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FINAL REPORT, November 2003 High-jlow Requirements for the Duchesne River for young pikeminnow (Osmundson and Kaeding 1991; Osmundson 2001). Loss of off-channel habitat has had a detrimental effect on razorback sucker (Xyrauchen texanus), another endangered native fish species known to use the lower Duchesne River (Osmundson 2001). Off-channel water bodies are also frequently sites of enhanced primary production, which is then exported to the main channel (Eckblad et al. 1984; Saunders and Lewis 1988; Tockner et al. 1999). Loss of high flows can cause the desiccation and death of existing riparian vegetation, or eventually eliminate certain species because of failure to regenerate (Rood and Mahoney 1990; Poff et al. 1997). Riparian vegetation, in turn, is a significant factor controlling the evolution of channel morphology. Riparian vegetation along stream banks helps prevent high rates of bank erosion and traps sediment to assist in building new bars and floodplains (Hicken 1984). The Necessity for an Historical Perspective Any imposed changes that modify stream flow or sediment flux in a stream reach, such as climatic fluctuations, water development such as dams and diversions, or other land use changes, may cause the stream channel to adjust. However, predicting the response ofa given stream to a particular environmental trigger is far from clear cut. Each stream system is unique and may exhibit a singular response to imposed changes that cannot be readily integrated into any general model (Carling 1988). Rivers are historical systems, in that present form is influenced strongly by variables passed down through geomorphic and geologic time (Schumm and Lichty 1965). F or example, the size of bed material available for transport may be dictated by the presence of relict alluvium in the floodplain, or the potential of a stream to adjust its slope is limited by the slope of the valley through which it flows. The response of a particular stream to external changes may be determined by autogenic factors internal to the stream system itself (Hooke and Redmond 1992). Such internal factors can be described in terms of intrinsic geomorphic thresholds (Schumm 1977) or chaotic system dynamics in which multiple equilibrium conditions exist so that, when perturbed, the stream system may evolve to a new equilibrium condition rather than returning to its initial state (Hooke and Redmond 1992). These issues complicate the problem of precisely identifying the relationship between cause and response, and underscore the need for an historical context in which to consider channel changes on an individual river. 3