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flows in the summer of 1993. These data were subsequently used to calibrate and run a standard <br />1-dimensional flow model (the step-backwater method; Henderson, 1966) to calculate changes in <br />the average boundary shear stress (T) over a range of discharge. The critical shear stress (T) for <br />the bed material was determined from the Shields' parameter: <br />?* = tic <br />(ps-p)gDi <br />(1) <br />where -r is the critical dimensionless shear stress, ps and p are the density of sediment and water, <br />respectively, g is the gravitational acceleration, and D; is the particle size. For this analysis, we <br />used D50 as the representative grain size and chose Tcso = 0.03 as the criterion for incipient motion. <br />At this level of I r*, most of the particles on the stream bed are immobile, and it is the sporadic <br />movement of a few individual particles that produces low but measurable bed load transport <br />(Parker et al., 1982). Andrews (1994) refers to this condition as "marginal transport". With an <br />increase in shear stress, more of the bed becomes mobile, until at T?so = 0.06, many particles are <br />in transport, and there is "significant motion" of the bed material (Andrews, 1994; Wilcock and <br />Southard, 1989). We believe this condition is most relevant to the problem of maintaining fish <br />habitat. The result of our hydraulic analysis is a relation that shows how the average boundary <br />shear stress changes with discharge. We are interested in defining not only the discharge requrred <br />for incipient motion (TAT, = 1), but also the discharge required to produce general motion (VT, = 2) <br />of the bed material. <br />7