Comparison of Reach-Scale Morphologic Adjustment in Confined and Unconfined Alluvial Mountain Rivers, Western Washington

Over human time scales (10-1 – 102 yr), alluvial mountain rivers respond to changes in sediment input and discharge through adjustments in reach-scale morphology (width, depth, grain size, and, to some degree, slope). Channel confinement (valley-width relative to the bankfull channel width) in these systems can strongly influence the magnitude of channel response. We compared channel responsiveness to flood events (50-100 yr) within the last 5 years in unconfined and confined valley segments on the Olympic Peninsula, western Washington. Field measurements of cross-sectional averaged width and depth in 20 confined and 20 unconfined valleys are compared to the bankfull dimensions predicted from established downstream hydraulic geometry relationships for the region. We expect that measured bankfull geometry of confined reaches will be significantly greater than the predicted bankfull dimensions, which would suggest that the morphology of confined channels is more responsive to flood events. In unconfined channels floodplains are large enough to disperse over-bank flows, which can limit the effect of peak discharges on channel morphology, whereas confined channels are forced to disperse the extra energy exerted by peak flows into increased shear stress along their bed and banks. Results from this study can aid modeling efforts to predict future changes in channel geometry and aquatic habitat in response to climate change or land use at the basin scale