The Columbia Slough is a narrow waterway, about 19 miles (31 km) long, in the floodplain of the Columbia River in the U.S. state of Oregon. From its source in the Portland suburb of Fairview, the Columbia Slough meanders west through Gresham and Portland to the Willamette River, about 1 mile (1.6 km) from the Willamette's confluence with the Columbia. It is a remnant of the historic wetlands between the mouths of the Sandy River to the east and the Willamette River to the west. Levees surround much of the main slough as well as many side sloughs, detached sloughs, and nearby lakes. Tidal fluctuations cause reverse flow on the lower slough. 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). Before European Americans explored the region, tribes of Native Americans fished and hunted along the slough. In the mid 19th century large migrations of settlers began arriving from the east who farmed, cut timber, built houses, and by the early 20th century established cities, shipping ports, rail lines, and industries near the slough. Increased investment in the floodplain led to larger losses during floods, and these losses prompted levee building that greatly altered the area. A flood pouring through a levee break in 1948 destroyed the city of Vanport, which was never rebuilt. Portland International Airport lies along the middle slough and marine terminals of the Port of Portland are near the lower slough. The Oregon Department of Environmental Quality and the city's Bureau of Environmental Services deal with environmental issues, many created by its use as a waste repository during the first half of the 20th century, causing the slough became one of Oregon's most polluted waterways. Early attempts to mitigate the pollution were unsuccessful, but in 1952 Portland began sewage treatment, and over the next six decades the federal Clean Water Act and similar legislation mandated further cleanup. The slough is frequented by more than 150 bird species and 26 fish species and animals including otters, beaver, and coyotes.