UPDATED: October 2019

This is a summary of conditions in the Columbia Slough watershed based on data from the 2019 Watershed Health Index. The scores are a snapshot of conditions across the entire watershed. Conditions can vary in smaller parts of the watershed.

Learn more about the Columbia Slough watershed program and projects

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.

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.

	Effective impervious area		4.8
	Stream connectivity		5.6

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

Despite these projects, stormwater treatment in the Columbia 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.

	Ammonia-nitrogen		9.7
	Dissolved copper		7.6
	Dissolved oxygen		4.7
	E. coli		8.0
	Temperature		1.0
	Total mercury		4.4
	Total phosphorus		5.5
	Total suspended solids		5.3

Land area in the Columbia 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 project at NE 33rd Avenue, improve water flow and fish access to habitat.

	Bank condition (hardening)		0.0
	Floodplain condition		6.7
	Large wood		2.6
	Riparian integrity		2.5
	Stream accessibility		1.7
	Substrate composition	Not applicable
	Tree canopy		3.9

The watershed is at the junction of the Columbia and Willamette rivers, 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.

	Birds		2.8
	Fish		1.1
	Macroinvertebrates	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.