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The Columbia Plateau is underlain by massive basalt flows of the Columbia River Basalt Group, with a composite thickness of about 10,000 feet. The oldest and most extensive basalt unit, the Grande Ronde Basalt, is estimated to be about 8,000 feet thick beneath the Columbia River. Overlying the Grande Ronde is the Wanapum Basalt, which in the study area reaches a thickness of about 1,000 feet near the Columbia River. The Saddle Mountains Basalt, overlying the Wanapum Basalt, is about 800 feet thick. Sedimentary deposits overlie the basalt, principally near the Columbia River, and reach a thickness of about 150 feet. Individual basalt flows within the massive outpourings range from a few feet to as much as 300 feet thick. Characteristically, the basalt flows have a dense center with vertical jointings and a scoriaceous or brecciated zone at the top and bottom. Porous zones between basalt flows are called interflow zones; they consist of weathered basalt flowtop breccia and scoria and may also contain sedimentary interbeds. Structural features in the study area include a series of anticline-syncline pairs. An arcuate topographic feature that extends east-west through the area is called the Willow Creek monocline. A head gradient as steep as 400 feet per mile across the feature indicates that it may be a hydrologic barrier. The main avenues of ground-water movement in the Columbia River Basalt Group are the interflow zones between basalt layers. These complex, poorly-interconnected zones may be rather extensive in the lateral direction but largely isolated from overlaying and underlying interflows by poorly permeable basalt flow centers. Wells in the Columbia River Basalt Group usually intercept more than one interflow zone. Another control on ground-water movement is the "barrier" effect of stratigraphic pinch-outs and offsets. Although these are not entirely impermeable to ground water, they do retard ground-water movement and may effectively isolate parts of the units. Geohydrologic units (aquifers) in the study area were delineated by vertical grouping of wells with similar head, separated by zones of low permeability. Four such units were defined for this report. The uppermost unit (layer 1) consists of unconsolidated deposits of gravel, sand, silt, and clay that overlie the basalt; layer 2 consists of the Saddle Mountains Basalt, layer 3 the Wanapum Basalt, and layer 4 consists of the upper thousand feet of Grande Ronde Basalt. A three-dimensional finite-difference model to simulate groundwater flow was calibrated for steady-state and transient-flow conditions. The steady-state model was used to simulate ground-water conditions prior to major ground-water development, which began about 1950. The transient model was used to simulate ground-water conditions from 1950 to 1982. Additionally, the transient model was used to predict possible future ground-water development alternatives. On the basis of model analysis, the major components of the water budget for inflow before ground-water development were recharge from precipitation about 146 ft3 /s (cubic feet per second) or 106,000 acreft/yr (acre-feet per year), ground-water leakage from streams--about 31 ft3 /s (22,400 acre-ft/yr), and subsurface flow entering the boundaries of the study area about 15 ft 3/s (10,900 acre-ft/yr). The major outflow component of the budget was leakage to streams--about 185 ft 3 /s (134,000 acre-ft/yr). During the period 1950-82, maximum water-level declines of about 300 feet occurred in parts of the Grande Ronde Basalt unit (layer 4). During the same period, ground-water withdrawal increased to about 182 ft3 /s (132,000 acre-ft/yr). On the basis of model analysis, return flow from surface irrigation increased 36 ft3 /s (26,100 acre-ft/yr), groundwater storage decreased by about 87 ft 3 /s (63,000 acre-ft/yr), groundwater leakage to streams decreased by about 42 ft3 /s (30,400 acreft/yr), and leakage from streams increased by about 17 ft3 /s (12,300 acre-ft/yr). INTRODUCTION Early water use in the Umatilla Basin consisted primarily of (1) municipal use, in towns such as Pendleton, Heppner, Hermiston, and Arlington; and (2) rural domestic and stock-watering use by dryland wheat farmers. Surface-water diversions, shallow wells, and sumps supplied water for flood and wheel-line irrigation systems in the valley bottoms. Irrigated crops commonly included pasture and alfalfa. In about 1964 higher prices for wheat encouraged additional irrigation and caused changes in cropping patterns. An irrigation "boom" in the 1960's and 1970's centered around Hermiston, Oregon. Owners of large corporate farms close to the Columbia River installed pumping stations to lift Columbia River water to their arrays of center pivots. Away from the rivers, development spread to the uplands as irrigators turned for large supplies of ground water to the basalt bedrock aquifers underlying Umatilla, Morrow, and Gilliam Counties. Dryland wheat farming gave way to irrigated wheat and row crops. Multiple center-pivot systems and greater depth to water in the uplands contributed to the drilling of deeper wells and installation of pumps of higher capacity than those used in the valley bottoms. Since 1960, more than 300 irrigation wells have been drilled into the basalt in the three-county area (Smith, Collins, and Olson, 1983). The development of this water source slowed in the late 1970's, due to increasing recognition of problems associated with large amounts of pumpage. These problems have included widespread water-level declines and well interference, increasingly expensive operation, and deteriorated performance of wells. Previous studies by U.S. Geological Survey and Oregon Water Resources Department (OWRD) indicated the need for OWRD to take regulatory action to control ground-water withdrawals in the area. However, little was known about the magnitude of recharge, discharge, vertical movement of water, and hydraulic characteristics of the groundwater reservoir. In 1980 the Geological Survey entered into a cooperative agreement with OWRD to describe and quantify the groundwater resources of the Umatilla Basin. Purpose and Scope The major purposes of this report are to describe the aquifer system (its boundaries, hydrologic properties and flow system) and to quantify recharge and pumpage. In order to enhance understanding of the ground-water system, a digital-flow model was developed and calibrated. The model was used to simulate ground-water levels for the period 19501982 and to predict ground-water levels to the year 2000. This report covers the'period from 1950 to 1982 and is based partly on data collected in the field and partly on data available in the files of OWRD, U.S. Geological Survey, and various other state and federal agencies. Field data collection by OWRD and the Geological Survey included locating wells, making water-level measurements in wells, obtaining power-consumption and flow-meter readings from wells, and mapping to supplement existing geologic maps. Available data included drillers' reports, streamflow measurements, and precipitation, temperature, land-use, and soils information. Previous Work and Acknowledgments Previous work in the area includes a study of the stratigraphy of the Columbia River Basalt Group (Swanson and others, 1979); reconnaissance geologic maps of the Columbia River Basalt Group (Swanson and others, 1981); a map study of geology and structure of The Dalles 1:250,000 quadrangle by Bela (1982); geologic studies by Shannon and Wilson, Inc. of Portland for Portland General Electric (1973 and 1974); and Newcomb's ^1967) discussion of The Dalles-Umatilla syncline. Wagner's (1949) hydrologic study of the Umatilla River basin includes a well and spring inventory with drillers' logs. Hogenson's (1964) discussion of the geology and ground-water resources in the Umatilla Basin includes a later inventory of wells. Robison's (1971) atlas of the hydrology of the Hermiston-Ordnance area contains information on geology, hydraulic heads, water chemistry, and carbon-14 dates. Water quality in the basalt is further described and summarized in a report by Newcomb (1972). Gonthier and Harris (1977) made an analysis of the water resources of the Umatilla Indian Reservation on the eastern edge of the study area. Ground-water studies by McCall (1975) and Bartholomew (1975) of Oregon Water Resources Department were compiled at the time the State held hearings on the proposed Ordnance and Butter Creek critical ground-water areas; these studies include tabulations of water rights. The most recent published well inventory is in the report by Smith, Collins, and Olson (1983). Land-use mapping has been done by several agencies, including the U.S. Army Corps of Engineers (Johnson and others, 1981), Battelle Pacific Northwest Laboratories for Rockwell Hanford Operations (Stephan and others, 1971), and Environmental Remote Sensing Applications Laboratory (ERSAL) in cooperation with the Soil Conservation Service and the U.S. Forest Service (Murray, 1981). Numerous studies describe the hydrology of other parts of the Columbia Plateau; the more extensive of these, such as that by Gephart and others (1979), include discussions applicable to northeastern Oregon. Special thanks are due to the many well owners in the study area who allowed access to their wells and power records; their cooperation has been indispensable. P. L. Oberlander, D. W. Miller, J. R. Bull, and R. Almy of OWRD shared the data they collected in the field and compiled from OWRD files. Mr. Oberlander (now at Battelle Pacific Northwest Laboratories) in particular shared his scientific observations and the results of his study of the area. Steve Applegate (then State Watermaster in Pendleton) and his assistant Tony Holcomb have been sources of practical, first-hand information on the wells in the area. Location of the Study Area The study area includes 5,800 square miles in northeastern

Geohydrology and digital simulation of the ground-water flow system in the Umatilla Plateau and Horse Heaven Hills area, Oregon and Washington