Interactions between vegetation and braiding vegetation leading to the formation of single-thread channels in a laboratory experiment.

Abstract

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. We show how laboratory experiments using real 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 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, bar development, and avulsion. The dominant channels that emerge from the vegetation-channel interactions organize to a geometry such that nearly all the flow is carried in the channels at high discharge. The organization to a bankfull geometry is directly linked to the tendency for plants to colonize exposed riverbed at low-flow and making it difficult for higher flows to reoccupy vegetated riverbed. The result is a reduction in wetted area with minimal difference between the wetted area at high flow and low flow. Vegetated islands that survive subsequent floods expand and merge with other islands and eventually form a continuous floodplain. Our methodology provides a basis for experimentally developing, self-sustaining high-amplitude meanders, and has applications for river management and basic research purposes.