Impervious surfaces, such as sidewalks, restrict infiltration. In this photo, water from a recent storm will largely be lost to evaporation instead of being used by plants or recharging the groundwater table.

Perhaps the most notable effect of an increasing urban footprint is the resulting stormwater runoff.  Concrete, asphalt, and other impervious surfaces restrict infiltration of water, thus reducing groundwater recharge and increasing downstream flooding and erosion.  Stormwater is typically treated as a pollutant and shed from an urban surface as quickly as possible.  Conveyance of stormwater usually occurs along gutters or through storm drains until it can be disposed of in arroyos or river channels.  This conventional method of addressing stormwater can lead to channel incision and downstream flooding due to the discharge of water exceeding channel dimensions.

Detention Basins

Occasionally excessive runoff from impervious surfaces is diffused by transporting it to detention basins.  A detention basin is a large pit, often lined with rock cobble and placed near a drainage point in a parking lot.  While detention basins can capture a large volume of runoff, they are generally devoid of vegetation, create drowning dangers, and provide breeding grounds for mosquitoes.

Despite holding large volumes of stormwater, detention basins can inundate vegetation and present mosquito problems.