Beyond the Long Profile

The intricate dissection of mountainous landscapes by rivers and water divides has fascinated geomorphologists for generations. Early workers focused on qualitative descriptions of network patterns but without necessarily relating them to specific environmental drivers. Following a dramatic increase in the availability of high-resolution topographic data and computational power in recent decades, we are now able to quantify metrics related to the linkages between tectonics, rock properties, and fluvial processes as revealed in the geometry of fluvial networks. These links are a consequence of river networks being imprinted on the top boundary of the upper crust. This means that in tectonically active landscapes, surface landforms can advect together with shallow crustal rocks in response to deformation induced by tectonic stresses and to reorganize when deformation is intense. In this contribution, we explore factors that drive changes in channel network patterns and review quantifiable metrics for identifying such changes. We focus on three major tectono-lithologic drivers of change. First, in landscapes featuring transpression, rotation of channels can reveal competition between horizontal motions and erosion, and in landscapes with pure shortening, basin shape and aspect ratio may change. Second, in extensional settings, network patterns evolve when fault arrays nucleate, ramify, and then coalesce, leading to different patterns at each stage. Tectonic tilting could manifest in widespread reorganization with migrating drainage divides. Third, the lithology through which rivers carve their valleys appears to have a first-order control on the fluvial network geometry and fractal dimension. The juxtaposition of lithologies along tectonic contacts can also leave a revealing imprint on river networks.