could cause some localized erosion due to saturation of the channel bed and banks. Specifically, a small low flow "pilot channel" might develop over time. However, the relevant reach of Lone Pine Gulch is well vegetated and does not exhibit significant bed or bank erosion. For example, little channel erosion is apparent immediateiy downstream from a 24-inch diameter CMP culvert, where the road up the gulch crosses the channel -yet, this is a location where erosion would normally be apparent in a channel subject 2.05-183 RevimdJrm. /995 PR06; RevisedNov. 1998 rR80,- IN8PR08