Chapter 5: Biological Processes in Riparian Areas

Spatial Diversity

  1. Longitudinal
    • Longitudinal diversity occurs along a riparian area from the headwaters to the mouth of a channel, or along a specific reach of interest.
    • The nature of the riparian area varies as one moves in the up- or down-stream direction because of:
      • Elevation/climate change.
      • Transition from narrow to broad valley channel  morphologies.
      • Changes in stream power associated with running water.
    • These changes drive differences in biological communities along the riparian area.

      Figure 5.4 Longitudinal view of a riparian system note the change in species, geology and movement of materials.


    • In broad valley systems:
      • The active channel is wider and slopes are lower resulting in lower stream power.
      • Floodplains in these reaches are frequently wide, and contain complex geomorphic surfaces with secondary channels and soils or substrates of widely varying texture.
      • Some low areas of the floodplain, such as channel cutoffs, may develop hydric soils, if saturated, hydraulic conductivity is low and biological demand for oxygen is high.
      • Vegetation in broad valley systems includes:  
        • Shallow-rooted herbaceous hydrophytes such as cattail (Typha spp.), bulrush (Schoenoplectus spp.), spikerush (Eleocharis spp.) knotgrass (Paspalum spp.) and Carrizo (Phragmites spp.) that tolerate wet, anoxic environments.
        • Tree species such as willow (Salix spp.) and cottonwood (Populus spp.) are typically found on the floodplain sites that do not experience prolonged anaerobic conditions.
        • Additional ecological information on the species listed above can be found at: http://plants.usda.gov/

      Figure 5.5. Views of riparian habitat. Left picture: Gleason Flat a reach of relatively unconfined geomorphology. The outer region of the riparian area at Gleason Flat is dominated by mesquite (foreground) while the inner edge is dominated by saltcedar and willows. Right picture: Downstream of Gleason Flat a confined high energy environment. In this environment, note the absence of fine sediments and therefore the absence and or reduced presence of mesquite, cottonwood and willow.



    • In confined reaches:
      • The active channel is narrower and slopes are greater resulting in higher stream power.
      • Most fine sediments are transported through the reach, with the remaining soils or sediments being coarse textured.
      • These reaches have limited riparian habitat diversity and width because of:
        • the low water holding capacity of the sediments of these reaches,
        • scouring action of high flows, and
        • confinement by adjacent valley walls.

  2. Transverse
    • Spatial diversity associated with riparian areas can also be observed along a cross-section running from the stream to the uplands.

      Figure 5.6. Transverse view of a riparian zone.


    • As one traverses the riparian system from stream to upland three major habitats are crossed:
      • Channel edge – that portion of the riparian area that abuts the channel, may contain hydrophytic species (hydroriparian).
      • Inner riparian – the riparian area that is dominated by riparian obligates such as willow or cottonwood (mesoriparian).
      • Outer riparian – the portion of the riparian that has a significant presence of species such a mesquite (xeroriparian).
    • If the floodplain contains multiple channels, it may be possible to pass through each of these habitats more than once.
    • In narrow confined reaches, the channel edge, inner and outer riparian habits may be greatly compressed.

      Figure 5.7. View of the Verde River downstream of Horseshoe Dam. Note the transverse spatial complexity of this river reach.



  3. Vertical
    • The vertical or structural component is another aspect of spatial diversity in riparian habitat that generally differs from surrounding uplands.
    • In the southwestern US, riparian vegetation is often denser than that found on adjacent uplands and therefore provides increased cover for many wildlife species.
    • The increased density and vertical diversity provides cover which provides for thermoregulation, hiding perching, and nesting.

Figure 5.8. Vertical structure at the upland riparian edge.

 

 

October 9, 2006