The District of Columbia is one of 772 older cities in the country with a combined sewer system. The system covers about a third of the city, and was built in the late 19th century to carry both sanitary sewage and storm water in the same pipe. The system operates well in dry weather. However, during rainstorms, the flow can exceed the capacity of the pipe. To prevent sewer backups and flooded streets, these combined sewers may discharge into the Anacostia and Potomac Rivers and Rock Creek, a phenomenon known as combined sewer overflows (CSOs).