About 98 times a year, the District gets enough rain that its aging combined sewer system overflows, dumping roughly 2.5 billion gallons of wastewater in the Anacostia and Potomac rivers, as well as into Rock Creek. But today, D.C. Water broke ground on a $2.6 billion project that will all but eliminate the overflow, making area rivers and the Chesapeake Bay significantly cleaner in the process.