Dynamic single-thread channels maintained by the interaction of flow and vegetation

Most rivers on Earth today flow as a single channel, in some cases with occasional islands, and follow a more or less sinuous course. However, single-thread channels have proven difficult to reproduce and study experimentally: experimental self-formed channels tend to widen and subdivide, leading to a braided pattern. Cohesive sediment has been the main mechanism studied for stabilizing banks and producing a single-thread channel. We show how laboratory experiments using vegetation to stabilize banks can organize the flow and convert the planform morphology from braided to single-thread. Our experimental strategy, a repeated cycle of short periods of high water discharge alternating with longer periods of low discharge accompanied by plant seeding and growth, leads to the evolution of a dynamic self-maintaining single-thread channel with well-defined banks and floodplain. By eliminating weak flow paths, the vegetation corrals the water into a single dominant channel until the reduction in total wetted width leads to a new self-organized state in which the flow removes vegetated area as fast as it is produced. The new channel is deeper and has a broader distribution of depths than the braided one, with channel size adjusted to carry almost all the flood flow. The resulting system maintains a dynamic steady state via similar mechanisms to those that operate in meandering channels in the field, specifically erosion at the outside of bends, bend growth, and bar development. Our methodology provides a basis for experimental development of self-sustaining high-amplitude meanders and has applications for river management and basic research purposes.