Why some alluvial rivers develop an anabranching pattern

Abstract

Anabranching rivers have been identified globally, but a widely accepted and convincing theoretical explanation for their occurrence has remained elusive. Using basic flow and sediment transport relations, this study analyzes the mechanisms whereby self‐adjusting alluvial channels can anabranch to alter their flow efficiency (sediment transport capacity per unit of stream power). It shows that without adjusting channel slope, an increase in the number of channels can produce a proportional decrease in flow efficiency, a finding particularly relevant to understanding energy consumption in some braided rivers. However, anabranching efficiency can be significantly increased by a reduction in channel width, as occurs when vegetated alluvial islands or between‐channel ridges form. The counteracting effects of width reduction and an increasing number of channels can cause, with no adjustment to slope, an otherwise unstable system (underloaded or overloaded) to achieve stability. As with other river patterns, anabranching can be characterized by stable equilibrium or accreting disequilibrium examples.

Least action principle, equilibrium states, iterative adjustment and the stability of alluvial channels

“…In an environment with lower gradients, meandering channels expend excess energy in a more modest and uniform way (Langbein and Leopold, 1966). Over even lower gradients, but where energy is still in excess, it is possible to consume this surplus by forming multiple channels (anabranching) without adjusting channel gradient (Huang and Nanson, 2007). This may occur where the channel boundary is well vegetated and therefore too resistant to allow meanders to readily form and reduce gradients.…”

“…This means that in situations where S V < S fmin some rivers can achieve stable equilibrium, not by increasing slope, but by reducing W/D. Anabranching is relatively common on straight low gradient rivers in Australia, a low gradient continent where increasing channel slope is barely an option but where the formation of islands or ridges stabilized by well adapted riparian vegetation can reduce W/D ratios Nanson, 1999, 2000;Wende and Nanson, 1998;Jansen and Nanson, 2004;Huang and Nanson, 2007). However, as with braided and meandering rivers, not all anabranching rivers achieve stable equilibrium, the Columbia River in Canada being the most studied example (Smith and Smith, 1980;Makaske, 2001;Tabata and Hickin, 2003;Abbado et al, 2005).…”

“…While the conditions of mSP and MSTC inherent in basic hydraulic relations for alluvial channel flow provide a convincing physical explanation for why regular regime relations and bankfull hydraulic geometry have been exhibited in a very wide range of environments, it needs to be stressed that these outcomes, and the recent progress made by Huang and Nanson (2007) in explaining the cause of anabranching rivers in terms of the variational analytical approach advocated by Huang and co-workers, have demonstrated that the basic hydraulic relations of flow continuity, resistance and sediment transport are sufficient to explain not only the regime relations but also the dynamic forms of river channel flow. In other words, there is no need to predefine any extremal hypothesis in order to gain a proper understanding of the behaviour of river channel flow.…”

“…The combined effects of both endogenous and exogenous factors on the formation of river channel patterns have been quantified with several integrated models (see, e.g., Schumm, 1977;Bettess and White, 1983;Wang and Zhang, 1989;Schumm and Winkley, 1994;Eaton et al, 2004;Millar, 2005). While most of these previous models have focused largely on the effect of valley slope, Huang and Nanson (2007) present a detailed mathematical analysis of the effects of the other exogenous factors, particularly non-uniform channel boundaries confined by riparian vegetation, on the development of anabranching and braiding rivers. …”

“…Nanson and Knighton (1996) hypothesized that in conditions where S V is only slightly below S fmin equilibrium could be achieved with a reduction in W/D caused by cohesive well vegetated banks and no increase in slope. Nanson and Huang (1999) and Huang and Nanson (2007) have mathematically modelled such conditions, and Jansen and Nanson (2004) physically modelled them and demonstrated the relative efficiency of anabranching compared to single-thread reaches in Magela Creek in northern Australia. This means that in situations where S V < S fmin some rivers can achieve stable equilibrium, not by increasing slope, but by reducing W/D.…”