Reducing CSO volumes In older parts of Seattle, pipes that carry both stormwater and wastewater can reach capacity during heavy rain, causing what’s called a combined sewer overflow, or CSO, to discharge into local water bodies including the Duwamish River. The system was long ago designed this way to prevent damage to facilities and protect homes and businesses from being flooded with stormwater and sewage during storms. 
This graph shows the amount of CSO reduction from the many control projects King County has completed.
A map of recommended CSO control projects as presented in the plan.
While CSOs are about 90 percent stormwater, uncontrolled CSOs can contribute to degraded water quality and public health risks. Since 1990, King County has reduced CSO volumes in the Lower Duwamish Waterway by more than 90 percent. With five of 10 Lower Duwamish CSOs controlled as of 2007, King County's CSO program is now focusing on the remaining Duwamish CSOs, with planning targeting the beginning of major work in 2012. King County is proposing a plan that would invest $500 million to control remaining Lower Duwamish CSOs by 2025. Selected King County studies and researchKing County monitors existing conditions of the Duwamish River and Elliott Bay both with CSOs and the conditions of those water bodies if CSOs were eliminated. Conclusions are presented in the Combined Sewer Overflow Water Quality Assessment Study. | 
