Draining about 51 square miles (130 km2), the Columbia Slough watershed lies in the floodplain of the Columbia River between the mouths of the Sandy River to the east and the Willamette River. Parts of Portland, Fairview, Gresham, Maywood Park, Wood Village, and unincorporated Multnomah County lie within the drainage basin (watershed). As of 2005, about 158,000 people, 5 percent of Oregon's population, lived in the basin.
The watershed includes residential neighborhoods, agriculture, the airport, open spaces, 54 schools, interstate highways, railways, commercial businesses, and heavy and light industry. In general, the northern part is industrial and commercial; the southern part is residential, and agricultural areas lie to the east. As of 2001, single-family residential zones covered 33 percent of the watershed, and mixed-use zones accounted for another 33 percent. The other zones were 12 percent industrial, 12 percent parks or open space; 6 percent multi-family residential, 3 percent commercial, and 2 percent were for farming or forests. As of 2005, about 3,900 businesses operated in the watershed and employed about 57,000 people.
Adjacent to the Columbia Slough basin are the watersheds of the Sandy River to the east, Johnson Creek to the south, the Willamette to the south and west, and the Columbia to the north. Lying slightly north of the watershed, large islands in the Columbia River include, from east to west, McGuire Island, Government Island, Lemon Island, and Tomahawk Island, which is connected to Hayden Island. Bordered by the Columbia and two arms of the Willamette, Sauvie Island lies just west of the mouth of the slough.
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