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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

Channel enlargement with urbanization

Two key changes drive stream channel alterations in urban systems:

  • ↑ sediment supply initially, followed by
    ↓ sediment supply over time
  • ↑ sediment transport capacity (i.e., stream discharge)

Early in urban development, soil disturbance commonly increases sediment supply and leads to channel aggradation (Wolman 1967, Chin 2006). Once development is more established, imperviousness and stream discharge commonly increase and sediment supply decreases, leading to channel degradation or incision (Wolman 1967, Chin 2006).

Photo courtesy of Susan Cormier

Thus, streams in urban catchments tend to widen and deepen. Trimble (1997) observed this process in Borrego Canyon Wash, CA (Fig 40), where erosion rates downstream of an urbanizing area were 20 m3 m-1 yr-1, versus 0.47 m3 m-1 yr-1 at a less urbanized site. In lowland streams of western Washington, Booth & Jackson (1997) found that channels generally exhibited stability thresholds (below which there was little or no bed and bank erosion) at 10% effective impervious area, or at increased discharge such that 10-year discharge in a forested catchment equaled 2-year discharge under current catchment land use (Fig 41).

Channel enlargement is common but not universal in urban streams. Whether channel enlargement occurs can depend on several factors (Bledsoe & Watson 2001, Chin 2006, Colosimo & Wilcock 2007), including:

  • Age and extent of urban development
  • Riparian condition
  • Connectedness of impervious areas and conveyance of stormwater to channel
  • Degree of channel entrenchment
  • Erodibility of bed and bank material

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Figure 40. Surveyed stream channel cross-sections taken downstream of an urbanizing area on Borrego Canyon Wash, CA.
From Trimble SW. 1997. Contribution of stream channel erosion to sediment yield from an urbanizing watershed. Science 278:1142-1144. Reprinted with permission from AAAS.
Figure 41. Observed stable and unstable channels, plotted by % effective impervious area in catchment and magnitude of simulated flow increases (ratio of modeled 10-year forested to 2-year current or urbanized discharges).
From Booth DB & Jackson CR. 1997. Urbanization of aquatic systems: degradation thresholds, stormwater detection, and the limits of mitigation. Journal of the American Water Resources Association 33(5):1077-1090. Reprinted with permission.

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