Publication Summary

Pacific and Wahkiakum Counties, in southwestern Washington, have an area of about 1,200 square miles. This semi-mountainous area is bordered by the Pacific Ocean. The area has a marine climate with precipitation averages ranging from 70 to over 120 inches per year, most of which occurs in winter.

For purposes of this study, rocks in the area are divided into three unconsolidated Quaternary units and two Tertiary bedrock units. Alluvium, consisting of peat, clay, silt, sand and gravel, occurs mainly in the lower parts of valleys, where it is relatively thin, and in a narrow strip near the beach north of Willapa Bay, where it may exceed 200 feet in thickness. The alluvium in the valleys generally yields only small amounts of water, but yields may vary from none to a few hundred gallons per minute. Irrigation pits in alluvium along the coast north of Willapa Bay yield from 100 to 500 gpm.