Eitan Shelef - Dissertation Defense
Constraints on the Form and Formation of Branched Channel Networks
Abstract: Branched Channel Networks (BCNs) are among the most striking geometric forms that define the surface of our planet, as well as the surfaces of some other planets. These networks are typically arranged in a branched, tree-like plan-view configuration, where the along-flow profile of segments composing these features are concave-up such that the channel slope (S) and drainage area (A) are related to one another by the concavity (θ) as S∝A^-θ . While θ likely reflects the mechanics of channel forming process, it is not clear if and how these processes are related to the plan-view arrangement of BCNs. In fact, many topologic measures fail to distinguish the plan-view configuration of natural networks from those generated by random walks, and so it is unclear whether the geometric properties of BCNs reflect the action of specific surface processes or simply result from the interaction of flows with pre-existing topography.
Here we demonstrate that the multi-scale plan-view structure of BCNs observed across our planet and expected from process-based simulations is unlikely to result from random-walk processes. Instead, we show that the multi-scale structure of BCNs reflects two coupled constraints: (a) the characteristic along-flow channel profile shaped by the channel forming processes (as reflected by the channel concavity; θ), and (b) the fact that two flows initiating at an infinitesimal distance apart on each side of a drainage divide must experience an identical elevation drop between the divide and the junction where these flows once again meet. We found that the degree to which these constraints are satisfied distinguishes random, natural, and modeled landscapes, and explains morphological differences between modeled landscapes of various concavities, as well as temporal changes in their plan-view network geometries. These plan-view forms may therefore divulge information about processes that transport mass across Earth's surface, and may reveal processes that sculpt BCNs on other planets.