Flow alteration in urban streams

Alteration of natural hydrologic regimes is a consistent and pervasive effect of urbanization on stream ecosystems, as discharge patterns—the amount and timing of water flow through streams—change with urban development. Key aspects of urbanization affecting hydrology may include:

• ↓ infiltration and ↑ surface runoff of precipitation associated with impervious (and effectively impervious) surfaces
• ↑ speed and efficiency of runoff delivery to streams, via stormwater drainage infrastructure
• ↓ evapotranspiration due to vegetation removal
• ↑ direct water discharges, via wastewater and industrial effluents
• ↑ infiltration due to irrigation and leakage from water supply and wastewater infrastructure
• ↑ water withdrawals and interbasin transfers

Commonly reported effects of urbanization on stream flow regimes include (but are not limited to):

STORMFLOW

• ↑ high flow frequency (Fig 33)
  [Roy et al. 2005, Schoonover et al. 2006, Brown et al. 2009]
• ↑ high flow magnitude (Figs 33 and 34)
  [Rose & Peters 2001, Burns et al. 2005, Schoonover et al. 2006]
• ↑ flashiness or rapidity of flow changes (Fig 33)
  [Roy et al. 2005, Schoonover et al. 2006, Chang 2007]
• ↓ high flow duration
  [Rose & Peters 2001, Poff et al. 2006, Chang 2007]
• ↓ lag time (Fig 34)
[Arnold & Gibbons 1996, Changnon & Demissie 1996]

BASEFLOW

• ↓ low flow magnitude (Fig 34)
  [Finkenbine et al. 2000, Rose & Peters 2001, Kaufmann et al. 2009]
• ↑ low flow magnitude
  [Burns et al. 2005, Riley et al. 2005, Poff et al. 2006]
• ↑ low flow duration (Fig 34)
  [Roy et al. 2005, DeGasperi et al. 2009]

These hydrologic changes can reduce habitat quality in urban streams, and adversely affect stream biota. For example, high flows can scour organisms and substrate from streambeds, while low flows can reduce habitat area and volume. See the Flow Alteration and Physical Habitat modules for further details on biotic responses to these changes.

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