Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

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The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches. In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels. The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.