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4.5 INTRODUCTION - HYDROLOGY • The description of the hydrological conditions that follows is a result of the accumulation of data and observations since the start o£ mining in 1981. This information is combined with studies conducted by McWhorter (Appendix A) during research of the sub-surface water conditions at the present day Keenesburg Mine. 4.5.1 DESCRIPTION OF HYDROLOGY Physiography Weld County lies in the phyaiographic area known as the Great Plains Region. The general slope of the plain is to the east and southeast. The South Platte River flows through Weld County creating the types of topography that are associated with flood plains, terraces, and uplands. The topography in the north portion of Weld County is gently undulating to rolling. South of the South Platte River volley it is rolling to hummocky. The river floodplein is level to gently undulating. Elevations in the county range from a low of approximately 4,400 feet above sea level at the point of egress of the Pawnee Creek to highs of approximately 6,200 feet above seal level in the northwest portion of the county. • Hydrology In addition to the South Platte, important streams in Weld County include the Cache La Poudre River, Vrain Creek, Crow Creek and Kiowa Creek, all of which flow into the South Platte River. Several water impoundments have been developed in Weld County. The larger reservoirs include the following: Empire, Riverside, Milton, New Windsor, Lower Latham and Black Hollow Reservoirs. The rolling ridges of the mining area are primarily blow sand with sage brush and prairie grass for vegetation. In the vicinity_of the mine, the area slopes in two general drainage patterns. To the west, the area slopes to Box Elder Creek which has an intermittent stream flow to the north and eventually loins with the South Platte several miles to the north. In the immediate vicinity of the mine site, subsurface flow end surface drainage patterns are to the east. Most mobile subsurface water would flow in an easterly direction towards Ennis Draw. Historically this draw has no surface evidence of water flow, as precipitation is absorbed by the porous surface sonde and creates a subsurface flow that supports the salt grass meadows of Ennis Draw. Subsurface flow in Ennis Draw is dischnrged in Box Elder Creek to the north of the proposed area. Refer to Map 4.6.3 (g). • -24-