Portland’s first sewers were made of wood and installed in an effort to keep the growing city’s dirt streets from turning to mud during wet weather. They drained into the nearest body of water, usually the Willamette River. As the city grew and indoor plumbing became more common, sanitary waste was directed into the existing sewers. For nearly a century the City’s wastewater flowed directly into the Willamette River and Columbia Slough. By the 1920s they were so badly polluted that mill workers refused to handle logs pulled from their waters, but it was 1938 before voters approved measures to provide sewage treatment.
In 1947 the first interceptor sewers were built to carry dry weather flows to a new sewage treatment plant in North Portland. Running parallel to the Willamette River and Columbia Slough, these large diameter sewers ‘intercepted’ sewage using a diversion dam placed in the existing sewer just upstream from the outfall. The interceptors and treatment plant could safely handle up to three times the average dry weather flow, but were designed to allow higher flows from more significant rainfall events to overflow into the River and Slough. These overflows of combined sanitary and stormwater flows are called combined sewer overflows or CSOs.
Combined sewers transport sanitary flows as well as urban runoff from roofs, streets, and other impervious surfaces. As Portland grew, the volume of runoff increased, and by the late 1980s CSO volume reached an estimated 6 billion gallons annually, about 20 percent of which was sanitary sewage. (BES, 1994) However, the impacts on water quality from urban stormwater were (and continue to be) significant.
Most areas of Portland developed before the 1960s were served by combined sewers until the mid-1990s, with the exception of the central business district. Additional impervious areas and sanitary loads resulting from increased development caused more frequent CSOs, and sewer separation began in the 1970s. Parts of southwest, southeast, and northeast Portland that were either developed after the 1960s or annexed into the City have always been served by separate sewer systems.
Under an agreement with DEQ in 1991, the City began work to control CSOs, and some north and southeast Portland neighborhoods within the combined sewer area have been separated into sanitary and stormwater systems. The city completed construction in 2011 on large tunnels that capture the remaining combined flows for treatment. The city's CSO control program reduced Willamette River CSOs by 94%.