Increased Runoff
Impervious surfaces are hard surfaces, such as sidewalks, streets, and rooftops, that don't allow water to seep into the ground. Water that does not soak into the ground becomes runoff and travels to the nearest body of water.
As the amount of impervious surfaces increases, more runoff is created and less water is able to sink in, or "infiltrate," into the ground. Infiltration is very important because water that travels slowly to creeks and streams helps sustain their water flows and aquatic life through drier spells. Water that travels slowly through the ground also gets filtered by natural processes before it reaches the water body.
Water that travels too quickly to creeks and streams can pick up and carry more sediment and other pollutants. It also hits creeks and streams in a rush, which worsens erosion and flooding.
Typical water cycle in an undeveloped area
Typical water cycle in an urban area
Source: www.nccwep.org
