Morphological Adaptation of River Channels to Vegetation Establishment: A Laboratory Study

While the scientific community has long recognized that alluvial rivers are the product of interactions between flowing water and bed material transport, it is increasingly evident that vegetation mediates these interactions and influences the stream channel characteristics. In a novel set of mobile bed laboratory experiments with variable discharge, we demonstrate that vegetation colonization affects bank erosion rates, channel shape, channel sinuosity, and bar pattern. Our analyses compared the morphological evolution of channels with initially steady bars considering the following three scenarios: (1) channel without vegetation, (2) channel with vegetation added to the floodplains, and (3) channel with vegetation added to both the floodplains and the bar surfaces that emerge at low flows. Absence of vegetation produced the widest and shallowest channel with the lowest sinuosity. Floodplain vegetation in the second scenario reduced bank erosion and resulted in a deeper and more sinuous channel with shorter bars. In the third scenario, vegetation establishment on emerging bar surfaces intensified erosion on the opposing bank, enlarging the amplitude of bends. Enhanced sedimentation on vegetated bar areas increased both bar elevation and bar length compared to the second scenario. The results show that the colonization of bar surfaces by plants creates the conditions for new floodplain and island formation, fostering channel meandering and anabranching. Finally, our experiments emphasize the role of alternating high and low flows on the morphological development of streams mediated by vegetation.