Statement of Problem: Habitats of many -- if not most rivers -- of the U.S. are perceived to be degraded as a result of human-induced stresses. Habitat, in general, describes the three-dimensional structure in which organisms live (Gordon and others, 1992). Habitat can have physical, chemical, and biological components; this task is concerned primarily with physical habitat as measured by depth, velocity, and substrate. The research is motivated, however, by establishing relevance of physical habitat to biota. Physical habitat in river corridors can be altered by human actions by changing flow regime or by changing channel morphology, or by changing both. In natural rivers, channel morphology is expected to be ¿equilibrated¿ to some range of flows that are responsible for the bulk of sediment transport and consequent geomorphic expression. When river discharges are altered by human actions, channel morphology and spatial and temporal distributions of habitats are changed. These changes may or may not be harmful to riverine species or intended uses of the river. In highly managed rivers, habitat can also be altered independently from flow regime by engineered structures, including bank revetments, levees, and channel-training structures. In these engineered rivers, channel morphology is independently imposed on the river, and physical habitats are not equilibrated to flow regime. Understanding the interaction of hydrology (flow regime) and geomorphology (channel morphology) is critical for evaluating habitat loss and degradation, and for guiding river restoration designs. Where it is reasonable to neglect the effects of sediment transport, the spatial and temporal distribution of physical habitat can be modeled with operational multidimensional hydraulic models that calculate changes in habitat with changing discharge hydrodynamic time scales. This technique is maturing, but challenges remain in the areas of optimizing model formulation (data density, calibration/verification standards, biologically meaningful model scales), defining biological significance of physical habitats, and understanding spatial and temporal patterns. More importantly, in most rivers significant sediment transport takes place and channel geometry changes over time. These geomorphic effects are poorly captured in available research models: there is a pressing need to address the coupling of geomorphic and hydrologic habitat dynamics. When time frames are extended, a great deal of uncertainty becomes apparent because of complex geomorphic adjustments of watersheds. Natural and human-influenced disturbances are transmitted through watersheds and may diminish or grow as the travel downstream (and sometimes upstream) over time. Geomorphic adjustments are characterized by complex response: thresholds, feedbacks, and sediment-routing within watersheds result in non-linear, lagged, and unexpected cumulative changes in physical habitat. There is a pressing need to understand broad-scale, long-term geomorphic responses to evaluate resource management and restoration options. Current understanding is so poor that in many rivers there is difficulty in identifying the signal of human disturbance from among natural variation.
1) Develop multi-scale understanding of the origins and transmission of physical disturbances through drainage basins, and how the disturbances affect river-corridor physical habitats.
2) Develop measurement and modeling tools and instrumentation to apply to hydrologic, hydraulic, and geomorphic assessment of habitat dynamics in river corridors ranging in size from wadeable streams to "Great Rivers".
3) Improve understanding of short- and long-term habitat dynamics through field-scale, adaptive-management experiments.
4) Link understanding of physical habitat dynamics to biotic responses.
5) Apply understanding to adaptive management of river systems.