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Ground Water Management Areas Overview

This page shows the 4 Ground Water Management Areas in the Tualatin Basin.
Ground water management areas

The colored outline areas on the map represent the boundaries of the GWLA’s (Ground Water Limited Area) and the CGWA (Critical Ground Water Area). Due to the small scale of this map, the boundaries are approximations. To get more specific information on a particular tax lot within one of these areas, please contact our office.

Red outline area

Cooper-Bull Mountain Critical Ground Water Area

Pink outline area

Chehalem Mountain Limited Ground Water Area

Gold outline area

Sherwood_Dammasch-Wilsonville Limited Ground Water Area

Light Blue outline area

Parrett Mountain Limited Ground Water Area

The northern Willamette Valley and much of the Columbia River Plateau contain many sources of ground water that are isolated in volcanic rock. These aquifers are in the Columbia River Basalt Group, commonly called the “basalt”. Heavy pumping has causes declines in the basalt. In 1992, the Water Resources Commission established eleven (11) “Ground Water Limited Areas” (GWLA) in the northern Willamette Valley. Three (3) of these areas are partially located within Watermaster District 18 boundaries. These 3 areas are the Chehalem Mountain GWLA, the Parrett Mountain GWLA, and the Sherwood, Dammasch-Wilsonville GWLA. Special restrictions are in place to help stabilize ground water levels in these areas.

District 18 also has one (1) Critical Ground Water Area (CGWA), the Cooper-Bull Mountain CGWA. State law requires that when pumping of ground water exceeds the long-term natural replenishment of the underground water reservoir, the Water Resources Commission must act to declare the source a critical ground water area and restrict water use.

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