Building out traditional "gray" infrastructure, like pipes and underground storage tanks, to handle the excess water can be hugely expensive – and we often outgrow them as the population increases. City officials in New York, Philadelphia, and dozens of cities around the country are finding that green infrastructure, which absorbs rain before it reaches the sewer system (lessening the load it needs to process so there are fewer overflows), can be a more cost-effective solution that not only slashes water pollution, but also provides flood protection, beautifies communities, improves air quality, and cuts energy costs. (Many of these cities are profiled in NRDC’s recent report, Rooftops to Rivers II: Green strategies for controlling stormwater and combined sewer overflows.) Green infrastructure also holds tremendous potential to augment over-stressed sources of municipal water supply, and can even be a driver for private investment in urban revitalization efforts.