Emergent geometries of groundwater-fed rivers

Abstract

Groundwater-fed rivers form stunning geometries over a range of scales. These rivers grow as water from an underground aquifer reemerges and erodes the overlying topography. Both the aquifer and the overlying topography generate flows along diffusive gradients. We study three features produced by these gradients over different scales: the shape of the valley that forms around a single stream, the network-averaged planform stream shape, and the shape of the drainage basin. First, we identify a new feature in stream valleys - a spatially variable diffusivity - that gives rise to a theoretical valley shape that agrees with the shapes of real valleys. Next, we present evidence and theory for a 120° opening stream confluence angle as a result of lateral rearrangement of streams in response to the pressure field generated by the aquifer. We then study how this mechanism exerts itself on the scale of the network. Finally, we widen our scope and analyze river planform morphology on a continental scale. We identify how branching angles can predict a river basin aspect ratio. We find a relationship between this aspect ratio and river basin scaling exponents with local climate.