REGIONAL GEOLOGY AND HYDROLOGY Sample Clauses

REGIONAL GEOLOGY AND HYDROLOGY. The City of Corvallis is part of the alluvial plan that lies between the Cascade and Coast Ranges in the central Willamette Valley in northwestern Oregon. Volcanic and marine sedimentary units exposed in the foothills range from Eocene to Oligocene or Miocene. The volcanic rocks are primarily pillow lavas and basalt flows, which yield only small quantities of water generally adequate for domestic and stock use. Marine-deposited sandstone, siltstone, and shale of the older sedimentary units are fine grained, poorly permeable, and generally yield small volumes of water to xxxxx. The alluvial deposits (sand and gravel) of the valley plain contain the most productive aquifers in the area and are considered to be the only units feasible for large-scale development of groundwater supplies. (Xxxxx, 1974) Aquifers in the area are recharged principally by direct infiltration of precipitation. Most of the precipitation (about 38 inches per year) occurs during late autumn and winter. Groundwater is discharged naturally from the area by seepage and spring flow to streams, by evapotraspiration, by underflow, and artificially through xxxxx. (Xxxxx, 1974) The area is drained primarily by the Willamette and Mary’s Rivers. These rivers are mature, low gradient rivers with significant meandering. The Willamette River, which is the master stream, enters from the south and flows generally northward east of the Coast Range foothills. The Mary’s River enters the area from the west and drains into the Willamette River near Corvallis. (Xxxxx, 1974)