Increasing demands for water supplies, plans for irrigating new lands, proposals to divert water from the Yakima River by users downstream from the reservation, and ground-water problems have made an accounting of the overall availability of water very important to water management on the reservation. This report, which broadly outlines the water resources of the Toppenish Creek basin, has been developed as part of the cooperative study by the Tribal Council of the Yakima Indian Nation and the U.S. Geological Survey.
Precipitation averages about 20 inches a year over the basin but only about 2 inches fall during the April-September growing season. However, diversions from the Yakima River make the Toppenish Creek valley one of the best agricultural areas in the State.
Ground water in the basin occurs in three distinct units: the basalt, old valley fill deposits, and young valley fill deposits. Each unit is capable of yielding more than 1,000 gallons per minute to properly constructed wells. Water levels in wells tapping the young valley fill are higher under present (1974) irrigation conditions than they were in the past, but some water-level declines have occurred in the old valley fill in response to pumping. Water levels in the basalt wells in some areas have been lowered as much as 80 feet by pumping during the past 15 years. Some additional pumping in these areas can be allowed without lowering the water level more than a few feet if the amount of pumpage and spacing of wells are carefully controlled.
The young valley fill in the eastern part of the lower valley is the best source of ground water in the basin. The zones of coarse sand and gravel to depths of about 150 feet in this area can produce large quantities of water. Irrigation water, from the Yakima River which seeps into the ground, yearly builds up this ground-water supply. About 120,000 acre-feet of water goes into temporary storage each irrigation season in the young valley fill.
