HR: 15:30h
AN: B44B-01    [Abstracts]
TI: Bed mobility: A Key Linkage Between Channel Condition and Lotic Ecosystems
AU: * Lisle, T E
EM: tel7001@humboldt.edu
AF: USDA Forest Service, Pacific Southwest Research Station, 1700 Beyview Drive, Arcata, CA 95521 United States
AB: Bed mobility is a key linkage between the physical dynamics of stream channels and lotic and riparian ecosystems over a range of scale. The depth, extent, and frequency of mobilization of bed material affect the variation of conditions providing the requirements of life stages of organisms living in and along the stream channel. Bed mobility thereby helps to define habitat patches supporting communities of organisms. At a larger scale, the rate of channel evolution and creation of new channel and riparian surfaces are governed by the ability of the channel to deform during sediment transport. Bed mobility is partly regulated by the tendency of coarse particles to interact to form structures (e.g., steps, clusters, cells) and sort themselves into coarse surface layers that resist transport. The capacity of the arrangement of bed particles to influence bed mobility generally declines with channels that have gentler slopes and better sorted (finer) bed material. But within channels having a common range of slope and bed material size, bed mobility can adjust to temporary changes in the caliber and rate of sediment supply. Thus bed mobility can be used as an integrative measure of physical habitat condition. Classification of the mobility of the surface of a gravel-bed stream channel is offered as an organizing framework to examine the influence of sediment supply and flow on benthic habitats. Degrees of bed activity can be related to particular `mobility fields' that are quantified by Shields stress (ratio of impelling to resisting forces acting on bed particles). Mobility fields are classified on the basis of the fraction of surface particles moved and the depth of bed mobilization, which limit the scale and severity of bed disturbance. As an approximation, the surface remains stable at Shields<0.03, is partially mobile at 0.03<Shields<0.06, fully mobile at 0.06<Shields<0.12, and unarmored at Shields>0.12. Only fully mobile beds are capable of significant evolution, and greater mobility is associated with channel instability. The proportion of patches having different degrees of mobility governs the dynamics of habitats at the reach scale. Bed mobility could be useful in evaluating channel condition and predicting channel stability following restoration.
DE: 0400 Biogeosciences
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Biogeosciences [B]
MN: 2005 Joint Assembly