ConserveOnline - Basin-scale availability of salmonid spawning habitat, by John Buffington et al.

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Basin-scale availability of salmonid spawning habitat, by John Buffington et al.

By Web Admin on 6/5/2007 | Keyword(s): Session ii: existing approaches

Basin-scale availability of salmonid spawning habitat as influenced by channel type and hydraulic roughness in mountain catchments: Examples from western Washington and central Idaho Availability of good quality spawning gravel has been identified as a key factor for the persistence and successful conservation of salmonid populations. A general framework is presented for examining the effects of channel type and associated hydraulic roughness on salmonid spawning-gravel availability in mountain catchments. Digital elevation models are coupled with grain-size predictions to provide basin-scale assessments of the potential extent and spatial pattern of spawning gravels. To demonstrate both the model and the significance of hydraulic roughness, we present a scenario for optimizing the spatial extent of spawning gravels as a function of channel type in Pacific Northwest catchments. Predictions indicate that hydraulic roughness could control more than 65% of the potential available spawning habitat at our study sites. Results further indicate that bar roughness can be important for maintaining spawning gravels in lower mainstem reaches, while wood roughness may be required for spawning-gravel maintenance in steeper, upper mainstem channels. Our analysis indicates that wood loss and consequent textural coarsening could deplete up to one third of the potentially-usable spawning area at our study sites. To further test the model, predicted spawning-gravel sites for chinook salmon were compared to an 8 year record of observed redd locations in the Middle Fork Salmon River, Idaho. Results demonstrate that nearly 90% of the redds (n=5135) occurred within reaches predicted to have suitable substrate sizes, with 98% of those reaches predicted to have a pool-riffle morphology. Moreover, redd densities were roughly 3 times greater in reaches predicted to be suitable. However, redds tended to be clustered in upstream portions of the accessible river network, rather than evenly distributed throughout the suitable reaches. This may be due to a variety of factors, including 1) depressed population sizes and preferential selection of the best spawning sites; 2) loss of chinook salmon phenotypes that once spawned in currently underutilized habitats, and fidelity of remaining subpopulations; or 3) constraints imposed by environmental factors other than substrate size (e.g., stream temperature, velocity, scour regime, hyporheic flow, or winter icing).

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