The morphodynamics of braiding rivers: experimental and theoretical results on unit processes

We give an overview of recent experimental and theoretical progress on the understanding of some fundamental (unit) processes which appear to control the morphodynamics of braiding rivers. In particular we analyse how bars, i.e. the basic constituents of braiding rivers, arise in response to intrinsic instabilities of the flow-sediment system (free bars) or to forcing mechanisms like channel curvature, channel widening and channel divergence (forced bars). We then proceed to discuss the mechanism of channel bifurcation and analyse the equilibrium configurations and stability of a basic unit of braiding rivers, consisting of a single loop formed by two channels bifurcating from an upstream node and merging into a downstream node. The picture drawn from the results of the above analyses, albeit incomplete, helps clarifying the highly dynamic and strongly fluctuating nature of braiding rivers. The paper is completed by a discussion of the basic ingredients that must be included in any sound model attempting to describe the development of a braiding network.