The Columbia Slough watershed is located in north and northeast Portland, Gresham and Fairview, just south of the Columbia River (see Figure 1). The watershed covers roughly 40,000 acres of land and is home to about 63,000 people. The Columbia Slough waterway begins at Fairview Lake and flows west for approximately 18 miles to Kelley Point Park where it empties into the Willamette River. The lower portion of the Slough, from about NE 13^th to the Willamette, is affected by tides and during high tides the water in this portion of the Slough backs up.

The Columbia Slough watershed used to be part of the Columbia River’s floodplain and, historically, a portion of the watershed flooded seasonally. Beginning in the 1920’s, levees were constructed to protect homes, farms and businesses from the flooding. Also, pumps were installed to move the stormwater from the watershed to the Willamette and Columbia River. The levees and pumps created opportunities for more development and today the Columbia Slough watershed is the most industrialized watershed in Oregon.

The Columbia Slough is an 18-mile channel that parallels the Columbia River. The Columbia Slough is divided into two sections, but can be managed as three. The Upper Slough starts at Fairview Lake and flows to the west for 13 miles to NE 18^th. There is a levee at roughly NE 142^nd that can be closed, creating a Middle Slough from NE 142^nd to NE 18^th. The Lower Slough begins at NE 18^th and flows for about 8 miles to the Willamette River.

Within the Columbia Slough watershed there are three drainage districts, which are managed by the Multnomah County Drainage District #1 (MCDD). Multnomah Drainage District # 1 starts at NE 223^rd and continues to NE 18^th, this is the Upper Slough. Portland City Limits end at NE 185^th, therefore 25% of MCDD is outside of City of Portland’s jurisdiction. The Cities of Troutdale, Fairview, Gresham, and Wood Village, and Multnomah County have overlying jurisdiction east of 185^th. Peninsula Drainage District #2 (Pen 2) begins at NE 18^th and continues to Interstate-5. Peninsula Drainage District #1 (Pen 1) begins I-5 and continues to approximately Portland International Racetrack. Both the Pen 1 and 2 are within the Lower Slough.

The Columbia Slough historically was part of the floodplain for the Columbia River and flooded frequently. The Lower Slough is also tidally affected and during high tides the water in both the Willamette and Columbia Rivers backs up and can prevent water from flowing out of the Slough. The elevation of the Upper Slough ranges from 5 to11 feet above mean sea level and the Lower Slough can range from elevation 4 to 30 feet. (The maximum elevation of 30 feet would occur during a 100-year flood event.)

In 1919, levees were constructed to reduce flooding and allow reclamation of land for agriculture and development of the watershed. The average water level elevation, within these levees, is lower than the Willamette and Columbia Rivers. Due to this elevation difference, stormwater pumps were installed to evacuate storm and groundwater from the drainage basins and pump it over the levees into the Lower Columbia Slough and the Columbia and Willamette Rivers. Because the levees and pumps provided protection from flooding, more land could be developed. Today there are about 27 miles of levees in the Slough. In the winter, MCDD manages the Upper Slough at a water elevation of 6½ to 7 feet above mean sea level. In the spring it is managed at an elevation of 8 to 8½ feet and in the summer it is managed at an elevation of 5 to 5½ feet, to reflect low-flow conditions.

The Columbia Slough is a 19 mile long complex of narrow and shallow channels located on the southern floodplain of the Columbia River between Fairview Lake and the Willamette River. The Slough was originally a series of wetlands and marshes; it is now a highly managed water system with dikes and pumps to provide watershed drainage and flood control for the lowlands surrounding it. The management of the Slough can have a significant impact on the water quality and uses supported by the Slough.

The Slough drains approximately 40,000 acres of land. Many kinds of land use are found within the watershed including heavy and light industries, residential areas, vegetable farming and the Portland International Airport. The Slough also serves as one of the City’s largest open space and wildlife habitat areas.

The Columbia Slough, from the mouth to Fairview Lake, has been placed on DEQ’s 1994/1996 303(d) list for multiple parameters. The Slough is water quality limited for chlorophyll a, pH and phosphorus from spring through fall, because of algal growth. This algal growth affects the aesthetic quality of the Slough and may affect such beneficial uses as fishing and boating. The dissolved oxygen criteria for cool water aquatic life is violated throughout the year; diurnal swings in dissolved oxygen during the summer months are most likely the result of algal growth, while winter violations are likely due to storm water runoff, including de-icing fluid. These dissolved oxygen criteria violations may prevent the Slough from supporting salmonid fish rearing as well as resident fish and aquatic life. Elevated bacteria levels through all seasons impact the water contact recreation use (swimming). In the spring through fall, high temperatures are also a concern, affecting salmonid fish rearing.

The Slough is water quality limited for DDE, DDT, PCBs and dioxin due to elevated levels found in fish tissue, impairing the use of the Slough for fishing. The State of Oregon Health Division and the City of Portland have issued recommendations against eating fish from the Slough due to contamination by PCBs, DDE and DDT. Review of data also indicates that dieldrin is present in fish tissue at elevated levels. The Slough is water quality limited for lead (Pb) because of levels above the fresh water chronic criteria for the protection of aquatic life.

The affected beneficial uses for the Columbia Slough, as identified in Oregon Administrative Rules, include salmonid fish rearing, resident fish and aquatic life, fishing, hunting, boating, water contact recreation and aesthetic quality.

The Slough has been divided into five reaches, based primarily on hydraulic characteristics. The reaches of the Slough are generally shallow with variable widths.

The Lower Slough (Reach 1) extends from the Willamette River to the Multnomah County Drainage District Pump Station No.1 at NE 13th Avenue (MCDD1) and includes the North Slough. The Lower Slough is tidally influenced, so the water quality in the Lower Slough is heavily influenced by that in the Willamette River. At MCDD1 there is a dike that physically separates the Lower and Middle Sloughs. Potential sources of pollutants to Reach 1 include sediments, combined sewer overflow events, groundwater, storm water, leachate from St. Johns landfill, industrial discharges, and water from Reach 2 and the Willamette River.

The Middle Slough (Reach 2) extends from MCDD1 to a cross levee (mid-dike) which has slide gates that can hydraulically isolate flows between the Middle and Upper Sloughs. About half the annual flow in the Middle Slough is due to groundwater. Potential sources of pollutants to Reach 2 include sediments, groundwater, storm water, illicit discharges, and contamination from the NuWay Oil Site and water from Reach 3.

The Upper Slough (Reach 3) extends from the mid-dike to the outlet of Fairview Lake. The Upper Slough receives considerably less groundwater than the Middle Slough. West of Four Corners, the Slough is subject to reversal of flows due to the operations of the Multnomah County Drainage District Pump Station No.4 (MCDD4) located on Marine Drive. MCDD4 discharges directly to the Columbia River. The arm of the Slough to MCDD4 often has little or no flow exchange with other portions of the reach. During the summer it is basically a stagnant waterbody. The Middle and Upper Slough are not driven by tidal influences or affected by CSO discharges. The system hydraulics are influenced by the MCDD pump stations and inflow of groundwater. Potential sources of pollutants include sediments, groundwater, storm water, industrial discharges and water from Reach 4.

Fairview Lake (Reach 4) is a shallow lake that covers about 105 acres. The lake is not considered part of the Slough, but contributes to the flow of the Slough. During the summer months, however, flow from Fairview Lake to the Upper Slough is negligible compared to the flow from groundwater (CH2MHill, Part 2, 1995). The flow from Reach 5 (Fairview Lake drainage basin, not pictured) includes the tributaries of Fairview Lake, which are composed of Fairview Creek, Osborn Creek, and No Name Creek. This reach is not part of the Slough, but it discharges to Fairview Lake.

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