INFLUENCE OF THE WILLAMETTE SILT ON GROUND-WATER FLOW IN SHALLOW BASIN-FILL SEDIMENTS OF THE WILLAMETTE VALLEY, OREGON

Recharge, discharge, and ground-water flow in basin-fill sediments of the central and southern Willamette Valley, Oregon, are influenced by the extent and thickness of the Willamette Silt, a unit of fine-grained, rhythmically bedded sediments deposited by multiple floods from Glacial Lake Missoula during the late Pleistocene. The silt mantles land surface throughout most of the valley lowlands and forms a poorly drained, low-permeability hydrogeologic unit that ranges up to 130 feet in thickness. The silt intercepts precipitation during winter months and stores large quantities of water for later release to streams and underlying aquifers.

In the floodplains of the Willamette River and its main Cascade Range tributaries, the Willamette Silt has been removed by erosion and the water table resides in late Quaternary and Holocene deposits of unconsolidated sand and gravel. The shallow sand and gravel aquifer in these areas is unconfined, well-connected to adjacent streams, and responds directly to precipitation or changes in stream stage. Elsewhere, local streams do not fully penetrate the silt, the water table resides near land surface within the silt, and the silt behaves as a leaky-confining layer overlying the gravels.

In the southern valley, where the silt averages 10 to 20 feet thick, the underlying aquifer is weakly confined by the silt and shows a direct response to short-term precipitation and a strong correlation to seasonal precipitation. In the central valley, where the silt ranges from 60 to 130 feet thick, the underlying aquifer is confined by the silt and shows no direct response to short-term precipitation but shows some degree of correlation to annual precipitation. Well hydrographs in this area are commonly dominated by regional pumping drawdown and exhibit sustained recovery curves.

In the central valley, several deeply incised streams, including the Pudding River, act as drains within the silt and influence ground-water flow paths in the shallow sand and gravel aquifer. Water-level contours define elongate recharge areas in the silt separated by narrow discharge zones that coincide with incised stream channels. However, field studies indicate that the silt has low vertical permeability and that streams are only weakly connected to the underlying sand and gravel aquifer.