Urban runoff is a major source of water pollution. A large portion of city surfaces are impervious. Rain water, rather than being absorbed into the ground, runs along these surfaces picking up pollutants before draining into storm sewers.
Rain gardens or swales can be an elegant solution for managing runoff, cutting down the amount of pollution reaching nearby waterways, and beautifying the streetscape.
Thanks to @M2JL on Twitter for submitting the idea