Influence of bank materials, bed sediment, and riparian vegetation on channel form along a gravel-to-sand transition reach of the Upper Tualatin River, Oregon, USA

We examined the role of bed sediment size, bank materials, riparian vegetation, and discharge on channel form along a 10-km reach of the Upper Tualatin River, encompassing both gravel and sand-bed reaches. Statistical correlations and analyses of downstream changes of channel morphology reveal that bank materials and bed sediment are the dominant controls on channel cross section form. A rapid downstream reduction in bed sediment size in the gravel-bed channel is followed by an abrupt transition to a narrower, deeper, and less sinuous sand-bed channel with more cohesive bank materials. The simultaneous reductions in channel slope, bed sediment size, and width-to-depth ratio indicate the channel crosses a geomorphic threshold to maintain continuity in sediment transport. This gravel-to-sand transition and associated change in channel form are induced by a break in valley slope and an increase in bank resistance from cohesive bank materials. Bank materials, measured as the average percent silt and clay in banks, are a primary influence on channel form within both subreaches but demonstrate a greater influence on channel width and vertical stability in the gravel-bed channel and on channel depth and lateral stability in the sand-bed channel. Riparian vegetation at current densities and compositions is not a significant control on contemporary channel cross section form and may be responding to the bank and channel stability provided by cohesive bank materials in the laterally dynamic gravel-bed channel.