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In some of the older sections of Corvallis, sanitary wastewater and stormwater are collected in the same pipe. During wet weather, these combined flows can exceed pipeline and wastewater treatment plant capacity. This results in the overflow of a mixture of untreated wastewater and stormwater (called "combined sewer overflows" or "CSOs") into the Willamette River. The City undertook a program to change this practice in order to respond to water quality issues and federal/state requirements that Corvallis remediate CSOs by December 31, 2001. 

Helping to prevent sewer overflows and backups is easy

Most of the time, the City's sewer pipes have adequate capacity to carry wastewater to the treatment plant. But during heavy, extended raining periods when the ground water is high, the capacity of some sections of the sewer system is exceeded. Ground water or surface water will enter older sewer pipes through cracks and, added to the wastewater flow, will exceed the space available in the pipe. When these conditions occur the water has to go somewhere. If it can't flow down the main pipe, it will back up into the service line and, possibly, all the way into the basement.