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Columbia Slough Watershed Council

State of the Slough

State of the Slough

The Columbia Slough has had a long history of environmental decline. But with the elimination of Combined Sewer Overflows (CSOs) in 2000, watershed wide efforts to revegetate the Slough with native plants, and the increasing awareness of businesses and residents, the Slough is cleaner today than it has been in over 100 years.

For more information about Oregon Department of Environmental Quality projects in the slough, please visit their website.


Combined Sewer Overflows (CSOs) occur in some areas of the City where the stormwater sewer and sanitary sewer are combined in one pipe - a combined sewer system. When it rains, combined sewer pipes fill up with both stormwater runoff and sewage. Once the pipes fill to capacity, the excess combined sewage overflows into the local river or slough.

The City of Portland used to have 13 CSO outfalls that discharged combined sewage into the Lower Columbia Slough. But in October 2000 the Bureau of Environmental Services completed its “Big Pipe” project and sewage stopped flowing in to the Columbia Slough for the first time in a century. The elimination of CSOs from the Slough has greatly increased the health of the waterway. Local agencies continue to monitor the Slough to measure the improvements of water quality since the elimination of CSOs from the Slough.

Fish Advisory

Columbia Slough Fish may be hazardous to your health. Fish in the Columbia Slough contain PCBs and pesticides. These chemicals may effect human development, reproduction and immune systems. These chemicals may also increase your chance of getting cancer.

People most at risk include: unborn babies, pregnant and nursing mothers, children and people who eat Slough fish often and for many years. Even though the concentrations of PCBs and pesticides in Slough fish are fairly low, they still pose a health risk because these chemicals increase in the body and my cause health problems many years after eating the fish.

PCBs and pesticides accumulate in the fatty parts of fish over the life of the fish. If you do choose to eat Columbia Slough fish, to reduce health risks you should avoid eating the fatty parts or the guts. Bake or broil the fish fillet on a rack so that the fat drips off, and then do not eat the fat drippings.

Water Quality

TMDL is a term from the Clean Water Act that stands for Total Maximum Daily Load. It is an estimate of how much of a particular pollutant a water body can absorb before it becomes a problem. TMDLs are established by Oregon DEQ for every water body that is listed in the Clean Water Act as a water-quality-limited water body .

In 1998 TMDLs were established for the Columbia Slough for pH, dissolved oxygen, phosphorus, chlorophyll-a, bacteria, lead, DDE/DDT, PCBs, dieldrin, and 2,3,7,8-TCDD. DEQ established the temperature for the Willamette Basin (including the Columbia Slough) in 2006.