CSOs are Combined Sewage Overflows, which occur when enough storm water (rainfall or snowmelt) enters a community's combined sewer system - in combination with the household and industrial sewage also flowing in these pipes - that it overloads a sewage treatment plant's capacity, and then by design overflows the excess directly into waterways.  As little as 1/10th of an inch of rainfall can trigger a CSO, and about half of all rainfalls in New York City trigger CSOs, which add up to 27 billion gallons of this raw sewage and storm water cocktail dumping into our waterways every year.  That's enough to fill a ten-foot diameter pipe from Times Square to Bangkok...surf's up!