HR: 09:00h
AN: H11F-05    [Abstracts]
TI: Considerations of Scale and Processes in Stream Restoration and Ecological Response
AU: * Simon, A
EM: asimon@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 United States
AU: Shields, D
EM: dshields@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 United States
AU: Kuhnle, R
EM: rkuhnle@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 United States
AU: Knight, S
EM: sknight@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 United States
AB: Stream restoration as a means of controlling accelerated channel erosion and improving biological function in streams has become pervasive in the United States over the past twenty years. A broad range of practices often involving direct modifications to stream channels and adjacent floodplains, including alterations to morphology and pattern have been used for stream restoration. Because alluvial-channel processes and biological functioning operate as linked, open systems, any restoration project must be placed in the context of existing watershed and channel processes with a quantitative understanding of the rates of transfer of flow energy and materials. This is particularly true of reach-scale projects where local stabilization and habitat improvements may be completely overwhelmed by watershed or channel-system scale instabilities. In this regard, it is unlikely that a reach-scale project will be successful in an unstable alluvial system. This is analogous to constructing bank-stabilization measures in an actively incising channel. A conceptual model of channel response and evolution that marks systematic shifts in channel processes over time and space has been linked to fish-community structure in Mississippi streams. This link reflects changing habitat conditions and sediment-transport regimes over the course of fluvial adjustment. Suspended-sediment concentrations that can increase by orders of magnitude for a given discharge during the incision and mass-wasting phases abrade fish gills and reduce the ability of fish to hunt for food due to reduced water clarity. Similarly, durations of high suspended-sediment concentrations are shown to be inversely related to numbers of benthic macro invertebrates. Streambeds experiencing active incision (Stage III) may be too mobile for benthic macro invertebrate communities to thrive. Channels dominated by mass-wasting processes (Stages IV and V) lose riparian vegetative cover and shading which may result in higher stream temperatures. Aggradation processes typical of Stage V result in loss of interstitial spaces for spawning, de-oxygenation of substrate and may suffocate organisms. Perhaps most importantly, channel widening produces shallower depths at base flow and renders streams less retentive of large wood. Ecological characteristics recover in advanced stages of channel evolution as baseflow channels are narrowed and berms re-vegetate (Stage VI), but full recovery to pre-incision (Stage I) conditions has not been observed for both ecologic and sediment-transport systems. The processes reflected by stages of evolution can operate over entire fluvial networks and over time scales in the order of 100 years. Issues regarding effectiveness or benefit of stream restoration practices, therefore, must address scale. Furthermore, site and approach selection for reach-scale restoration projects should be guided by knowledge of watershed-scale processes. As an example, a grade control structure installed on Hotophia Creek, Mississippi successfully eliminated upstream-progressing incision and resulted in locally improved aquatic populations in the stilling basin. However, the trapping of hydraulically-controlled sediment on the upstream side of the structure resulted in streambed incision, de-stabilization of streambanks and degraded aquatic habitat in downstream reaches not protected by other grade-control structures.
DE: 0481 Restoration
DE: 1815 Erosion
DE: 1856 River channels (0483, 0744)
DE: 1861 Sedimentation (4863)
DE: 4804 Benthic processes, benthos (0408)
SC: Hydrology [H]
MN: Fall Meeting 2005