Modeling Length Adjustment in Alluvial Anabranching Units

Anabranching units form when the stream flow divides into two channels that reconnect downstream. Recent observations indicate that these patterns tend to develop fairly regular planforms, characterized by an average length that scales with reach-averaged channel geometry and by weak asymmetry between the anabranches. The question remains as to how such systems evolve toward these regular geometries, and why some of them fail to maintain a multi-thread morphology after their formation. Here, a dynamic model for a solitary anabranching unit is formulated, in which the planform evolution is described in terms of temporal variations in the nodal length separating the upstream bifurcation from the downstream confluence. The governing equation expresses a balance between competing processes acting at different flow stages: elongation during base-flow conditions, assumed to be linked to sediment flux partitioning at the bifurcation, and recession driven by erosion during flood events. The analysis reveals three possible evolutionary trajectories: (i) the attainment of an equilibrium length, in which both branches are capable of carrying sediment over long timescales; (ii) the transition to a single-thread configuration through island removal under dominant erosion; or (iii) the closure of one anabranch following unbounded elongation. The evolutionary pathway depends on reach-averaged hydraulic geometry, bank and floodplain properties, and basin hydrology. The predicted range of lengths for long-term persistence of the multi-thread configuration is broadly consistent with satellite-derived observations.