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Columbia Slough Report Card

Photo of paddling on the Columbia Slough

This is a summary of conditions in the Columbia Slough watershed based on data from the 2015 Watershed Health Index. The scores are a snapshot of conditions across the entire watershed. Conditions can vary in smaller parts of the watershed.
Link to introduction page on the watershed report cards


Learn more about the Columbia Slough
Watershed Program and Projects

Columbia Slough Hydrology Score: D

Over half of the Columbia Slough system is highly altered with levees and pumps for drainage and flood control. Reducing impervious surfaces and better stormwater management can improve hydrology. The Mason Flats Wetland, for example, manages runoff from more than 600 acres of development. Stormwater management projects that let runoff soak into the ground and replenish groundwater will help improve the impervious area score, water quality and habitat.

The stream connectivity score is good. Although levees block migrating fish access to half of the slough, most of the waterways within the levee system are open with very few piped sections.

Columbia Slough
Hydrology Average Score

Hydrology score: 5.8


What is this?
Effective impervious area
Effective impervious area: 5.0
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Stream connectivity
Stream connectivity: 6.7

Columbia Slough Water Quality Score: B

Projects and investments in the last two decades have significantly improved the slough’s water quality, making it a good place to explore nature by foot, canoe or kayak. The city’s completion of projects in 2000 to reduce combined sewer overflows to the slough, and projects to connect property to the public sewer and abandon septic tanks and cesspools have greatly reduced E. coli bacteria and nitrogen in the slough. Port of Portland improvements in managing de-icing chemicals from airport runways have improved dissolved oxygen levels.

Despite these projects, stormwater treatment in the slough watershed remains a priority. Reducing total suspended solids and pollutants like pesticides, PCBs, and toxic organic compounds that bind to soil, will help improve water and sediment quality. Increasing vegetation along the banks and restoring riparian buffers to shade the water will reduce water temperature and provide other benefits.


Columbia Slough
Water Quality Average Score

Water quality score: 5.7


What is this? Ammonia-nitrogen Ammonia-nitrogen: 9.1
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Dissolved copper
Dissolved copper: 7.0
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Dissolved oxygen
Dissolved oxygen: 5.9
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E. coli
E. coli: 7.6
What is this? Temperature Temperature: 1.4
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Total mercury
Total mercury: 4.0
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Total phosphorus
Total phosphorus: 5.9
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Total suspended solids
Total suspended solids: 5.0

Columbia Slough Habitat Score: 2.6

Land area in the slough watershed is highly developed with roads, homes, businesses, industries and port facilities, leading to poor overall habitat scores. However, the watershed also has significant habitat resources that are protected as natural areas. Smith and Bybee Wetlands, Vanport Wetlands and Big Four Corners Natural Area are examples.

The poor riparian score is due to the history of development right up to the slough banks leaving little to no buffer of plants and trees. The Watershed Revegetation Program’s work with private property owners over the last two decades has had a positive impact, but additional efforts are needed.

City restoration projects such as the Columbia Slough Confluence, Ramsey Refugia and Lower Slough Refugia are adding in-stream large wood. Culvert replacements, such as a recent project at NE 33rd Avenue, improve water flow and fish access to habitat.


Columbia Slough
Habitat Average Score

Columbia Slough Habitat Score: D-


What is this? Bank condition (hardening) Bank condition: 0.0
What is this? Floodplain condition Floodplain condition: 6.7 6.7
What is this? Large wood Large wood: 2.0
What is this? Riparian integrity Riparian integrity: 2.6
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Stream accessibility  Stream accessibility: 1.4 1.4
What is this? Substrate composition Not applicable  
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Tree canopy
Tree canopy: 2.9

Columbia Slough Fish and Wildlife Score: F

Because the watershed is at the junction of the Columbia and Willamette rivers, it is a key area along migratory routes for birds and fish. 170 species of birds and 29 species of fish are found in the slough watershed, including salmon and steelhead that use the lower slough while migrating to the ocean. There are also resident native fish species in the middle and upper sloughs. While a few large natural areas support fish and wildlife, inadequate riparian buffers and tree canopy limit overall conditions.

Projects to improve habitat and water quality will help increase abundance of fish and wildlife. Monitoring at the Ramsey Refugia project, 1.5 miles up the slough from the Willamette River, shows that salmon from both the Willamette and Columbia basins use the area.


Columbia Slough
Fish and Wildlife Average Score

Columbia Slough Fish and wildlife score: F


What is this? Birds Birds: 3.9
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Fish: 0.9
What is this?
Not applicable

For more information about what we measure and where these scores come from, visit the About Watershed Report Cards page.

See some of what Environmental Services is doing for healthy rivers and streams on the Explore Portland’s Watersheds map.