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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

Urbanization & riparian hydrology

Increased stormwater flows associated with urban development can scour stream channels and increase channel incision, especially in systems with limited sediment inputs (e.g., highly impervious watersheds, which often occur in older urban areas).

Channel incision and reduced infiltration (again, due to impervious surfaces) act to lower riparian water tables (Fig 6), thereby altering riparian hydrology. For example, Hardison et al. (2009) examined six Coastal Plain streams in North Carolina, ranging from 3.8-36.7% catchment impervious area. They found that:

  • Channel incision increased with total impervious area (TIA)
  • The duration of shallow riparian groundwater throughout the year decreased as TIA increased
  • Sites with higher TIA had greater depths to riparian groundwater (Fig 8)

This “urban riparian drought” can have significant repercussions for the structure and function of riparian areas (Groffman et al. 2002, 2003; Hardison et al. 2009), including:

  • Shifts in riparian vegetation from wetland to upland species, or from diverse to limited size distributions
  • Changes in nitrogen uptake and cycling, such that urban riparian areas may be sources of, rather than sinks for, nitrate

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Figure 6. Cross-sectional view of typical groundwater tables (dotted lines) in (a) rural and (b) urban streams underlain by a shallow confining unit.
Figure 7. (a) Mean riparian zone groundwater depths, June 2006-June 2007, for six sites varying in catchment impervious area (rural = 3.8-12.4% total impervious area, urban = 22.1-36.7%). (b) Half-hourly riparian zone groundwater depths, over the same period, at the most rural (Phillippi) and most urban (Fornes) sites.
Figures 6 and 7 from Hardison EC et al. 2009. Urban land use, channel incision, and water table decline along Coastal Plain streams, North Carolina. Journal of the American Water Resources Association 45(4):1032-1046. Reprinted with permission.

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