Tuolumne River Report - U.S. Fish and Wildlife Service

TUOLUMNE RIVER TECHNICAL ADVISORY COMMITTEEHABITAT RESTORATION PLAN FOR THE LOWER TUOLUMNE RIVER CORRIDORclasses only, from approximately 8 mm (1/4 in)diameter to 128 mm (6 in) diameter. These sizeclasses are most useful to salmon, and are smallenough to be mobilized by the contemporary highflow regime.Coarse sediment is also naturally recruited fromchannel migration (bank erosion), so channelmigration should be allowed to occur whereverpossible. However, as channels migrate, thereneeds to be coarse sediment replenishment fromupstream sources to rebuild point bars on theinside of bends. Channel migration does notnecessarily provide a net input of coarse sedimentwhen it occurs, but is a mechanism for sedimentstorage and routing. Maintaining coarse sedimentsupply in upstream reaches of the TuolumneRiver is therefore necessary. Tributary contributionsof coarse sediment is also negligible. Thenext priority is then filling bedload impedancereaches (see Section 3.1.1) that trap coarsesediments in transport. Restoring these reaches toa morphology properly sized to the contemporaryhigh flow regime will allow coarse sediment fromupstream sources to continue the depositionmobilization-redepositionprocess that “recycles”introduced gravel year after year. Severalconceptual restoration designs developed by theRestoration Plan target these impedance reaches.4.3.7. Microhabitat restorationMicrohabitat restoration on a river the size of theTuolumne River is typically a short-term strategyat best, because periodic high flows still occurwith the capacity to destroy the restoration effort.Typical microhabitat restoration includes replacingsubstrate in riffles with clean gravel, constructingboulder weir grade control structures,placing boulder clusters for instream habitat, andmechanical gravel flushing. Most microhabitatrestoration projects have had limited success (e.g.,Kondolf et al. 1996), and should only be implementedif project benefits outweigh the costs of ashort project life-span.Mechanical gravel flushing may be a feasibleshort-term strategy for reducing fine sediment inCHAPTER 4Figure 4-6. Suggested “gravel transfusion” insertion method to quickly restore and replenish instream alluvial deposits.156