Red outline area:
Cooper-Bull Mountain Critical Ground Water 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 caused declines in the basalt. In 1992, the Water Resources Commission established eleven "Ground Water Limited Areas" (GWLA) in the northern Willamette Valley. Three of these areas are partially located within Watermaster District 18 boundaries. They 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 Critical Ground Water Area (CGWA), the Cooper Mountain-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.