Urbanization - Riparian/Channel Alteration
- 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 (see Figure 4).
These alterations can contribute to multiple instream stressors, including:
Water/sediment quality: decreased nutrient uptake and retention, increased erosion of bank sediments (and associated contaminants)
Temperature: decreased shading and thermal buffering
Hydrology: decreased woody debris inputs, decreased interception of surface and groundwater flows
Physical habitat: increased erosion of bank sediments, decreased woody debris inputs
Energy sources: decreased leaf inputs, increased algal biomass (due to decreased shading), increased dissolved organic carbon
Direct modification of stream channels is common in urban systems. These direct alterations of channel morphology often are the most damaging changes urban streams experience (see the Physical Habitat module and the Physical Habitat section of this module).
- 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
Riparian Zones and 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.
- 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 Figure 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. These factors include stream size, stream gradient, surrounding geology, and riparian vegetation type.
Urbanization and Riparian Hydrology
Increased stormwater flows associated with urban development can scour stream channels and increase channel incision. This is especially true 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 (see Figure 6), thereby altering riparian hydrology.
- 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 (Figure 7).
- 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
- Drainage density of natural channels was approximately ⅓ less in urban and suburban vs. forested catchments in Atlanta, GA (Meyer and Wallace 2001).
- Approximately ⅔ of all streams were buried in Baltimore City, MD (Elmore and Kaushal 2008).
- 93% of ephemeral channel length and 46% of intermittent channel length were lost to burial and piping associated with urbanization in Hamilton County, OH (Roy et al. 2009, Figures 8 and 9). As a result, drainage areas for remaining ephemeral and intermittent channels were larger in urban areas.
Interestingly, Roy et al. (2009) found that perennial channel length actually increased with urbanization (see Figure 8), although approximately 40% of perennial channels originated from pipes. This increase in perennial channel length was due at least in part to increased baseflow stemming from reductions in forest cover and evapotranspiration.