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The Molalla-Salem Slope area of the Northern Willamette Valley includes the lower foothills of the Cascade Range and a narrow strip of valley plai^ of the Willamette Valley. Annual precipitation on the area ranges from about 41 inches at the westernmost side of the valley plain to about 80 inches in the foothills at the eastern side of the area, providing ample recharge to the aquifers that underlie the area. Volcanic and sedimentary rock units exposed in the foothills range in age from Oligocene to Holocene. Most of these units have been penetrated by wells in the valley-plain area, where the older units are overlain by the Willamette Silt and alluvial deposits. The older units of a given rock type generally are less permeable than the younger. In general, water levels in wells in the area fluctuate seasonally in response to recharge to, and discharge from, the ground-water reservoirs, and F"us are highest following periods of greatest precipitation and lowest following periods of least precipitation. Moderate year-to-year declines of water levels in a few wells that tap aquifers in the basalt of the Columbia River Group indicate that water is being removed from those aquifers at a faster rate than it is being replenished by natural recharge. The principal water-yielding units in the foothills area, in descending order of average specific capacity of existing wells, are the Columbia River Group, Little Butte Volcanic Series, marine rocks, Troutdale Formation, and Sardine Formation. Ground water occurs under perched, confined, and unconfinei conditions, and at most places supplies of water adequate for domestic and stock use can be obtained from wells less than 400 feet deep. The principal water-yielding units in the five siubareas of the transition zone are the Troutdale Formation, alluvium, Boring Lava, Columbia River Group, Little Butte Volcanic Series, and marine rocks. In the northern part of tve transition zone, many wells draw water from perched aquifers capable of yielding small to moderate quantities of water. In the central part of the transition zone, most wells draw water from water-table and confined aquifers capable of yielding small to large quantities of water. In the southern part, alluvial aquifers yield large quantities of water to wells, whereas bedrock aquifers yield small quantities of water from perched-water zones. Subareas of the transition zcne that are underlain by sedimentary and alluvial materials having large storage capacities can safely accommodate considerable additional withdrawal® of water. 2 GEOLOGY, GROUND WATER, MOLALLA-SALEM SLOPE AREA, OREG. The principal water-yielding unit beneath the valley plain is tl*3 Troutdale Formation; the Columbia River Group is of secondary importance. In the valley-plain area, the average depth of wells that tap the Troutdale is about 150 feet, and the average yield of irrigation and public-supply wells is about 300 gallons per minute. The average depth of wells drilled into the Colombia River Group is about 330 feet, and the average yield is about 200 gallons per minute. Annual pumpage from the Troutdale aquifers could be increased several fold without causing overdraft in the area. Chemical quality of all water, except the connate water derived from the marine sedimentary rocks, is generally excellent and within desirable ranges of hardness and salinity for public supply, industrial, and irrigation uses. Connate water from the marine rocks, which at places has migrated into permeable zones in the Little Butte Volcanic Series and Columbia River Group, is too saline for many uses and toxic to many crops. The volume of water pumped for all uses in 1966, mostly from the aquifers beneath the valley plains, totaled about 12,000 acre-feet. Of this to^al, about 73 percent, or 8,600 acre-feet, was pumped for irrigation; about 2& percent, or 2,600 acre-feet, was for domestic use; and about 4 percent, or 500 a?re-feet, was for municipal use. The volume of manageable stored water in the 0to 200-foot depth zone of the 186-square-mile valley area is about 2,900,000 acre-feet, and additional water is stored in the Troutdale Formation below the 200-foot depth hor'zon beneath much of the area. The minimum recharge to the ground-water body beneath the valle^ plain from precipitation, infiltration from irrigation, and fluid-waste disposal directly on the valley plain is estimated to be 180,000 acre-feet, which is equal to 1.5 feet of water over the 186-square-mile area. Recovery of water levels eacl year to the level of the previous year indicates that recharge equals discharge, which takes place through seepage to streams, evapotranspiration, and pumpage. The quantity of ground water available for use on a sustained annual basis from aquifers beneath the valley-plain area far exceeds the 12.000 acre-feet pumped for all uses in 1966 and amounts to about 180,000 acre-feet. If supplemental irrigation water were applied to all 119,000 acres in the valley-plain area at the average rate of application used in 1966 (1 acre-foot per acre), then annual replenishment would exceed the amount pumped by about 60,000 acre-feet. Well-construction methods commonly used in the area are adequate for production of large quantities of water from clean gravel aquifers but inadequate for the production of similar quantities of water from sand aquifers. Because nearly 90 percent of the valley-plain area is underlain by alluvial iraterials that contain less than 20 percent gravel in the 0to 200-foot depth zone, future development of ground water in much of the valley plain will be accomplishei by utilizing gravel-envelope and screened wells. INTRODUCTION PURPOSE AND SCOPE OF THE INVESTIGATION In recent years, the demand for ground water for domestic, irrigation, industrial, and public supplies has increased greatly in the Molalla-Salem Slope area. This report was prepared to answer, in part, the questions:

Geology and ground water of the Molalla-Salem slope area, northern Willamette Valley, Oregon