THE EFFECT OF QUATERNARY STRATIGRAPHY ON GROUND WATER AND SURFACE WATER RESOURCES, WILLAMETTE VALLEY, OREGON

Surficial deposits are important in understanding recharge to aquifers, streamflow, and the effect of ground-water withdrawals on aquifer water levels and streamflow. Previous studies have identified material from six Quaternary episodes of deposition in the central and southern parts of the Willamette Valley that can be grouped in three hydrogeologic units. Gravel fans between 400 and 22 ka form an aquifer underlying most of the valley. Silt and clay deposited as backwater deposits from cataclysmic releases of ice-dammed Glacial Lake Missoula between 15 and 12.5 ka form a low permeability unit that overlies and confines most of the alluvial fan aquifer. In the flood plains of the Willamette River and its major tributaries, the silt and clay were removed and Holocene gravels were deposited forming the most productive aquifer in the valley. Availability of ground water for irrigation, and therefore the distribution of crop types and well locations, is correlated with the thickness and permeability of these deposits. Where older gravels are thin, ground-water withdrawals are limited to the Holocene gravels in the modern floodplain. Where older gravels are thick, ground-water withdrawals are widespread and a variety of crops are cultivated. Recharge and baseflow to streams are affected by the thickness and extent of the confining unit consisting of silt and clay. Streams incised into the confining unit are flashy and receive less ground-water discharge from the underlying aquifer than streams underlain by younger gravel. Ground-water ages indicate that precipitation and possibly stream water readily recharge the Holocene gravels. Water levels in older and Holocene gravels differ markedly. Ground-water levels in the Holocene gravels do not respond significantly to pumping because the aquifer is unconfined and has a good hydraulic connection to streams. Where older gravels are overlain by thick silt and clay, ground-water levels decline substantially in response to irrigation withdrawals each summer because the aquifer is confined by the silt and clay, and hydraulically isolated from streams.