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Cow Valley is a small upland basin in a semiarid region of eastern Oregon. The withdrawal of ground water for irrigation in the area increased rapidly from 1949, when the first irrigation well was drilled, until 1955, when nearly 7,000 acre-feet of ground water was pumped from 14 wells. The basin is isolated hydraulically from adjacent areas by its relatively high topographic position as well as by hills and buried ridges of impermeable bedrocks of Triassic and Jurassic (?) age, which surround the valley. The principal aquifers are alluvial sand and gravel of Pleistocene and Recent age, and lava flows and associated pyroclastic rocks of Pliocene or Pleistocene age, that overlie the impermeable bedrock.Precipitation within the basin is the only source of water for the area; evapotranspiration and outflow through Cow Creek are the only modes of discharge from the basin. The recharge was less than the total ground-water discharge during 1951-59, even though the potential recharge from precipitation was generally above average during that period. The withdrawal of about 50,000 acrefeet of ground water during 1951-59 caused a decline of the water table, ranging from about 10 to 15 feet, which indicates that an appreciable part of the water pumped wss removed from ground water in storage. The amount of recharge that normally replaces the ground water pumped from storage now is, and probably will continue to be, considerably less than 5,500 acre-feet per year.Chemical analyses indicate that the ground water is of generally good quality, not only for irrigation, but for most other purposes as well. However, the water from some of the wells is harder than is desirable for domestic use. INTR ODUCTION PURPOSE AND SCOPE OF INVESTIGATION120 119" 50 MILES FIGURE 1. Map of Oregon showing area of investigation. ' Stose, George W., 1932, Geologic Map of the United States: U.S. Geol. Survey (4 sheets).