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O~ ~ /, 8 r', U 11 . 17. ., Occurrence of Ground Water in the Ogallala and several consolidated formations in Colorado A report to the Ground-water Codification and Research Studies Committee by Edward A. Moulder . The aquifers herein discussed differ from the alluvial aqui- fers found along the principal streams. Withdrawals from these aquifers have very little affect on the surface-water resources of Colorado compared to those from the alluvial aquifers of the South Platte, Arkansas, and San Luis Valleys. The Ogallala formation re- ceives very little recharge from surface streams and contributes very little water to the streams in Colorado. The perennial stream s are in contact with the Ogallala and the principal consolidated water-bearing formations in Colorado only where they cross the out- crops of the formations; the contact area is very small compared to the areal extent of the aquifers. Therefore, in most areas, it may be possible, from a practical standpoint, to satisfactorily develop and manage the ground-water resources from these aquifers inde- pendent of the surface water. High Plains aquifer. --Of the four major ground-water provinces in Colorado (South Platte Valley, Arkansas Valley, San Luis Valley, and High Plains), the High Plains province is the largest. It covers about 12, 000 square miles including all or parts of 15 counties in eastern Colorado. The province has been separated into three major parts by Big Sandy Creek and the Arkansas River. The largest part is northeast of Big Sandy Creek and centers around Yuma County. The part southwest of Big Sandy Creek is irregularly shaped, being eroded, in part, by tributaries to the Big Sandy and the Arkansas. The third part lies south of the Arkansas in the southeast corner of the State. .' Deposits of the Ogallala formation make up the surface of the High Plains and form the aquifer. However, a part of the Ogallala formation is overlain by younger windblown deposits generally less than 50 feet thick. The Ogallala consists chiefly of sand and gravel, although it contains numerous beds of finer-grained material consisting of clay, silt, cemented sand, and limestone. In a few places the formation is more than 400 feet thick, but more generally it ranges in thickness from 200 to 300 feet. From one-third to one- half of the material is saturated.