Equilibrium of morphological units in anabranching rivers

Abstract

Anabranching rivers are characterized by multiple sequences of interacting channels. These patterns consist of morphological units in the form of closed loops, with an upstream bifurcation controlling the partition of water and sediment fluxes, and a downstream confluence where the two anabranches reconnect. Bifurcation-confluence loops can also be encountered in single-thread rivers showing a transitional planform between meandering and anabranching, often associated with width oscillations and chute cutoffs. Individual channel loops display an average length that is proportional to the reach-averaged bankfull depth of alluvial rivers. The existence of a characteristic length scale reflects a hydro-morphodynamic interaction taking place between the constitutive elements of these morphological features, bifurcations and confluences. However, it is not clear why channel loops should organize themselves to attain a certain spatial scale, and how their planform shape is related to the morphodynamical processes governing the distribution of water and sediment between the anabranches. In this work we tackle these issues through a comparative analysis of theoretical findings and remotely sensed data from natural gravel-bed and sand-bed rivers. Two missing ingredients are included with respect to previous analyses: the case where one of the anabranches stops transporting sediment, and the case of loops with unequal lengths of the bifurcates. The theoretical model suggests that four distinct types of long-term equilibrium states can be identified, depending on the reach-averaged bankfull properties and on the planform shape of the loop. The comparison between model results and field data reveals that most of the observed river loops place themselves consistently in the region of the parameter space where theory predicts that both branches keep open in the long term.