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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/channel alteration

Intact riparian zones, or vegetated areas adjacent to stream channels, can serve several functions (Allan 1995), including:

  • Provide organic matter for stream food webs
  • Provide habitat (e.g., woody debris, bank vegetation)
  • Reduce bank and channel erosion
  • Moderate stream temperatures
  • Intercept and process groundwater nutrients and pollutants

Urbanization typically reduces the extent and quality of riparian areas, via the removal of native vegetation and the development of near-stream areas (Fig 4). These alterations can contribute to multiple instream stressors, including:

Water/sediment quality: ↓ nutrient uptake and retention, ↑ erosion of bank sediments (and associated contaminants)

Temperature: ↓ shading and thermal buffering

Hydrology: ↓ woody debris inputs, ↓ interception of surface and groundwater flows

Physical habitat: ↑ erosion of bank sediments, ↓ woody debris inputs

Energy sources: ↓ leaf inputs, ↑ algal biomass (due to ↓ shading), ↑ dissolved organic carbon

Photo by Bob Davis, courtesy of NOAA

Direct modification of stream channels is common in urban systems, and these direct alterations of channel morphology often are the most damaging changes urban streams experience (see the Physical Habitat module, as well as the Physical Habitat section of this module).

Typical channel alterations in urban streams include:

  • Channelization (i.e., channel straightening)
  • Channel hardening or armoring (e.g., lining channels and banks with concrete and riprap)
  • Creation of dams and impoundments
  • Stream piping and burial

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Removal of riparian vegetation and channel hardening in an urban stream.
Courtesy of U.S. EPA.
Figure 4. Spearman’s rank correlations between riparian urbanization (building area within 250 m radius of stream site) and riparian vegetation characteristics, at 71 sites near Cincinnati, Ohio. Many of these characteristics (e.g., riparian tree density and cover) showed negative relationships with urbanization.
From Pennington DN et al. 2008. The conservation value of urban riparian areas for landbirds during spring migration: land cover, scale, and vegetation effects. Biological Conservation 141:1235-1248. Reprinted with permission from Elsevier.

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