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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

Riparian zones & channel morphology

Forested riparian zones play a key role in determining stream channel morphology. Their root structures can help stabilize streambanks, and the woody debris they contribute to streams can protect banks by absorbing flow energy.

Because urbanization often results in riparian alteration, it is difficult to separate the effects of general watershed urbanization (e.g., increased stormflows) on channel morphology from those of riparian alteration. Hession et al. (2003) tackled this issue, using a paired design that considered forested and nonforested riparian reachs on both urban and nonurban streams. They examined the effects of urbanization and riparian vegetation on channel morphology in 26 unchannelized mid-Atlantic streams (Fig 5), and found that:

  • Urban streams were generally wider than nonurban streams, especially for smaller streams.
  • Forested urban streams were generally wider than nonforested (i.e., grassed) urban streams.
  • Differences between forested and nonforested reaches (i.e., the vertical arrows in Fig 5) were generally similar for urban and nonurban streams—illustrating that even in urban systems, riparian vegetation influences channel morphology.

In extrapolating these results to other sites, however, keep in mind that relationships between riparian alteration and channel morphology in urban streams depend upon numerous other factors, including stream size, stream gradient, surrounding geology, and riparian vegetation type.

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Stream with reduced riparian tree cover and an eroded bank.
Courtesy of U.S. EPA.
Click here for larger version.
Figure 5. Bankfull width in urban and nonurban streams, with forested and nonforested riparian reaches, as a function of drainage basin area. Vertical arrows indicate the effect of riparian vegetation on bankfull width in urban and nonurban streams.
From Hession WC et al. 2003. Influence of bank vegetation on channel morphology in rural and urban watersheds. Geology 31(2):147-150. Reprinted with permission.

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