Publication Summary

This report characterizes and documents the distribution of the surficial aquifers within the Chehalis River Watershed. Theoretically, if the location and character of surficial aquifers is known, local governments can use such information to help plan development to protect ground-water supplies.

This report characterizes and documents the distribution of the surficial aquifers within the Chehalis River Watershed. Theoretically, if the location and character of surficial aquifers is known, local governments can use such information to help plan development to protect ground-water supplies.

The Chehalis River Watershed is the second largest watershed in Washington State, encompassing an area of approximately 2,711 square miles. It is surpassed in size only by the Columbia River system. As human populations increase in western Washington, development increases within the watershed. Development has occurred mostly along the major river valleys where the depth to ground water is shallowest, the soil permeability is highest, and where risks of ground-water contamination are greatest. This development pattern is expected to continue.

Surficial aquifers are defined here as the uppermost saturated zone, typically an unconfined aquifer, of mappable extent. We further restrict the definition to include important water-supply aquifers that are the most likely to be degraded by human activities.

Surficial aquifers were delineated and mapped based on comparisons of physical properties such as depth to ground water, surficial geology, soil properties, and the presence or absence of near-surface aquitards.

The diverse geology of the Chehalis River Watershed controls the occurrence and movement of groundwater. The watershed′s principle surficial aquifers are contained within the thick, unconsolidated glacial and alluvial deposits that underlie the major river valleys and upland prairies. Surficial aquifers in the watershed typically lie only a few feet below land surface and extend to a depth of no more than 100 feet. A notable exception is near Aberdeen where the alluvial aquifer is about 200 feet thick. Depth to the water table varies by location, but ranges from less than ten feet to a maximum of about 50 feet. The principle alluvial and glacial aquifers are capable of sustained well yields of 200 to more than 3,000 gallons per minute.

Bedrock units produce ground water locally, but well yields are generally low. Bedrock aquifers are not classified as surficial aquifers in this study.

Comparisons of ground-water levels (measured at time of drilling) in wells across the watershed show that water-level altitude and hydraulic heads are higher in the upper tributary stream valleys and along the upland aquifer perimeters and lower along the principal river valleys. Ground water generally moves from upland recharge areas near the aquifer perimeter, toward natural points of discharge along the rivers and tributary streams. Water also moves vertically downward to recharge underlying regional aquifers.