The sediments, morphological features and riverflows that define the hydrogeomorphology of natural river channels provide physical habitat diversity that sustains the aquatic biodiversity of river ecosystems. This simple concept underpins the large number of contemporary ecohydraulic models that are available in the literature. Such models have been widely used to predict how morphological diversity (taken here to encompass channel sediments, topography and flow velocity) influences habitat quality for target species at the reach scale. The accuracy of these predictions is a matter of considerable practical importance, as the results are frequently used as the basis for restoration or rehabilitation. However, such models are limited in that they do not account for dynamic changes in river morphology, which themselves are triggered by changes in the flows of water and sediment delivered from the watershed upstream and stimulated by climatic, tectonic or land cover perturbations across a wide range of temporal scales. Accordingly there is an urgent need to combine the outputs of catchment-based geomorphological models with ecohydraulic models, so that predictions of habitat quality focused on specific reaches can be placed into their appropriate (i.e., the watershed) spatial context. To address these issues we herein present preliminary simulations from a case study of the Sulphur Creek watershed, a 24.2 sq. km., third-order catchment draining one of 47 tributaries to the Napa River, which empties southerly into the San Francisco Bay of northern California. Therein, the influence of catchment-scale geomorphic dynamics on reach-scale fish habitat is investigated using the ooCAESAR landscape evolution model. This cellular automaton model, based on its predecessor CAESAR, was chosen because it can be run at spatial resolutions (1 to 5 m.) that are ecologically meaningful and at temporal resolutions that capture both individual event dynamics and long-term evolutionary history. Water depths, velocities and surface grain size distributions produced by the ooCAESAR simulations are used to model habitat suitability for spawning and rearing lifestages of Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) using traditional habitat suitability curves. To drive the simulations, scenarios were developed that express a combination of plausible future events and time series based on climate (rainfall), land use, restoration and seismic variants. A subset of an ensemble of scenario-driven simulations are used to illustrate how non-linear catchment-scale dynamics influence the quality of fish habitat expressed at the reach-scale, in sometimes non-intuitive ways. Sediment budgets derived from the simulations help segregate processes and explain how the delivery and storage of sediment within the catchment combine to change physical habitat in reaches utilized by fish.
Available at: http://works.bepress.com/joseph_wheaton/119/