U.S. Geological Survey

By Dennis G. Woodward, Marshall W. Gannett and John J. Vaccaro

USGS Professional Paper 1424-B, 82 pages, 1 plate, 30 figures, 16 tables

Order from U.S. Geological Survey, Branch of Information Services, Box 25286, Denver, CO 80225 (ph: 303-202-4700, e-mail infoservices@usgs.gov ).

Abstract

The Willamette Lowland in Oregon and Washington encompasses 3,700 square miles and includes the low-lying parts of the Willamette Valley in Oregon and most of Clark County, Washington. About 70 percent of the population of Oregon and Clark County, Washington, reside in the Willamette Lowland, and the burgeoning population is increasing the demand for available water. The lowland is 145 miles long and averages 10 to 15 miles in width. Outcrops of folded and faulted basalt within the Willamette Valley divide the lowland into four separate areas or structural basins-from north to south, the Portland Basin, the Tualatin Basin, the central Willamette Valley, and the southern Willamette Valley. Each of these areas has decidedly different hydrologic and hydrogeologic properties.

The 3,700-square-mile aquifer system within the Willamette Low land is composed of five hydrogeologic units, from oldest to youngest: (1) the basement confining unit, (2) the Columbia River basalt aquifer, (3) the Willamette confining unit, (4) the Willamette aquifer, and (5) the Willamette Silt unit. The Willamette aquifer, the principal aquifer unit in the Willamette Lowland, consists primarily of coarse-grained proximal alluvial-fan and braided-stream deposits. The greatest thicknesses and coarsest materials of the Willamette aquifer outside of the Portland Basin occur in six major alluvial fans that were deposited where major streams from the Cascade Range enter the Willamette Lowland. The aquifer system in each basin, although hydraulically connected through a series of restrictive water gaps, is distinctive. The Columbia River basalt aquifer and the Willamette confining unit underlie most of the Portland Basin; the Willamette aquifer includes the basin-filling deposits above the Willamette confining unit. The Columbia River basalt aquifer and the Willamette confining unit are the only regional hydrogeologic units above the basement confining unit in the Tualatin Basin.

All five hydrogeologic units occur in the central Willamette Valley. The Columbia River basalt aquifer underlies the entire central Willamette Valley, except for small areas along the far eastern margin, where it thins out against the underlying basement confining unit. A number of faults have been mapped in the central Willamette Valley, some of which offset the aquifer, and numerous other faults have been mapped in the uplands surrounding the basin where the aquifer crops out. The Willamette aquifer in the central Willamette Valley contains three major alluvial fans-the Salem fan, the Molalla fan, and the Canby fan. The Willamette Silt unit overlies most of the central Willamette Valley, has a maximum thickness of about 130 feet near the center of the basin, and generally thins toward the south and near the margins of the basin.

In the southern Willamette Valley, all of the regional hydrogeologic units are present; however, the Columbia River basalt aquifer occurs only in the Stayton Subbasin. The Willamette confining unit is thinner in the southern Willamette Valley than elsewhere in the Willamette Lowland. The Willamette aquifer contains the Lebanon fan and the Stayton fan. The Willamette aquifer is much thinner (averaging only about 20 to 40 feet thick) between the alluvial fans of the southern Willamette Valley than in the central Willamette Valley. The Willamette Silt unit covers most of the southern Willamette Valley and generally thins toward the south.

Ground water in the Willamette aquifer generally occurs under unconfined conditions. The regional water-table map shows an overall pattern of ground-water flow to the major streams, indicating that the base flow of these streams is sustained by ground-water discharge. The hydraulic gradient of the Willamette aquifer ranges from more than 60 feet per mile near the western part of the central Willamette Valley to less than 2 feet per mile in the flood plain of the Willamette River north of Salem, Oregon. On the basis of average values of the hydraulic gradient and the hydraulic characteristics of the Willamette aquifer, the velocity of water moving through the aquifer ranges from 3 to 30 feet per day, which is typical for sand and gravel aquifers.

Long-term hydrographs for observation wells completed in the Willamette aquifer confirm that, on a regional basis, the aquifer is in equilibrium. Water is recharged to the Willamette Lowland aquifer sys tem primarily through the direct infiltration of precipitation on the lowland. An analysis of ground-water recharge from precipitation done for this study showed that about 21,346 ft³/s (cubic feet per second) of precipitation falls in the study area, of which about 13,186 ft³/s falls on the aquifer system. Of the latter quantity, about 5,462 ft³/s is estimated to recharge the aquifer system. The regional estimate of mean annual recharge is about 42 percent of the mean annual precipitation and includes a 280 ft³/s reduction due to land use and land-cover effects. Excluding recharge derived from sources other than precipitation, recharge varies seasonally from about 0.05 inches per month in the summer to about 3 to 6 inches per month in the winter. Because most of the low streamflow (during August) in the Willamette River is accounted for by streams entering the lowland, the mean annual recharge helps support base flow from about December through July.

Water is discharged from the Willamette Lowland aquifer system primarily by flow to surface-water bodies (streams, reservoirs, and springs) but also by evapotranspiration and by pumpage through wells. Regionally, ground water flows toward streams and is discharged to the streams through springs and seeps. This ground-water discharge fully supports the base flow of streams that head in the lowland and partially supports the base flow of the other streams. Low-flow discharge measurements were made during August to September 1992 on the Willamette, McKenzie, and Santiam Rivers, and high-flow discharge measurements were made during June and September 1993 on the Willamette River to determine minimum ground-water seepage into and out of the rivers. The seepage estimates suggest that as streams cross the proximal part of the buried alluvial fans, they lose water to the aquifer, but as the streams cross the distal part of the fans, they gain water from the aquifer.

A considerable volume of ground water in the Willamette Lowland is discharged by evapotranspiration from both the soil root zone and the aquifer system in areas where the water table is near the land surface. On the basis of the results of the cross-sectional ground-water flow models, from 15 to 16 inches per year of evapotranspiration is supported by the aquifer system.

Throughout the Willamette Lowland, an estimated 464 ft³/s of ground water was withdrawn in 1990 for all uses. For comparison, this quantity is about 4 percent of the mean annual precipitation falling on the aquifer system, about 1 percent of the mean annual flow of the Willamette River, and about 8 percent of the estimated mean annual recharge to the aquifer system.

Most of the shallow ground water throughout the Willamette Low land is of good chemical quality and is suitable for most uses. Median values of all constituents and properties for the samples from the Columbia River basalt and the Willamette aquifers are similar, although maximum concentrations of calcium, sodium, and chloride are more than 10 times higher in the samples from the Columbia River basalt aquifer, and maximum concentrations of bicarbonate and sulfate are at least 2 times higher in the samples from the Willamette aquifer. On the basis of 75 water samples, ground water in the Columbia River basalt aquifer is predominantly a calcium-magnesium-bicarbonate type, but a few samples were a calcium-sodium-chloride water type. Ground water in the Willamette aquifer, on the basis of 181 analyses, is homogeneous and is predominantly a calcium-magnesium-bicarbonate type, although a few samples are a chloride-dominant (calcium-magnesium-chloride) type.

The occurrence of saline (chloride-dominant) ground water in the Willamette Valley has caused problems and has caused speculation regarding its origin for many years. This study suggests that saline ground water in the lowland is marine connate water, exists at depths in Tertiary marine rocks, and migrates upward along faults and compact folds. The saline water in the Willamette aquifer and Columbia River basalt aquifer, as well as the more dilute chloride-dominant ground water in shallow marine rocks, generally occurs near faults or folds and results from the mixing of shallow meteoric water with deep connate water brought near the surface along the fault zones.

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