1.2.1 Hydrologic Effects of Urbanization

Changes in the landscape as a result of urbanization may affect the natural hydrology by:
  • Increasing the peak flow rates of runoff.
  • Increasing the total volume of runoff.
  • Decreasing the time it takes for runoff to reach a natural receiving water.
  • Increasing stream velocities.
  • Reducing groundwater recharge.
  • Increasing the frequency and duration of high stream flows.
  • Altering the frequency and extent of wetland inundation and saturation.
  • Reducing stream flows and wetland water levels during the dry season (Ecology, 2004).

Figures 1.2 and 1.3 illustrate a relatively natural hydrologic condition compared with that of an urbanized one. As a consequence of these cumulative changes in hydrology, stream channels may experience both increased flooding and reduced base flows. Natural riffles, pools, gravel bars, and other areas may be altered or destroyed. Increased channel erosion, loss of hydraulic complexity, degradation of habitat, and changes in the composition of species present in receiving waters may follow. Table 1.1 provides a summary of typical watershed impacts resulting from urbanization and stream channel responses.


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Figure 1.2
Natural Water Cycle
Source: AHBL, Inc.

Figure 1.3
Urban Water Cycle
Source: AHBL, Inc.