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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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About the Watershed

Columbia Slough Watershed

The Columbia Slough watershed drains 32,700 acres of land. The watershed includes Fairview Lake, Fairview Creek, and portions of Fairview, Gresham, Maywood Park, Wood Village, and Multnomah County. The waterway is located within the former floodplain of the Columbia River between the mouths of the Willamette and Sandy Rivers. Before  construction of flood control levees in the watershed, the spring freshets inundated the floodplain, cutting new channels and depositing sediment. Native Americans used these waterways and the uplands for fishing, hunting, and gathering food.

Over the years, the watershed and waterway were heavily altered to accommodate industry, transportation and agriculture. Beginning in 1918, levees were built to provide flood protection. Wetlands and side channels were drained and filled to allow for development. The waterway was channelized, and dozens of streams were filled or diverted to underground pipes. This resulted in a massive loss of habitat, flood storage capacity, and ability to filter sediments and pollutants.

Today the Columbia Slough comprises a 19-mile main channel that parallels the Columbia River and about 30 miles of secondary waterways. The Upper and Middle Sloughs are highly managed with piped surface water, dikes and levees, and a system of pumps that provide watershed drainage and flood control. The Lower Slough is tidal. The watershed provides important habitat, jobs for about 60,000 people and is home to nearly 160,000 residents. 

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Physical Habitat

The Columbia Slough watershed provides critical habitat at both the local and regional level. The Lower Slough is connected directly to the Willamette/Columbia River system and provides estuarine type refuge for salmonids. The Slough waterway provides a thin ribbon of connectivity within the floodplain for a wide variety of wildlife. The watershed also contains several significant habitat blocks that provide significant benefits to wildlife.

There are a wide variety of habitats in the watershed. They include:

  • In-Channel: tidal, non-tidal, creeks, springs, water column and benthos
  • Riparian Habitat
  • Wetlands Habitats (forested, scrub/shrub, emergent and exposed mudflats
  • Upland Habitats (mixed canopy, non forested open area, landscaped areas
  • Unique Habitats
  • Off Channel Open Water
  • Listed Species

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Loss of habitat to urban development is the greatest single influence on physical habitat and biological communities in the Columbia Slough Watershed.  Over the last several decades, agricultural lands, open space, and wetlands have been altered to industrial and commercial land uses. Remaining habitats are fragmented, making them susceptible to invasion by non-native plant species and reducing habitat quality for native wildlife species.

Extensive populations of several invasive plant species, including reed canarygrass, Japanese knotweed, Himalayan blackberry, and purple loosestrife, dominate in the watershed. They form dense stands that provide very limited food and cover value for native wildlife. The Slough’s altered hydrology, narrow riparian corridors, and disturbed and fragmented habitat provide suitable conditions for these invasive species.

Several large habitat areas in the watershed are protected by environmental zones and public ownership. The City of Portland created environmental zones in 1989 to protect existing natural resource lands throughout the City. The Bureau of Environmental Services began the Watershed Revegetation Program in 1993 to plant native vegetation and increase habitat for native fish and wildlife. In 2004, Metro will adopt a Goal 5 Fish and Wildlife Habitat Protection Program for regionally significant natural resource lands.  The Columbia Slough watershed provides an ideal test case for the region to balance industrial development and jobs with natural resource protection and wildlife habitat.

The Columbia Slough is largely disconnected from its floodplain. Construction of levees and management of water levels in the Slough for flood control, as well as the hydrologic management of dams on the Willamette and Columbia Rivers, is now required to protect extensive urban development. As a result, the floodplain no longer provides important ecological functions: trapping sediment, providing nutrients, storing floodwater, providing refuge habitat for salmonids, and controlling erosion.

The upland area of the watershed has little native forest and mainly contains fragmented habitat areas such as city parks and street trees. These habitats offer limited value for native wildlife and lack safe wildlife travel corridors. These conditions are primarily caused by urban development and tree removal on individual private lots and streets.   Scattered mature Oregon white oak trees can still be found around the watershed and provide habitat for oak-dependent species. 

Several culverts in the Middle and Upper Slough are undersized or impassable, impeding stream connectivity.  As a result, aquatic and terrestrial species have limited mobility, and impounded water provides ideal conditions for the growth of algae and macrophytes (water vegetation).

Excessive fine sediments often settle in the streambeds of Columbia Slough tributaries (Wilkes Creek, Osborn Creek, and Fairview Creek). This is caused by erosion in upland areas and sediments settling out of stormwater that enters the streams. These sediments degrade native fish spawning areas and limit suitable habitat for benthic macroinvertebrates (bottom-dwelling aquatic insects).

Wapato Wetlands and over 1,000 acres of Smith and Bybee Lakes in the Lower Slough provide off-channel habitat for salmonids. Smith and Bybee Lakes became available as off-channel habitat when the water control structure was replaced in December 2003.  The Lower Slough is also considered off-channel habitat from the Willamette and Columbia Rivers. Filling and levee construction have disconnected other off-channel areas (such as secondary channels) from the slough.

Public Health and Safety

  • 54% of the watershed is impervious surface.
  • 426 acres are not served by sanitary sewers.
  • Aging pipe systems need maintenance and replacement.
  • Slough combined sewer overflows (CSOs) are 99% controlled.

The Columbia Slough Watershed has both combined and separated sanitary and stormwater collection systems. These systems are complex, and charting the flow of stormwater from where it lands on the ground to where it discharges is difficult.

Some of the sanitary sewer system is approaching the end of its life expectancy.  Increased maintenance may be needed to repair older pipes. 

Several large-scale projects (such as the Columbia Slough Big Pipe, sewer separations, and the Downspout Disconnection program) have controlled 99% of CSOs to the Columbia Slough. Thirteen CSO outfall pipes into the Lower Slough remain, and may overflow once every 10 years in summer and once every five years in winter.

There are 473 acres of unsewered areas that may be contributing bacteria to the Slough. The Slough is water quality limited for bacteria, which means it does not meet water quality criteria and may pose human health risks.

Managing stormwater with a piped collection and disposal system interrupts the natural hydrologic cycle. Stormwater pipes collect stormwater and discharge it directly to the Slough. This bypasses the natural system, which would have infiltrated stormwater into the ground. Other stormwater management approaches, including sumps, surface infiltration and constructed wetlands, do not interrupt the hydrological cycle.  Environmental Services strives to manage stormwater as close to its source as possible and use approaches that mimic the natural hydrologic cycle.

Street trees are effective at collecting rainfall and keeping it out of the sewer system.  In doing so, they reduce stormwater runoff into the Slough and help maintain the natural hydrologic cycle. Street trees also provide aesthetic and urban heat island benefits.

Automobiles deposit oil, grease, dirt, brake dust, trash, vehicle parts and heavy metals (copper, lead and zinc) on the roadways. When it rains, stormwater can pick up these pollutants and transport them via pipes to the Columbia Slough. Some stormwater pipes have water quality facilities that allow some of the pollutants to settle out of the water before it enters the slough.

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Biological Communities

Urban development in the Columbia Slough Watershed has resulted in extensive habitat loss and negative impacts to native fish and wildlife species. Native habitats have been removed and converted to urban uses, and corridors that connect habitat areas have been lost. The resulting fragmented habitats and impacted water bodies tend to favor non-native species and generalist native species that are tolerant of the disturbed conditions.

Non-native species often out-compete more sensitive native species for feeding and breeding habitats. They may also directly prey upon native species.  Several native species have become locally extinct as breeding species, and others are coming very close to local extinction.

During the breeding season, the Columbia Slough watershed hosts a number of species listed as threatened or endangered by the state and federal government, including the bald eagle, willow flycatcher, painted turtle, and western pond turtle. This is because the watershed’s remnant high-quality habitats are protected and managed for these species.

Endangered coho and Chinook salmon use the Lower Slough for refuge in fall, winter, and spring. This is because the Lower Slough provides suitable refuge conditions during these seasons: calm water, cool temperatures, shallow depth, and available food resources.  Anadromous salmonids are no longer present in the Middle and Upper Slough because levees and pump stations remove the connection of the Middle and Upper Slough to the Willamette and Columbia Rivers. This reduces the overall diversity of native fish species in the Columbia Slough.

The diversity of benthic macroinvertebrates (bottom-dwelling aquatic insects) is low throughout the Columbia Slough system, limiting the available food resources for other aquatic and terrestrial species. The low diversity is caused primarily by the abundance of fine sediments in the water, low nighttime oxygen levels, and impaired water quality.  Polluted sediments may also be a factor.

Most of the historic cold-water tributaries to the Columbia Slough have been piped or filled.  Cutthroat trout are currently present in only two of the remaining tributary streams, Fairview Creek and Osborn Creek.

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Water Quality Standards

The Columbia Slough is a highly managed, low-gradient, shallow body of water that lacks significant shading and has few cold water inputs. It has warm water temperatures (except in specific locations), excessive growth of algae and macrophytes (water vegetation), and wide pH fluctuations. The Slough also can have high levels of total suspended solids (TSS) —fine soil particles that are suspended in the water column.  These characteristics can be aesthetically displeasing, but are generally harmless to human health.

Eutrophication is the primary water quality issue for the Columbia Slough. Eutrophication is a natural process where nutrients and organic substances enter the system and increase biological productivity. Human sources of nutrients can greatly accelerate this process. The main problem with eutrophication is excessive algal and macrophyte growth. That causes wide variations in pH during spring, summer, and fall, which make it hard for aquatic species to survive.

Another effect of eutrophication is the level of oxygen dissolved in the water. Dissolved oxygen (DO) in the Columbia Slough would be expected to be low in summer because of warmer water temperatures and high in fall and winter because of cooler water temperatures. However, DO is in fact high in summer because macrophytes and algae add (pump) oxygen into the system. DO is low in fall and winter, when the vegetation dies off, and the Slough sometimes does not meet water quality standards. Low DO is one of the limiting factors for cold-water fish, including salmonids.

The Upper and Middle Slough usually meet water quality criteria for E. coli bacteria.  Since 2000 when combined sewer overflows were controlled, the Lower Slough has also generally met water quality criteria for E coli bacteria with a few exceptions. However, it is still advised that people wash their hands after recreating in the water.

Urban land use pollutants are transported to the Slough via stormwater. Motor vehicles contribute heavy metals (copper, lead and zinc), oil and grease, brake and tire dust, dirt and debris to the waterway. Other urban pollutants include fertilizers, pesticides, herbicides, and sediments.

TSS can cause turbidity and siltation that cause breathing problems in fish and limit the ability of macroinvertebrates (aquatic insects) to find food. They also reduce dissolved oxygen and decrease sunlight availability to aquatic life. Sources of TSS include sediments transported in stormwater from streets, parking lots, driveways, agriculture runoff, and construction activities.
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Sediment Quality

Sediments are soil particles, sand, clay, or other substances that settle to the bottom of a water body. Some sediments are generated naturally, such as through weathering of rocks and breakdown of vegetative debris. Others are from human activities related to industrial discharges, improper management and disposal of chemicals, land development or construction, soil erosion, transportation, and runoff from impervious surfaces.

Chemicals, such as heavy metals and toxic organic chemicals, bind to sediments. Some of these chemicals can leave the sediment particles, dissolve in water, and eventually become accumulated by biological organisms.

Because the Columbia Slough is a highly managed water body in an urban watershed, the sediments contain contaminants that are above DEQ’s screening levels. Extensive sampling of the Slough has revealed that there are widespread and somewhat elevated levels of contamination of both sediments and fish. 

Contamination levels are quite high in a few locations. In those areas, the Oregon Department of Environmental Quality (DEQ) is working with the responsible parties to determine the extent of contamination and clean it up.

Heavy metals (lead, zinc, chromium, copper) and toxic organic chemicals (pesticides, PAHs (polycyclic aromatic hydrocarbons), and PCBs (polychlorinated biphenyls) are common contaminants. The heavy metals most likely are from transportation-related activities.

Most of the toxic organic chemicals are too low to detect in sediments, but are found in fish tissues. These organic chemicals may be moving up the food chain from plants to insects to fish through a process called bioaccumulation.

The levels of pesticides and PCBs in fish pose a human health risk if a person eats fish from the Slough often and for many years.

Since 1994, the City of Portland (through a Consent Order with DEQ) has been conducting a remedial investigation and feasibility study to determine the extent of contamination and find ways to clean up the slough. The City and DEQ are adopting an approach that includes reducing pollutant sources, cleaning up specific sites, and long-term monitoring to track how the system is responding to watershed management actions.

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  • There are 30 miles of levees in the watershed.
  • The Lower Slough's 8.5 miles are tidal.
  • Portland's drinking water wells tap a deeper aquifer than the slough aquifer.
  • Low gradient stream.
  • Upper and Middle Slough flows are pumped.
  • Few natural springs, seeps and streams remain in the watershed.
  • Columbia and Willamette dams affect the Slough.
  • 54% imperviousness has changed Slough flow and volume.
  • Water rights requirements keep summer elevations high.
  • The Slough's topography and gradient are flat.
  • Three drainage districts provide flood control and water management services.

The Columbia Slough is a low-gradient waterway. The flat topography of the floodplain influences the slough’s appearance and character. However, human activities also affect the nature of the slough.

Flood control and hydroelectric facilities on the Willamette River and Columbia Rivers have greatly altered the slough’s historic character. Water control regiments on the rivers affect water flow and volume in both the tidal and non-tidal portions of the slough. 

Flood control levees and water level management in the slough also affect water flow and volume.  In addition, the piping of streams, springs, and seeps has limited cold water inputs to the slough.

Impoundments for surface water rights have altered the flow regimen in the Upper and Middle Slough.

Impervious surfaces cover 54 percent of the watershed. This has increased stormwater runoff volumes, decreased water quality, and changed the timing of peak flows. The removal of trees and other vegetation has further decreased stormwater infiltration and retention. These conditions have caused the Slough to evolve into a 'flashy' urban stream, where a surge of stormwater enters the slough within hours of rainfall, instead of over the course of days as would occur in undeveloped conditions.

The slough’s flat topography, changed hydrology, and highly managed system result in slow water movement. Sluggish water forms ponds behind culverts and levees. These physical conditions contribute to warm water temperatures and allow excessive growth of algae and macrophytes (water vegetation). The vegetation negatively impacts dissolved oxygen and pH levels in the water, with adverse effects on fish and other aquatic species.

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Public Involvement and Education

Public participation is a critical element of watershed restoration. The effective identification of problems, solutions, and opportunities depends on citizen input. 

  • Portlanders highly value education programs that increase waterway, watershed, and environmental awareness for all ages.
  • Stewardship activities promote awareness of watershed health and improve watershed conditions.
  • Partnerships enable the City to undertake watershed projects it is unable to conduct on its own.

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