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Chapter 5: Biological Processes in Riparian Areas
Temporal Diversity
- Riparian habitats often experience significant, frequent change over time resulting in a large number of habitat patches of differing ages in a small spatial area.
- Variation of habitat over time is high due to the random nature of two abiotic drivers:
- floods and
- droughts.
- Because of the location of Arizona relative to global circulation patterns, the climate of Arizona is characterized by periods that may be relatively wet or dry.
- This climate pattern produces stream hydrographs that are characterized by long periods of low flow punctuated by brief high flow events.
Figure 5.9. Hydrograph of the Verde River showing extended periods of low flow punctuated by brief high flow events. It is during periods of low flow that riparian habitat of the inner riparian area can encroach into the stream channel.
Floods
- Floods are a regenerative mechanism for riparian habitat.
- Floods:
- by removing existing vegetation, they create new habitat for woody species establishment
- deposit new sediments necessary for establishment of riparian species, especially willows (Salix spp.) and cottonwoods (Populus spp.), and
- redistribute nutrients.
- Flood disturbance varies with respect to:
- Timing – season of flooding.
- Frequency – number if events per unit time.
- Magnitude – controls stream power at any point in the riparian area.
- Duration – length of time flood water is present.
- Spatial impact – many instances where flooding is localized in watersheds and impacts are restricted to a single tributary and possible where the tributary joins the main stem.
Droughts
- Droughts also impact riparian systems by allowing encroachment of vegetation into the main channel.
- Majority of vegetation encroachment into the channel during drought occurs in lower gradient reaches.
- Typical species include that encroach are:
- bulrushes (Schoenoplectus spp),
- cattail (Typha spp),
- knotgrass (Paspalum spp) and
- carrizo (Phragmites ssp).
- These species trap fine sediment and the net result is an increase in the extent of anaerobic habitat/function along stream margins.
- Droughts may also reduce riparian habitat width due to mortality of species, especially in the outer riparian zone.
Figure 5.10. Verde River at Sheepbridge, left, April 2002, note encroachment into channel by riparian vegetation coincident with low discharges of the previous years. Right, same location September 2005, note the removal of herbaceous and woody vegetation.
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