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Elk • ~~ja W nr (lu itv Streams Based upon the monitoring program previously described, this section provides an overview of the existing streamflow regime for the watersheds in the vicinity of the West Elk Mine. Included are discussions on average annual flows, low flows, peak flows, and issues relating to channel morphology and sediment yield. Streamflow -Most of the streams in the permit area are ephemeral, flowing only in response to snowmelt or significant rainfall events. Using Sylvester Gulch as an example, "significant" rainfall (based on the conceptual-level application of the U.S. Soil Conservation Service curve number (CN) approach), means that a precipitation amount or snowmelt equivalent of about one inch in 24-hours is required to generate runoff. The distribution of flows for the North Fork and Minnesota Creek is shown on Figure 8 and Figure 9, respectively. These figures indicate the similarities in the monthly distribution of the annual flows for the North Fork and Minnesota Creek. For both watersheds, over 80 percent of the annual yield occurs from April through July which demonstrates the influence of snowmelt on these streams. While snowmelt provides the greatest yields, summer rainfall can cause lazge peak flows. These storms typically occur during late summer months when many of the ephemeral streams have diminished flows. The peak flows resulting from significant storm events aze the primary mechanism for sediment transport and defining the channel geometry. Within the pemut area, the watersheds appeaz to yield smaller amounts of streamflow than do any of the gaged basins that aze tributary to the North Fork upstream from Somerset. To provide a comparison between the gaged streams on the lease area and other neazby gaged watersheds with longer periods of record, the monthly yield of nine watersheds in the North Fork drainage area were plotted. As shown in Exhibit 18A, the drainage azeas of these watersheds ranged from 1.3 to 63 square miles. Only the two perennial streams, South Prong and Horse Creek, can be properly included in such a comparison. Exhibit 18A shows that these two coal lease area watersheds fall well below the driest annual runoff values of the regional watersheds. Indications aze that these watersheds typically have less runoff per square mile than most watersheds in the region. The gage records support the conclusion that the watersheds in the permit area have very little flow and typically yield less per squaze mile than other regional basins. There are a number of factors potentially responsible for the low annual flows occurring in these streams. The combination of low precipitation and heavy vegetation is important. The dense vegetation leads to a high evapotranspiration rate. The permit azea watersheds receive low precipitation compazed to much of the North Fork basin. Within the North Fork basin, annual precipitation varies from up to 50 inches in the high mountains to 20 inches at Somerset. Drainages within the permit area receive about 22 to 30 inches of precipitation per year. The _ combination of these factors results in a diminution of excess precipitation available for runoff. ~~~ 2.04-93 Revised November 1004 PRI G