SIMULATION OF GROUND-WATER FLOW IN THE WILLAMETTE BASIN AND CENTRAL WILLAMETTE SUB-BASIN, OREGON

The demand for water in the Willamette basin due to an increasing population and irrigation, and the full appropriation of tributary stream flow during the summer months creates an increasing demand for ground water in the region. An increase in ground water use potentially creates further depletion of stream flow, seasonal and long-term declines in ground water levels, and limitations due to low-permeability aquifers suitable for low demand uses only. In 1996, the U.S. Geological Survey and the Oregon Water Resources Department began a cooperative study to develop a quantitative conceptual understand of the ground water flow system of the Willamette River basin and central Willamette valley sub-basin. Regional and local models of the Willamette basin, and central Willamette sub-basin show a significant amount of discharge to the Willamette River is captured by wells located throughout the basin. Transient modeling of the central Willamette sub-basin indicate a buffering effect on smaller streams in the basin from the lower permeability Willamette silt unit when pumping from the lower sedimentary unit; however, this effect is diminished when pumping from the middle or upper sedimentary units. Temporal effects of pumping are demonstrated with most summer pumping initially being supplied by water released from aquifer storage; however, average annual discharge from and recharge to storage will go to zero over time, and total stream capture will equal average annual pumping. Aquifer geometry and stream incision control the ultimate effects of well pumpage on streams in the Willamette basin.