An increase in the amount of impervious surfaces within a watershed increases both the volume of stormwater and the speed at which that water reaches the stream. Because impervious surfaces prevent or inhibit the flow of water into the earth, more water is left on the surface as runoff.  Impervious surfaces are typically also much smoother than natural pervious surfaces, and without impedance the runoff quickly flows into storm drains and then into the streams.  The combination of an increased stormwater volume rapidly entering into the stream causes stream erosion as evidenced by channel widening and down-cutting.  This in turn leads to loss of trees, loss of in-stream habitat, damage to infrastructure (particularly sewer mains which may then leak untreated sewage into the stream), and in some cases loss of private property as streams erode into property lines.  Additionally, because impervious surfaces block rainfall from entering into the groundwater table, streams receive minimal groundwater recharge.  This leads to very low water levels between rainfall events, when naturally the water levels would have been maintained by groundwater.

Increased impervious surface area also decreases the water quality of the stormwater reaching the stream.  Runoff from these areas is much more likely to carry pollutants such as oil, grease and grit from roadways, fertilizers, herbicides and pesticides from lawns, and bacteria from pet waste.  The temperature of impervious surface runoff is also generally increased relative to runoff from natural surfaces.  Lastly, increased erosion leads to increased sediment loads in the streams.  All of these confounding factors lead to streams impaired in their ability to support life, and unhealthy conditions for humans and wildlife interacting with the streams