Background

The burgeoning population of the Willamette River Basin is putting unprecedented demands on the water resources of the region. Because surface-water resources are largely allocated, ground-water resources are being increasingly looked upon to meet the growing demand. The increasing demand on these finite resources poses a particular challenge to resource managers and planners, and to society as a whole. Developing a sound, quantitative technical understanding of the ground-water hydrology of the region is of paramount importance for the successful management of water-resources and providing for the variety of demands.

The Willamette River drains the area of northwestern Oregon between the Cascade and Coast Ranges. The Willamette River drainage basin encompasses approximately 11,500 square miles extending north-south from the Columbia River to the southern drainage divide with the Umpqua River. Approximately 2,700 square miles of the basin consists of a relatively flat, broad valley, generally referred to as the Willamette Valley, which lies between the foothills of the adjacent ranges.

The regional geology of the Willamette Basin was recently studied as part of the USGS Regional Aquifer System Analysis (RASA) program. The basin is underlain by a variety of rock types. The Coast Range, which forms the western part of the basin, is predominantly marine sedimentary rock such as sandstone, siltstone, and mudstone. The Cascade Range, which forms the eastern part of the basin, consists of a variety of lava flows and volcanic sediments. The Willamette Valley, which lies between the two ranges, is a lowland that has accumulated a substantial thickness of sediment. The basin-filling deposits of the Willamette Valley also include a substantial thickness of basalt lava that flowed into the region during the early stages of basin development. This lava, known as the Columbia River Basalt Group, occurs in the northern two thirds of the Willamette Valley. The basalt lava has been folded and faulted, and now forms a series of uplands that separate the Willamette Valley into a series of sediment-filled sub-basins. The basalt lava is exposed in the uplands separating the sub-basins, and lies beneath the valley-filling sediments in the intervening areas.

Approximately 2 million people, about 70 percent of the population of the State, live in the 9 counties that lie wholly or principally in the Willamette Basin. Most of these people live in the 3 percent of the State that is the Willamette Valley.

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