The Columbia Slough is a narrow waterway, about 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 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. Drainage district employees control water flows with pumps and floodgates. 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 . 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.
Long before non-indigenous people explored the region, tribes of Native Americans fished and hunted along the slough. In the early 19th century fur trappers and explorers including Lewis and Clark visited the area before large migrations of settlers began arriving from the east. The newcomers farmed, cut timber, built houses, and by the early 20th century established cities, shipping ports, roads, 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.
Used as a waste repository during the first half of the 20th century and cut off from the Columbia River by levees, the slough became one of Oregon's most polluted waterways. Early attempts to mitigate the pollution, which included raw sewage and industrial waste, were unsuccessful. However, in 1952 Portland began sewage treatment, and over the next six decades the federal Clean Water Act and similar legislation mandated further cleanup. State and local governments, often assisted by community volunteers, undertook projects related to public health, natural resources, and recreation in a region with many homes, industries, businesses, and roads. The businesses and industries in the watershed employ about 57,000 people, which is also frequented by more than 150 bird species and 26 fish species and animals including otters, beaver, and coyotes. One of the nation's largest freshwater urban wetlands, Smith and Bybee Wetlands Natural Area, shares the lower slough watershed with a sewage treatment plant, marine terminals, a golf course, and a car racetrack. Watercraft able to portage over culverts and levees can travel the entire length of the slough. The 40-Mile Loop and other hiking and biking trails follow the waterways and connect the parks.
Name Slough usually rhymes with shoe in the U.S. except in New England, where it usually rhymes with now, the preferred British pronunciation. Slough may mean a place of deep mud or mire, a swamp, a river inlet or backwater, or a creek in a marsh or tide flat. The Columbia Slough is classified as a "stream" in the Geographic Names Information System (GNIS) of the United States Geological Survey (USGS). The slough takes its name from the Columbia River, of which it was historically a side channel or anabranch. Robert Gray, a Boston fur trader and whaler who sailed partway up the Columbia River in 1792, named the river after his ship, Columbia Rediviva. The Columbia part of the ship's name belonged to the tradition of naming things after explorer Christopher Columbus.
Course
The Columbia Slough flows roughly parallel to and about south of the Columbia River in Multnomah County. It begins at Fairview Lake in the city of Fairview and immediately enters the city of Gresham. Less than later, it enters the city of Portland and continues generally westward for about another to its confluence with the Willamette River. Throughout its course, the slough is nearly level, above sea level at the source and at the mouth. The mouth elevation was determined by entering the GNIS coordinates in Google Earth. Semidiurnal tides cause reverse currents on the lower of the slough.
Running slightly north of and parallel to U.S. Route 30 (Sandy Boulevard), the slough flows by Zimmerman Heritage Farm on the left bank (south) about from the mouth, Big Four Corners Wetlands on the right bank shortly thereafter, and receives Wilkes Creek on the left shortly after that. At about river mile (RM) 15.5 or river kilometer (RK) 24.9, it passes through a gated levee that separates the upper slough from the middle slough. Soon it passes Prison Pond Wetlands near Inverness Jail and connects to Johnson Lake Slough, all on the left. Shortly thereafter, it flows under Interstate 205. From here and for most of the rest of its course, the slough runs parallel to and slightly north of Columbia Boulevard. Passing Johnson Lake on the left, it crosses the Colwood National Golf Course and flows by Portland International Airport and an Oregon Air National Guard base on the right. On the left is Whitaker Ponds Natural Area. Shortly thereafter, it receives Whitaker Slough on the left and crosses the Broadmoor Golf Course. Between from the mouth, it receives Buffalo Slough from the left and passes by the defunct Peninsula Drainage Canal (City Canal), which lies to the slough's right. At this point, it passes through a second gated levee that separates the middle slough from the lower slough and its tidal flow reversals.
In the next stretch, the Columbia Slough crosses under Interstate 5 at about RM 7 (RK 11). Beyond the interstate, to the slough's north lies Delta Park, Portland International Raceway, and the Heron Lakes Golf Course. Until flooding destroyed it in 1948, the city of Vanport occupied this site. To the south is the Columbia Boulevard Wastewater Treatment Plant.
The slough flows through the Wapato Wetland and by the Smith and Bybee Wetlands Natural Area, including the former St. Johns Landfill, on the right at about RM 3 (RK 4.8) from the Willamette River, and by Pier Park on the left. Shortly thereafter, it turns sharply north for the rest of its course. It receives North Slough, connected to Bybee Lake, on the right, and passes through the Ramsey Lake Wetlands and Kelley Point Park before entering the Willamette River about from its confluence with the Columbia River. The mouth of the Columbia River is about further downstream at Astoria on the Pacific Ocean.
Discharge Since 1989, the USGS has monitored the flow of the Columbia Slough at a stream gauge from the mouth. The average flow recorded at this gauge between water years 1990 and 2021 was from a drainage area of undetermined size. The maximum flow was on December 5, 1995, and the minimum flow (biggest reverse flow) was on February 7, 1996, which coincided with a flood on the Columbia and Willamette rivers.
Watershed Draining about , 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.
Jurisdiction Many governmental entities share responsibility for the slough and its drainage basin. Decisions by the municipal governments of Portland, Fairview, Gresham, Maywood Park, and Wood Village, and the government of Multnomah County affect the slough. Metro, the regional governmental agency for the Oregon portion of the Portland metropolitan area, is involved in acquiring and protecting wildlife habitat in places like Big Four Corners Wetlands and Smith and Bybee Wetlands Natural Area, acquiring property to close gaps in the 40-Mile Loop and other trails, and creating additional water access along the slough. The United States Environmental Protection Agency (EPA) has designated the Oregon Department of Environmental Quality (DEQ) to regulate the slough under provisions of the federal Clean Water Act. The Port of Portland, a regional agency run by commissioners appointed by the Oregon governor, owns and manages about 11 percent of the land in the slough watershed. Three drainage districts that manage water flows in the slough floodplain overlap parts of these other jurisdictions.
Annual report card In 2015, Portland's Bureau of Environmental Services (BES) began issuing annual "report cards" for watersheds or fractions thereof that lie within the city. BES assigns grades for each of four categories: hydrology, water quality, habitat, and fish and wildlife. Hydrology grades depend on the amount of pavement and other impervious surfaces in the watershed and to what degree its streams flow freely, not dammed or diverted. Water-quality grades are based on measurements of dissolved oxygen, E-coli bacteria, temperature, suspended solids, and substances such as mercury and phosphorus. Habitat ranking depends on the condition of stream banks and floodplains, riparian zones, tree canopies, and other variables. The fish and wildlife assessment includes birds, fish, and macroinvertebrates. In 2015, the BES grades for the Columbia Slough are hydrology, B− ; water quality, B− ; habitat, D− , and fish and wildlife, F.
Geology