AuCoin's environmental record earned him the endorsement of major environmental organizations in each of his House elections. In addition to blocking offshore oil exploration, AuCoin prevented mining in the center of Oregon's Three Sisters Wilderness area by buying out a mining claim in the area's geologically significant Rock Mesa and served on the committee that helped write the 200-mile offshore economic zone, which would become known as the Magnuson Act. Although the Port of Portland shipyards, a major Oregon employer, stood to benefit from oil drilling in the Arctic National Wildlife Refuge, AuCoin opposed the plan on environmental grounds. He also helped preserve Cascade Head on the Oregon Coast, supported the Columbia Gorge Scenic Protection Act, helped stop the construction of Salt Caves Dam on the last free-flowing stretch of the Klamath River, co-authored the 1988 bill quadrupling the designation of National Wild and Scenic Rivers in Oregon, and fought the construction of a plant at the Umatilla Chemical Depot to incinerate excess chemical weapons.