No abstract

A series of laboratory experiments demonstrates that riparian vegetation can cause a braided channel to self-organize to, and maintain, a dynamic, single-thread channel. The initial condition for the experiments was steady-state braiding in noncohesive sand under uniform discharge. From here, an experiment consisted of repeated cycles alternating a short duration high fl ow with a long duration low fl ow, and uniform dispersal of alfalfa seeds over the bed at the end of each high fl ow. Plants established on freshly deposited bars and areas of braidplain that were unoccupied during low fl ow. The presence of the plants had the effect of progressively focusing the high fl ow so that a single dominant channel developed. The single-thread channel self-adjusted to carry the high fl ow. Vegetation also slowed the rate of bank erosion. Matching of deposition along the point bar with erosion along the outer bend enabled the channel to develop sinuosity and migrate laterally while suppressing channel splitting and the creation of new channel width. The experimental channels spontaneously reproduced many of the mechanisms by which natural meandering channels migrate and maintain a single dominant channel, in particular bend growth and channel cutoff. In contrast with the braided system, where channel switching is a nearly continuous process, vegetation maintained a coherent channel until wholesale diversion of fl ow via cutoff and/or avulsion occurred, by which point the previous channel tended to be highly unfavorable for fl ow. Thus vegetation discouraged the coexistence of multiple channels. Varying discharge was key to allowing expression of feedbacks between the plants and the fl ow and promoting the transition from braiding to a single-thread channel that was then dynamically maintained.