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

TUOLUMNE RIVER TECHNICAL ADVISORY COMMITTEEHABITAT RESTORATION PLAN FOR THE LOWER TUOLUMNE RIVER CORRIDORSEDIMENT SIZE FINER STREAM SLOPE STEEPERCHAPTER 2SEDIMENTDEGRADATIONAGGRADATIONWATERFigure 2-13. Conceptual flow-sediment balance necessary for channel equilibrium, and channel response todisequilibrium (from Lane 1955).Lane (1955) provided a simple formula to illustratethe way in which rivers balance slope, sedimentparticle size, sediment transport, and streamflow tomaintain a dynamic equilibrium:Q w* S µ Q s* D 50(2-1)where Q w= stream discharge, S = slope, Q s=sediment discharge , and D 50= median grain size(Figure 2-13).A balance in the sediment transport capacityprovided by the high flow regime with sedimentsupplied from the watershed over time anddistance results in a “dynamic quasi-equilibrium,”in which sediment is mobilized, transported, anddeposited. The channel migrates (dynamic), butthe size and shape of the channel remain similar(equilibrium) (Schumm 1977). While thispresentation of the quasi-dynamic equilibriumconcept of sediment supply and transport isconsiderably simplified, its use as a quantifiableobjective to balance flow and sediment supply is apowerful tool for assessing fluvial processes. Italso helps predict channel responses to alteredflow and sediment regimes.Dams, aggregate extraction, agricultural andurban encroachment, and other land uses havecaused sediment imbalances in the TuolumneRiver channel. For example, reduced magnitude,duration, and frequency of high flows imposed byupstream dams has allowed fine sediment toaccumulate in the river. Elimination of coarsesediment supply below the dams and accumulationof fine sediment from reduced flood flowshave degraded chinook salmon spawning habitat.The impact of these imbalances is also a functionof distance along the Tuolumne River because the28