The New York City Department of Environmental Protection (DEP) is undertaking a number of projects aimed at improving water quality in the Gowanus Canal. Several of these projects are intended to reduce combined sewer overflows (CSOs) into the canal. CSOs are prone to occur during heavy rains when stormwater overwhelms the capacity of the local sewer system.
Because Prospect Heights is part of the watershed for the Gowanus Canal, DEP plans to install a network of bioswales to absorb stormwater. Bioswales are tree pits with a curb cut to absorb runoff.
The map above (full PDF version here) shows the locations of approximately 40 bioswales to be created in Prospect Heights (marked by green dots). Some locations in the neighborhood have been painted with green markings, as shown in the photo below, taken on St. John's Place between Underhill Avenue and Plaza Street.