The Piceance and Yellow Creeks drainage area is about 900 square miles (2,330 square kilometres) and is referred to as the Piceance basin, or simply as the basin. The surface-water and ground-water systems in the Piceance basin are intimately related. The annual volume of runoff from the basin (Piceance and Yellow Creeks) is estimated to be 15,650 acre-feet (19.2 cubic hectometres). About 80 percent of the annual runoff is supplied by ground-water discharge.

Runoff from the basin is affected by irrigation diversions and consumptive use by crops, native vegetation, and evporation. Streamflow depletions resulting from irrigation are estimated to be 4,800 acre-feet (5.9 cubic hectometres) per year. In the absence of irrigation, the mean annual runoff from the basin would be 20,450 acre-feet (25.2 cubic hectometres). The period of lowest flow normally occurs during spring and summer when irrigation diversions are greatest. Peak flows from snowmelt and thunderstorms also occur during this period. A regional analysis, using the index-flood method, was made to estimate flood frequencies in the absence of irrigation diversions for the gaging stations Piceance Creek at White River and Yellow Creek near White River. The estimated mean annual floods are 800 cubic feet per second (22.7 cubic metres per second) for Piceance Creek and 390 cubic feet per second (11.0 cubic metres per second) for Yellow Creek. The peak flow observed during the 5 years of record on Piceance Creek at White River was 407 cubic feet per second (11.5 cubic metres per second) or about one-half the estimated mean annual flood. Yellow Creek is only slightly affected by irrigation diversions and the peak flow for the single year of record was 468 cubic feet per second (13.3 cubic metres per second).

Irrigation return flows and ground-water discharge affect the quality of surface water in the Piceance basin. The concentration of dissolved solids ranges from less than 500 milligrams per litre in the upper reaches to more than 5,000 milligrams per litre in the lower reaches of Piceance Creek and from about 700 to 3,000 milligrams per litre in Yellow Creek. Water quality decreases in the downstream direction due to ground-water discharge from the Green River and Uinta Formations.

The ground-water system in the basin consists of two principal aquifers separated by the Mahogany zone in the Green River Formation. Recharge to the aquifers occurs mainly from snowmelt along the basin margins above 7,000 feet (2,130 metres) altitude. Ground water flows from the basin margins toward the north-central part of the basin where it is discharged in Piceance and Yellow Creek valleys as evapotranspiration and streamflow. Recharge and discharge from the aquifer system are estimated to average 26,100 acre-feet (32.2 cubic hectometres) annually. About 20 percent of the recharge is discharged in Yellow Creek drainage. Estimates of the volume of water in storage in the aquifers range from 2.5 to 25 million acre-feet (3,100 to 31,000 cubic hectometres).

Sodium minerals in the aquifer below the Mahogany zone are actively being dissolved by ground water. The Mahogany zone impedes the flow of water between the aquifers and large chemical differences have developed. Water in the upper aquifer generally has less than 2,000 millgrams per litre dissolved solids while that in the lower aquifer exceeds 30,000 milligrams per litre dissolved solids in the northern part of the basin.

Digital models were used to simulate the hydrologic system. A watershed model was adapted to the drainage above the gage on Piceance Creek below Ryan Gulch to evaluate the possible effects of precipitation changes on the hydrologic system due to the introduction of atmospheric pollutants from oil-shale development or cloud seeding. A 10-percent decrease and 10- and 20-percent increases in the October to May precipitation were examined. It was found that each 10-percent change in precipitation results in a 40-percent change in ground-water recharge. The model study indicates that a 10-percent decrease in October-May precipitation results in a 30-percent decrease in mean annual runoff while 10-and 20-percent increases in precipitation result in 40- and 85 percent increases in mean annual runoff.

A digital model of the ground-water system was used to evaluate the effects of mine dewatering on the hydrologic system. Hypothetical mines in oil-shale lease tracts C-a and C-b were considered. Both mines were assumed to be in the Mahogany zone and to be 4 square miles (5.2 square kilometres) in area. Dewatering of the mines was assumed to occur simultaneously for a period of 30 years. For the hypothetical dewatering scheme simulated, the model study indicates that the mine in tract C-a will not produce enough water to meet the demand for processing and disposal of oil shale while the mine in tract C-b will produce water in excess of the demand. The concentration of dissolved solids of the water discharged from the mines may not exceed 5,000 milligrams per litre for the hypothetical dewatering scheme considered.

Dewatering the hypothetical mines will only slightly affect groundwater discharge in the Yellow Creek drainage. However, after 30 years of dewatering, the model indicates that ground-water discharge will cease in a 10-mile (16-kilometre) reach of Piceance Creek near tract C-b.

The decrease in ground-water discharge in this reach could cause an increase in the concentration of dissolved solids in the downstream reach of Piceance Creek. After 30 years of dewatering, the hydraulic head in the aquifers is decreased in 75 percent of the basin area and about 500,000 acre-feet (620 cubic hectometres) of water are removed from storage in the aquifers.

It is concluded that oil-shale development will have significant effects on the surface- and ground-waters systems in the Piceance basin.