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2.04.5 GENERAL DESCRIPTION OF HYDROLOGY AND GEOLOGY <br />1. Physiography <br />Weld county lies in the physiographic area known as the Great Plains Region. <br />the general slope of the plain is to the east and southeast. the South Platte <br />River flows through Weld County creating the types of topography that are <br />associated with flood plains, terraces, and uplands. <br />The topography in the north portion of Weld County is gently undulating to <br />rolling. South ofthe South Platte River valley it is rolling to hummocky. The <br />river floodplain is level to gently undulating. Elevations in the county range <br />from a low of approximately 4,400 feet above sea level at the point of egress of <br />the Pawnee Creek to highs of approximately 6,200 feet above sea level in the <br />northwest portion of the county. <br />2. Hydrology <br />In addition to the South Platte, important streams in Weld County include the <br />Cache La Poudre River, Vrain Creek, Crow Creek and Kiowa Creek, all of <br />which flow into the South Platte River. Several water impoundments have <br />been developed in Weld County. The larger reservoirs include the following: <br />• Empire, Riverside, Milton, New Windsor, Lower Latham and Black Hollow <br />Reservoirs. <br />The rolling ridges of the mining area are primarily blow sand with sage brush <br />and prairie grass for vegetation. In the vicinity of the mine, the area slopes in <br />two general drainage patterns. To the west, the area slopes to Box Elder <br />Creek which has an intermittent stream flow to the north and eventually joins <br />with the South Platte several miles to the north. In the immediate vicinity of <br />the mine site, subsurface flow and surface drainage patterns are to the east. <br />Most mobile subsurface water would flow in an easterly direction towards <br />Ennis Draw. Historically this draw has no surface evidence of water flow, as <br />precipitation is absorbed by the porous surface sands and creates a subsurface <br />flow that supports the salt grass meadows of Ennis Draw. Subsurface flow in <br />Ennis Draw is discharged in Box Elder Creek to the north of the proposed <br />area. Refer to Appendices D-1., F-4., and Q-1. <br />There is no evidence of surface runoff in the vicinity of the site. Essentially all <br />precipitation apparently infiltrates the highly permeable mantle of sand. No <br />water -formed erosional features are evident, not even small rills or gullies. <br />Ennis Draw is a broad, flat-bottomed depression with no observable channel or <br />gully and shows no evidence of the existence of surface flows. <br />39 <br />