Dam-induced base-level rise effects on the gravel-bed channel planform

•Little is known about gravel-bed channel response to the dam-induced base level rise•Initially multi-thread gravel-bed stream narrowed and its sinuosity increased in the reservoir backwater•This narrowing in 81-90% depends on initial river morphology that provides accommodation space for vegetation expansion•A single-thread channel is stabilized in backwater due to fine sediments deposition and vegetation expansion•Conceptual model of gravel-bed channel planform adjustments in the dam reservoir backwater is proposed

Little is known on the planform evolution of gravel-bed rivers in base-level raised backwater zones upstream from dam reservoirs. The general model of river channel response to base-level rise predicts a decrease in river sinuosity. However, the observations of channel adjustments in the reservoir backwater document a narrower and more sinuous channel developed as a result of fine sediment deposition and vegetation expansion. Here, the long-term aerial photo-based observations (1963-2015) of two gravel-bed rivers of different initial channel pattern (the Dunajec and its tributary, the Smolnik) located in the base-level raised zone of the Rożnów Reservoir in the Polish Carpathians were analyzed. The results show that the initially multi-thread river in backwater was significantly narrowed and its sinuosity increased (phase 1), which was followed by the stabilization of channel planform (phase 2). However, in the initially single-thread river only phase 2 occurred. The rate of channel narrowing observed on the initially multi-thread river was positively related to the initial channel width (R2 = 0.90, p < 0.0001) and bar width (R2 = 0.81, p < 0.0001). The increase in sinuosity was higher in the previously wider channel section and did not occur in the valley-confined zones of the single-thread river. The results are conceptualized in a two-phase conceptual model, which hypothesizes that the trajectories of the long-term planform adjustments of gravel-bed river in backwater are controlled by the initial river morphology which creates accommodation space for fine sediment deposition and associated vegetation expansion.

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