Definition: Urbanization refers to the development of roads, buildings and other infrastructure that converts land uses such as agriculture, pasture and forest to human infrastructure uses that are typically a high percentage of impervious surface. As populations continue to grow and urbanization increases, the tendency in the country is to build out, not up, and with such a strategy, formerly rural or pristine areas are being paved over at remarkable rates. Every day the United States is losing approximately 3000 acres of productive farmland and rural areas to sprawling development. There are various theories (socioeconomic, educational, pollution, density, poor zoning) for the reasons for sprawl, but ensuing damage to rivers and streams is well documented.
Stormdrains funnel precipitation directly into streams, increasing flooding rates and transporting pollutants from roads and parking lots into streams. Credit: Nat Gillespie
Actual Ecosystem Stress: Development results in more pavement, increased water consumption, creation and movement of wastewater, increased traffic, and a variety of other ecologically damaging consequences. Runoff pollution from pavement and other impervious surface (rooftops and compacted lawns) is now the country's number one threat to water quality, degrading approximately 40% of the nation's rivers and streams. This runoff (stormwater) represents precipitation (rainwater and snowmelt) that, in a rural setting, would normally filter into the ground and reach waterways gradually as shallow or deep groundwater.
In a developed area, this precipitation instead is trapped above impervious surfaces and is routed off quickly into surface waterways, often picking up various pollutants (road oils, herbicides, pesticides, sediment) along the way. This increase in surface runoff leads to an increase in the velocity and volume of surface waters, which increases the flood peaks and flood occurrence. Increased runoff corresponds with increased stream power, so stream banks become eroded and stream channel bottoms become incised (cut down) as the stream channel enlarges to accommodate larger water volumes. The result of these changes to the stream channel are increased sedimentation and siltation, which buries aquatic macroinvertebrate habitat and kills fish eggs, as well as decreased groundwater recharge as the water table is lowered by stream incision. (See watersheds and hydrology)
Stream above incised many feet because of increased stream power caused by development upstream. Credit: Mark Hudy, USFS
Recent studies indicate that degradation such as this begins to occur when impervious surfaces cover 10% of a watershed. Above this percentage, fish species start to die off. Brown trout and brook trout generally are decimated at levels of 10-12% imperviousness. At 30% impervious watersheds are generally are considered severely degraded. These levels are easily met by most commercial and residential subdivision developments.
It is important to realize that while grass or lawn is not impervious, it absorbs less precipitation than a forest, natural grass meadow or some agricultural crops. As rain drops fall at terminal velocity and strike the ground directly, they compact the soil underneath the lawn. The compacted soil quickly becomes saturated with water, which then has less of a chance to infiltrate into the subsurface soil. Precipitation then quickly travels as overland runoff into the nearest drainage sewer or stream.
The problems caused by runoff are further exacerbated by the increase in water consumption (and lower streamflow) corresponding to development and the movement of wastewater around or completely out of a watershed. The ecological problems from water consumption, formerly limited to the arid west, are now prevalent throughout the water-rich northeast. Hundreds of rivers, and especially their delicate headwater areas, are now drying up every summer due to overdevelopment, excessive lawn watering, irrigation for golf courses, and other recreational purposes.
Measures to Stop or Mitigate Threat:
Measuring Impact of Development of Maine Surface Waters, Senator George J. Mitchell Center for Environmental and Watershed Research