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diff=rent par~rae'ars asd, therefore, have different areal sizes. The area of the surface water basin i; larger than that of the ground water basin (Figur? 1). The surfacedater area is defined by topography. The extent of the ground water aquifers is defined by geologic structure and stratigraphy. Anticipated mining is defined by the existing permitted coal mines, those for which completed permit applications have been submitted to the Division, and leased federal coal for which there is mine plan information available. Coal mining operations in the study area (r'igure 2) are summarized in Table 3. This assessment is divided into three major subsections: Description of the Hydrologic Environment, Probable Hydrologic Consequences of iAining, and Cumulative Hydrologic Impacts of Mining. Surface and ground water are discussed separately in each subsection. A. Description of the Existing Environment 1. Regional Geology The sedimentary rock strata drained by Trout Creek and its tributaries have been folded artd faulted. Trout Creek, Oak Creek and their tributaries flow through the folded synclinal Twentymile Park Basin. This folding is related to the uplift of the Park Range to the east and the uplift of the Williams Fork Mountains and White River Plateau ':: ~''^e south. This basin is an eastern extension of the larger Sand Wash Basin. The Twentymile Park Basin is a doubly-plunging synclinal basin. The deep central basin extends to the north and southeast. The southwestern part of the basin is structurally connected to the Sand Wash Basin by the doubly-plunging Hayden Syncline. The Hayden Syncline plunges to the northwest into the Sand Wash Basin. Predominantly north-northwest trending normal faults are present throughout the basin (Figure 3). The geologic units occurring within the Twentymile Park Basin range in age from late Cretaceous to Quaternary (Figure 4). The oldest exposed sedimentary rock unit is the dlancos Shale, deposited in a marine and near-marine environment. Sediments of the Iles and Williams Fork Formations of the Mesaverde Group were deposited in primarily terrestrial environments above the Mancos Shale. The Lewis Shale was deposited in a predominantly marine environment (USBLM, 1976). Younger sedimentary rocks have been removed by erosion. The following description of the stratigraphic units occurring within the Twentymile Park Basin are modified from 'darner and Dale (1982) and Brodgen and Giles (1977). The tdancos Shale, the oldest unit exposed in the area, is an approximately 5,000 foot thick, homogeneous, light-gray to dark-gray, fossiliferous, marine shale with interbedded sandstones and limestones (Figure 4). The sandstones are generally thin-bedded, fine-grained, tan, fossiliferous, and form resistant ledges in the basal and upper parts of the formation. Areas underlain by the Mancos Shale often display rolling or hummocky topography. The Mesaverde Group is approximately 3,000 feet thick and conformably overlies Athe t4ancos Shale (Figure 4) and consists of the Iles and Williams Fork Formations. LJ