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M c- ) ~ N C;, o TIll' ddld obt,linl'd were plotted on ,1 lopo'lt'dphic b,lS(, IIldp pnJduc- i ng genet'd 1 i zed ground-wa ter contours. Ground-wa tpr tl ows t rom th(' Virgin Mountains to the northwest and from the area northwest of the river to the Virgin River. It was estimated that approximately 59,400 acre-feet per year of regional ground water enter the Virgin River in the study reach, based upon streamflow measurements taken in December 1980, and January and February 1981. Analyses of the streamflow data for the river itself, in conjunc- tion with the ground-water data, indicate that surface flow recharges the ground-water system in the river in the vicinity of the river channel and some adjacent phreatophyte flood plain areas during parts of the year. Also, ground-water underflow parallel to flood plain channel can exist, but does not appear to be quantitatively important. Irrigation water also locally recharges the shallow ground-water system underlying the irrigated areas as shown by water table fluctu- ations at irrigated areas. A portion of ground-water flow discharges from the basin to Lake Mead as underflow under the river channel and associated flood plain. There is no evidence of ground-water flow passing from one side of the valley to the other. Thus, it is assumed that the river acts as the regional drain. The quality of the ground water in agricultural areas is a calcium sulfate type with TDS values of 3,300 to 8,700 mg/L from just below the water table to about 10 feet below the water table. The depth to water table varies from about 2 feet to more than 20 feet, depending on the height of the land above the river level. The ground wa ter was found to have TDS values of 3,200 to 7,000 mg/L in the slightly deeper sand points. The water beneath the agricultural area appears to indicate the presence of calcium sulfate surface water recharged by irrigation. 22