Geologic framework of the Willamette Lowland aquifer system, Oregon and Washington

Abstract

The RASA Program represents a systematic effort to study a number of the Nation's most important aquifer systems, which, in aggregate, underlie much of the country and which represent an important component of the Nation's total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and, accordingly, transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies to develop an understanding of the natural, undisturbed hydrologic system and the changes brought about in it by human activities and to provide a means of predicting the regional effects of future pumping or other stresses.The final interpretive results of the RASA Program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer system. Each study within the RASA Program is assigned a single Professional Paper number beginning with Professional Paper 1400.

Location and timing of river-aquifer exchanges in six tributaries to the Columbia River in the Pacific Northwest of the United States

“…Its western margin was formed by the uplift of marine sediments of the Coast Range. The Willamette Lowland, which is an erosional and structural basin between the Coast and Cascade Ranges that extends north to the Columbia River, has an extensive unconsolidated sediments (Pliestocene and Holocene) deposited as basin fill and alluvial fans that form the Willamette aquifer (Gannett and Caldwell, 1998). The Willamette aquifer is generally less than 20 m thick except at the northern end of the lowland and around six alluvial fans, where sediments are thicker (as much as Figure 6 Lemhi River from RK 1 to 90 with geology, selected ground-water elevations, and the reach with largest gain in streamflow.…”

Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data

“…1B; Yeats et al, 1996;Gannett and Caldwell, 1998), but gravity data show that the Eocene to middle Miocene basin fi ll is signifi cantly deeper than the CRBG. We simultaneously modeled residual gravity and aeromagnetic data along a profi le across the Tualatin basin, using available well and surfi cial geologic constraints (Fig.…”

Hydrogeology and Water Resource Potential of Neogene Sediments in the Tualatin Basin and Comparison with the Neighboring Portland Basin, Northwest Oregon

“…The Willamette Lowland of northwest Oregon developed in the Late Miocene, on either Middle Miocene CRBG or older Paleogene sedimentary rocks, as a forearc depositional center in response to tectonic uplift of the Oregon Coast Range to the west and continued construction of the Cascade Range to the east (Niem and Niem, 1984;Gannett and Caldwell, 1998). Five nonmarine depositional basins are distinguished within the lowland (Figure 1; Yeats et al, 1991).…”

“…An unconfined aquifer created by the Late Pleistocene catastrophic Missoula floods composes the uppermost hydrogeologic unit in all basins ( Figure 2). This unit, known as the Willamette Silt in the Southern Willamette, Stayton, and Northern Willamette basins, unconformably overlies Pleistocene fan-gravel deposits derived from the Cascade Range, termed the Willamette aquifer, and Miocene to Lower Pleistocene fine-grained fluvial sediments of the Willamette confining unit (Yeats et al, 1991;Gannett and Caldwell, 1998). Underlying the Willamette Silt in the Tualatin basin is the Upper Miocene to Pleistocene Hillsboro Formation aquifer (Wilson, 1998a), which lacks the upper fan-gravels found in the other four basins.…”