"The Piceance basin extends over an area of about 1,600 square miles in northwestern Colorado and includes the drainage basins of Piceance, Yellow, Roan, and Parachute Creeks. Beneath the drainage basins lie the Uinta and Green River Formations of Eocene age and older rocks. The Uinta and Green River Formations consist of marlstone, sandstone, and siltstone and include large reserves of oil shale. Extensive fracturing and leaching of the formations has increased their permeability and resulted in aquifers that lie within, above, and below the oil-shale deposits. The hydrologic system of the basin consists of natural recharge from precipitation, circulation through fractured aquifers and confining beds, and discharge to stream valleys or seepage faces. Previous models were utilized to simulate the flow systems in the northern part of the Piceance basin. A preliminary three-dimensional, five-layer simulation model was prepared for the entire basin using available hydrologic data. The model was used in a steady-state error analysis to assess the degree of error in the hydrologic parameters used in the model. This analysis indicated that simulated hydrologic characteristics are plausible, all layers exhibit impaired vertical hydraulic conductivity, and one layer may exhibit lateral directional transmissivity. However, the model could not be calibrated because of the paucity of data in some regions of the basin."