Defining large river channel patterns: alluvial exchange and plurality

Many of the world's largest rivers have anabranching channels with components that may be straight, braided, anastomosing or meandering, although single-channel rivers also exist. But the term 'anabranching' insufficiently expresses the variety, internal complexity and sediment-processing activities of these complex systems. Following a review of pattern definition, variability along and between generally sand-bed and low-gradient large rivers is interpreted here in terms of sediment feed rates with sets of element-level channel/floodplain alluvial exchanges producing overall composite patterns. The presence/absence of elements shows little commonality between large rivers. In natural states, big rivers can be plural systems in which main, accessory, tributary and internal flood-drainage channels differ functionally. Additionally, there are ponded waterbodies that are variously connected and replenished at different flow levels. Even in extreme events, there may be partial decoupling of water-transfer and sediment movement both within main river branches and between them and accessory channels. Contrasted rates of geomorphological activity, as well as patterning elements, exist within hydraulic corridors. Dune migration and century-scale island formation (with widening and contraction of nearstraight channels downstream and over time) characterise many main channels, with meander scroll development in accessory ones. Some large rivers actively meander or braid in single or multiple channels such as to dominate the whole of valley-floor alluvial activity. By the time some rivers become large, they may be sediment-poor for their size, and sedimentation may not fill hydraulic corridors, either locally by overbank dispersal of fine materials, or by channel migration across and filling tectonic troughs. Ponded waters in less geomorphologically active environments, at scales from swales to flood basins, appear more significant along large rivers compared to smaller ones. The plural, mainstream-anabranching, and partially decoupling nature of many large river hydraulic and sediment systems is both of significance to human use and management, and to freshwater biology. Past practice has been to reduce or constrain these characteristics of large rivers for purposes of water transport and power generation, and for flood control and agricultural development. Future management requires a sensitive awareness of the highly varied natural geomorphological dynamics of large river channel patterns so that ecological cost, irreversible change and unsustainable maintenance expense are minimised.