Chapter 5: Biological Processes in Riparian Areas

The Riparian Corridor

  1. Corridor structure
    The structural attributes affecting function of riparian corridors include:
      1. continuity,
      2. shape, and
      3. width.
    1. Continuity
      • Riparian corridors are not uniform or continuous for great distances in the southwestern United States.
      • Discontinuities occur in the riparian corridor for a number of reasons including stream meanders, geological confinement for example a cliff face, and human built structures including roads, dams, cities and agricultural fields.
      • Naturally occurring breaks in the corridor provide edges for species to enter or exit the corridor. For example point bars create breaks in the channel edge habitat and provide a site important for willow seeds to enter the channel edge habitat, germinate and establish.
 

Figure 5.11. Examples of discontinuities of riparian corridors. Left to right: meander cutbank and point bar; cliff face and break in the corridor; and a dam.

 

    1. Shape and orientation
      • Shape and orientation on the landscape are important to determine riparian corridor function.
      • Riparian corridors generally have a branching structure on the landscape.
      • This allows a funnel effect that can disperse or concentrate moving species. In the funnel effect:
        • species moving upstream along the corridor are dispersed into new habitats,
        • or concentrated when moving downstream.
      • Orientation of the corridor relative to prevailing winds is an important feature for species that utilize wind to disperse seeds, such as willow (Salix spp.) and cottonwood (Populus spp.).
        • If the riparian corridor is perpendicular to prevailing winds, the bulk of these seeds would fall outside the corridor in unsuitable seed beds.
        • Prevailing winds parallel to the corridor will increase the effectiveness of wind as a dispersal mechanism.
    2. Width
      • Corridor width varies as a result of factors such as:
        • geology,
        • climate,
        • hydrology and
        • human activities.
      • The necessary width to maintain corridor function depends on:
        •  the ecological process of interest and
        • the size of the river system.
        • For example, corridor width needed to maintain effective movement of bird species may be quite different from that needed to trap or detain eroded particles from the uplands.
  1. Corridor function
    Riparian corridors function as conduits for movement, filters for non-biological and biological materials, sources of materials, and as sinks for materials.

 

Figure 5.12. Four corridor functions, A. conduit for movement, B. as a filter, C. as a sink, and D. as a source of materials to the uplands or the corridor.

 

 

    1. Conduit
      Riparian corridors can function as conduits for the movement of materials and species.
      • Much of the species dispersal and migration that takes place along a riparian corridor occurs at the upland edge or the channel edge.
      • Interior habitats have several disadvantages for dispersal including:
        • The highly heterogeneous habitats that must be crossed,
        • the discontinuous nature of the corridor, and
        • the change in corridor structure, induced by floods although the severity of these constraints is related to the species of interest.
    2. Filter/Barrier
      • Most corridors including riparian corridors can act as a filter or barrier to select against certain species of materials from crossing.
      • The intensity of the filter effect depends on the width of the riparian system and its associated river, low order streams may present little difficulty to species crossing.
      • Larger river systems can have a significant filter effect restricting gene flow between populations.
    3. Sink
      • Riparian corridors can also act as a sink where materials entering do not leave.
      • This may include wind deposited material such as dust and seeds or sediment from adjacent uplands.
      • Riparian corridors can be strong sinks for nitrate.
        • Nitrate entering riparian corridors that contain bodies of anaerobic soils undergoes the process of denitrification and is converted to nitrogen gas.
    4. Source
      • The riparian corridor functions as an important source of materials to adjacent uplands, the riparian habitat, and the aquatic habitat. These materials include:
        • seeds
        • sediments and
        • nutrients
      • During flood events where scour in the riparian habitat produces sediment to the river system, deposition of this sediment provides new sites for vegetation establishment.
      • After sediment deposition, the site is usually repopulated by corridor species for example cottonwood (Populus spp.) seeds from adjacent adult trees.