EXOGENIC FORCING AND AUTOGENIC PROCESSES ON CONTINENTAL DIVIDE LOCATION AND MOBILITY

The position and mobility of drainage divides is an expression of the dynamic feedback of exogenic climatic and tectonic drivers and autogenic channel network and hydrologic processes integrated across a range of scales. Here we explore those feedbacks at the large scale, represented by major rivers and continental drainage divides where channel network topology and divide migration reflect broad and commonly gentle topographic gradients that are influenced by unsteady tectonic, isostatic, and/or dynamic topography drivers. Our analysis utilizes long-wavelength topography synthesized by a low-pass filter to document the expression of transient landscape response to exogenic drivers as a measure of how drainage and erosion conform to topograpy. This provides a novel framework for predicting the direction of divide movement, as well as the the stable divide location, that is complementary to recent studies focusing on the χ channel metric. The Gibraltar Arc active plate boundary and Appalachian stable plate interior, two tectonically diverse settings with ongoing drainage system reorganization, are chosen to explore the length scales of exogenic forcings that influence continental drainage divide location and migration. The major watersheds draining both the active and decay-phase orogens studied here are organized by topographic gradients that are expressed in long-wavelength low-pass filtered topography (λ ≥ 100 km). In contrast, the river network and divide location is insensitive to topographic gradients measured over filtered wavelengths < 100 km, which are set by local crustal structures and rock type. The lag time between exogenic forcing and geomorphic response and feedbacks cause divide migration to be unsteady, and occur through pulses of drainage capture and drainage network reorganization that are recorded in sedimentological, geomorphic, or denudation data.