The Early Stages of Groundwater-fed River Bifurcation

Recent work shows, both theoretically and empirically, that river networks fed by subsurface flow bifurcate on average at an angle of 2π/5 [1]. However, the network's existence within a complex natural framework obscures the emergence of this pattern. Fortunately, this ambiguity betrays the presence of processes that have had some effect on the channels during the network's long history. In particular, we concern ourselves with the signature of the third dimension - the topographic relief - on the early stages of channel bifurcation. While, on average, channels grow in a direction dictated by the shape of the groundwater table, we hypothesize that the valley relief plays a crucial role in determining the opening angle and its relaxation to 2π/5 in this regime. A network-wide averaging of several thousand channel bifurcations driven by subsurface flow on the Florida Panhandle reveals that rivers on average branch initially at an angle wider than 2π/5, yet quickly relax to 2π/5 after a few meters. We hypothesize that this initial wide growth direction is governed by the shape of the topography. As these channels form independent valleys, the Laplacian field prevails, yielding an emergent 2π/5 branching angle. Our results therefore suggest that the path-selection of incipient channels fed by subsurface flow is coupled both to the local topography and the surrounding groundwater field. 1. Devauchelle, Olivier, et al. "Ramification of stream networks." Proceedings of the National Academy of Sciences 109.51 (2012): 20832-20836.