Research has shown that as impervious surfaces such as parking lots, roof tops, streets, etc reach 10% stream ecosystems begin to show evidence of decline. As impervious surfaces reach 30% the degradation to the stream ecosystem becomes much more severe and practically irreversible.
The first flush of stormwater is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. These first flush waters may have a level of contamination much higher than normally present in sewage. Consequently these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.
For a great National Geographic article on urban sprawl in Cincinnati please click here