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-zz- 4) The Coal Basin Mines will not have any surface facilities (disturbances) in the North Fork drainage. All mining operations in the general area are underground operations. The life-of-mine areas of these mines are plotted on Figure 2. In addition, the coal seams to be mined, the area of surface disturbance, the area to be undermined during the first 5-year permit period and the area to be undermined during the life-of-mine are tabulated in Table 1. This assessment is divided into the three major subsections: Description of the Hydrologic Environment, Probable Hydrologic Consequences of the Orchard Valley Mine and Cumulative Hydrologic Impact Assessment. DESCRIPTION OF THE HYDROLOGIC ENVIRONMENT The first part of this assessment describes the hydrology of the drainage basin of the North Fork of the Gunnison River and the mining activity occurring in the basin. This includes 1) a description of the regional geology; 2) a description of the ground water regime; 3) a description of the surface water regime; and 4) a discussion of precipitation and runoff in the basin. Regional Geology The Somerset Goal Field lies on the southeast margin of the Piceance Basin, and just south of Grand Mesa. The sedimentary strata exposed in the Somerset Coal Field dip at 3 to 5 degrees to the north and northeast. Only minor faulting of limited vertical displacement has been observed in the existing Blue Ribbon, Bear and Hawk's Nest Mines. However, in the Orchard Valley Mine, a fault with a displacement of 5U feet was encountered during mining and drill hole data indicates the presence of other faults in the life-of-mine area with similar displacements. One major fault has been encountered in the Somerset Mine. The faults which have been encountered in existing mines tend to be high angle normal faults. The steep slopes of the stream valleys and the instability of the rock strata in the North Fork Drainage Basin has contributed to numerous landslides, mud flows and rock falls. These mass wasting features have been mapped by W.R. Junge of the Colorado Geological Survey and published as an open file report, entitled "Geologic Hazards, North Fork Gunnison River Valley, Delta and Gunnison Counties, Colorado". Geologic units exposed in the North Fork Drainage Basin consist of Late Cretaceous to Early Tertiary Age sedimentary strata, Tertiary Age igneous intrusives, and Quaternary Age alluvial and colluvial deposits. A generalized stratigraphic column of Late Cretaceous units can be found in Figure 3. The units are described below in ascending order. The Mancos Shale is the oldest strata exposed in the region, and is of Late Cretaceous Age. This unit is composed of over 4,000 feet of gray marine shales and minor interbedded buff sandstones. This unit is highly erodible and unstable. Erosion and oversteepening of slopes in this formation produce the numerous rock falls and landslides observed in the lower North Fork Drainage Basin (Junge, 1978).