"For decades, stormwater, or runoff, was considered largely a problem of excess rainwater or snowmelt impacting communities. Prevailing engineering practices were to move stormwater away from cities as rapidly as possible to avoid potential damages from flooding. More recently, these practices have evolved and come to recognize stormwater as a resource that, managed properly within communities, has multiple benefits. Stormwater problems occur because rainwater that once soaked into the ground now runs off hard surfaces like rooftops, parking lots, and streets in excessive amounts. This runoff flows into storm drains and ultimately into lakes and streams, carrying pollutants that are harmful to aquatic life and public health. Traditional approaches to managing urban stormwater have utilized so-called 'gray infrastructure,' including pipes, gutters, ditches, and storm sewers. More recently, interest has grown in 'green infrastructure' technologies and practices in place of or in combination with gray infrastructure. Green infrastructure systems use or mimic natural processes to infiltrate, evapotranspire, or reuse stormwater runoff on the site where it is generated. These practices keep rainwater out of the sewer system, thus preventing sewer overflows and also reducing the amount of untreated runoff discharged to surface waters."
CRS Report for Congress, R43131