Combined sewer overflows, or CSOs, are remnants of the country's early infrastructure. In the past, communities built sewer systems to collect both stormwater runoff and sanitary sewage in the same pipe. These systems transport wastewater directly to the sewage treatment plant during dry weather. In periods of rainfall or snowmelt, however, the wastewater volume can exceed the capacity of the sewer system or treatment plant. For this reason, the systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams or rivers.