 The Columbia floodplain, formed by geologic processes including lava flows, volcanic eruptions, and the Missoula Floods, is part of the Portland Basin, which extends across the Columbia River from Multnomah County, Oregon, into Clark County, Washington. Five percent of Oregon's population, about 158,000 people, live in the slough watershed of about 51 square miles (130 km2). Municipal wells near the upper slough provide supplemental drinking water to Portland and nearby cities. The cities, the drainage districts, the county, and a regional government, Metro, have overlapping jurisdictions in the watershed. A regional agency operates Portland International Airport along the middle slough and marine terminals near the lower slough. The Oregon Department of Environmental Quality (DEQ) and the city's Bureau of Environmental Services (BES) deal with environmental issues.
