Causes of fluctuations in the rate of discharge of Clear Lake Springs, Millard County, Utah

The Clear Lake Springs in southeastern Millard County are the source of water for the maintenance of the Clear Lakes Migratory Waterfowl Refuge. Seasonal declines in the rate of discharge were noted during 1959-60.

Fluctuations in the flow of Clear Lake Springs are caused both by natural variations in the quantity of recharge and by variations in the quantity of water pumped from an increasing number of irrigation wells in the southern four districts of adjacent Pavant Valley.

The springs are the principal discharge point for an aquifer in a complex of highly permeable basalt flows. Water enters the basalt aquifer as direct recharge from precipitation, as interformational leakage from a contiguous artesian aquifer in lake and alluvial sediments, and as infiltration of infrequent flood runoff and of unconsumed irrigation water in the lowlands of Pavant Valley.

A hydrograph of the flow of the springs indicates that precipitation on the basalt outcrop recharges the aquifer; this conclusion is strengthened by fluctuations in the chemical quality of the spring water. The effects due to precipitation, however, are partly masked by the larger effects due to the pumping of ground water for irrigation in southern Pavant Valley. Withdrawal of ground water from wells in the southern four districts causes seasonal reductions in the flow of the springs by reducing the hydraulic gradient between the wells and the springs.

Statistical analysis of three parameters - the (1) October-April precipitation, (2) annual pumpage, and (3) annual lowest rate of spring discharge - shows that a departure of 1 inch from the normal October-April precipitation at Fillmore is accompanied by a change of 0.41 cubic feet per second in the low flow of Clear Lake Springs. Similarly, a departure of 1,000 acre-feet from the 1961-64 average annual pumpage causes the low flow of the springs to change by 0.23 cubic feet per second.

The average annual volume of discharge from Clear Lake Springs during 1960-64 was 14,900 acre-feet. The equation derived from the statistical analysis shows that of the average annual discharge, 3,000 acre-feet of water was derived from precipitation on the basalt, 9,000 acre-feet, from underflow from Pavant Valley, and 2,900 acre-feet, from undetermined sources.