A Continuous Simulation Model of Fluvial Geomorphic and Ecologic Processes

We present a physically-based simulation model of unsteady water flow, sediment, morphology and substrate composition that can be used to assess physical habitat conditions in streams on a continuous time basis. The model overcomes one of the most common shortcomings of current morphologically-based habitat assessment practices in which static stream morphology is assumed and only hydrological variability is considered. Furthermore, the model tracks the history of the physical habitat conditions everywhere in the stream, allowing for the analysis of ecological processes that are not only function of magnitude and frequency of flow events but also depend on the actual succession of events, like ecotone recovery or establishment. Depending on the particular process of interest, thresholds in the environmental inputs can be identified (e.g. magnitude of pool and riffle habitat maintenance floods) and used as a basis for the definition of reference conditions for restoration projects. Selective grain size fraction erosion, deposition and transport processes are simulated to provide a realistic description of the substrate distribution, which is particularly important in situations of marked geomorphic non equilibrium that are often present in degraded or recently restored streams. Since the links between geomorphic and ecologic process take place over a wide range of time scales, the model is designed to simulate periods of several years with a time resolution that can be sub hourly. Habitat conditions can be assessed based on flow and geomorphic information via species-specific preference curves or using other indicators of habitat quality like diversity index. To show the capabilities and limitations of our approach we present applications to both real and hypothetical situations.