U.S. Geological Survey
BACKGROUND
In 1991, the U. S. Geological Survey (USGS) began a full-scale National Water-Quality Assessment (NAWQA) program. The long-term goals of the NAWQA program are to describe the status and trends in water quality of large, representative parts of the Nation's surface- and ground-water resources, and to provide a sound, scientific understanding of the primary natural and human factors affecting the quality of these resources. The program will produce water-quality information useful to national, state, and local policy makers and managers.
The NAWQA program will collate water-quality information at various areal scales. Sixty study-unit investigations comprise the principal building blocks of the program and provide the foundation for regional and national assessment activities. The study units are hydrologic systems that range in size from 1,200 to more than 65,000 square miles and that incorporate about 60 to 70 percent of the Nation's water use and population served by public water supplies. They include parts of most major river basins and aquifer systems in the Nation. In 1991, the Willamette Basin was among the first 20 NAWQA study units selected for investigation under the full-scale implementation plan.
STUDY UNIT DESCRIPTION
The Willamette Basin NAWQA study unit includes the Willamette and Sandy Rivers, which are tributary to the Columbia River. The Willamette Basin drains an area of 12,000 square miles, contains Oregon's three largest cities (Portland, Eugene, and Salem), and is home to 68 percent of the State's population (about 1.9 million people). Approximately 70 percent of the basin is forested (largely tributary basins), about 22 percent is farmed, and the remaining 8 percent is urbanized or in other uses.
The Willamette Basin can be divided into three north-south physiographic provinces. The Cascade Range, consisting of volcanic rocks and exceeding elevations of 10,000 feet, forms the eastern boundary of the basin. The Coast Range, comprised of marine sedimentary and volcanic rocks at elevations over 4,000 feet, constitutes the western boundary. Between these mountain ranges lies the Willamette Valley -- an elongated structural and erosional lowland filled with flows of Columbia River Basalt (in the northern half of the basin) and younger unconsolidated sediment. The unconsolidated material is a major source of ground water, with wells commonly producing up to 300 gallons of water per minute. The Columbia River Basalt also is an important ground-water source, although wells completed in this material generally produce less than 100 gallons of water per minute.
The climate of the Willamette Basin is a modified, maritime temperate regime that is characterized by cool, wet winters and warm, dry summers. Approximately 10 percent of the average annual precipitation of 63 inches occurs between May and September. Precipitation varies markedly with altitude and ranges from about 40 inches at lower elevations to greater than 200 inches in the mountains. Most of the streamflow in the Willamette River typically occurs between November and March in response to persistent winter rain and spring snowmelt; however, melting of late spring snow in the high Cascade Range can prolong runoff into June or July. Summer and fall low flow periods are significantly affected by reservoir operations.
The 11 major reservoirs in the Willamette River basin have a combined usable storage capacity of nearly 1.9 million acre-feet. The reservoirs are operated primarily to maintain a minimum navigable depth in the Willamette River upstream to about Newberg. They also are operated for power generation, recreation, flood protection, anadromous fish propagation, irrigation, and public-water supply. As a result of these operational considerations, river quality is enhanced by augmentation of streamflow from reservoir releases. The Sandy River basin includes three storage reservoirs on the Bull Run River, with a usable storage capacity of 37,000 acre-feet. The Bull Run River is the major source of drinking water for the Portland metropolitan area and serves about 700,000 customers.
Water use in the Willamette Basin is estimated to be 800 million gallons per day, with 70 percent supplied by surface water. Surface-water uses include commercial and industrial (40 percent), irrigation (25 percent), domestic (25 percent) and other (10 percent). About 65 percent of the basin population relies on surface water for drinking. Ground water (30 percent of total water use) is used for irrigation (60 percent), domestic supply (25 percent), commercial and industrial applications (10 percent) and other purposes (5 percent).
WATER-QUALITY ISSUES
Water-quality issues in the Willamette Basin are difficult to categorize separately because they are, to a large extent, interrelated. However, based on previous studies in the basin and on discussions at the first meeting of the liaison committee (see section on COMMUNICATION AND COORDINATION), the following issues have been identified:
Communication and coordination among USGS personnel and other interested scientists and representatives of water-management organizations are critical components of the NAWQA program. Each of the 60 study-unit investigations will have a local liaison committee consisting of members who represent national, state, and local agencies, universities, and the private sector, and who have responsibilities with regard to issues related to water resources. Specific activities of each liaison committee will include exchange of information pertaining to water-quality issues of regional and local interest; identification of sources of data and information; assistance in the design and scope of project products; and review of project planning documents and reports. The liaison committee for the Willamette Basin study unit was formed in early 1991 and contributed significantly to formulating the water-quality issues discussed previously.
Information on technical reports and hydrologic data related to the NAWQA program can be obtained from:
District Chief, Water Resources Division
U.S. Geological Survey
10615 SE Cherry Blossom Drive
Portland, OR 97216
Open-File Report 91-167, D. A. Wentz and S. W. McKenzie, 1991
(revised, D. A. Wentz, 1995)
