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Grams and Schmidt 20 does not have a direct influence on channel geometry and slope. However, because the detailed profile of the alluvium-bedrock interface is not known, the possibility remains that the present longitudinal profIle is inherited from the profile established when the river did flow in contact with bedrock. Maintenance of an inherited profIle would occur only if the river had aggraded the same amount everywhere. An adjusted profile would exist if differential aggradation had occurred; borehole data show that depth to bedrock and therefore aggradatio.n since the river flo.wed in contact with bedrock has no.t been the same throughout all the canyo.ns. Aggradation may have occurred in response to a change in mainstem hydro.logy fo.llo.wing deglaciation o.f the surro.unding mo.untains or a change in tributary sediment delivery by debris flo.ws. Moreover, the o.bserved relatio.n between debris fans and slope indicates that steep-gradient reaches are related to. high debris-fan frequency. As sediment accumulated in the canyons, slope increased until streamflo.W competence matched the characteristics of tributary supply. The gravel bars with the largest mean particle sizes occur in the reaches of highest flood-flow average bo.undary shear stress. Reworking o.f gravel bars and downstream transport of coarse-grained alluvium probably o.ccurs during floods o.f about a pre-regulation 10 yr recurrence interval in the modern climate. Observations of lithologies contained in gravel bars SUPPo.rt the 'conclusio.n that gravel is rarely transported through the study reach under either the current regulated streamflow regime or the historic unregulated streamflow regime. The litho.logic compositio.n o.f gravel bars should reflect the distribution o.f litho.Io.gies o.f all upstream debris-contributing tributaries, no.t just the immediately upstream tributary basin if gravel was being transPo.rted thro.ugh the canyons. Our limited investigatio.n of lithologies contained in two. debris fan-gravel bar pairs indicates that gravel bars are, in fact, do.minated by the resistant litholo.gy mo.st abundant in the tributary basin immediately upstream. If attritio.n were the dominant process we Wo.uld expect to. fmd a do.wnstream fining of lithologies that did tlQt occur. Thus in the debris-fan do.minated canyons where "