Cities are full of impervious surfaces like roof tops, sidewalks, and roadways that prevent rain water from soaking into the ground. Rain flows over streets picking up grease, oil, metals, and other contaminants and then flows into our rivers and the Sound untreated. In cities like New Haven, where stormwater and sewage travel through the same pipes called combined sewers, which are designed to overflow, even small storms can cause untreated effluent to overflow into rivers and the Sound.