Scale‐Dependent Flow Directions of Rivers and the Importance of Subplate Support.

Large rivers play crucial roles in determining locations of civilization, biodiversity, and efflux to the oceans. The paths they take across Earth's surface vary with scale. At long‐wavelengths rivers can have simple flow paths. At smaller scales, in meanders for example, their paths change rapidly as a consequence of lithology, biota, and other environmental variables. It is not straightforward to identify the scales at which river planforms are set. We overcome these issues by developing a spectral (wavelet) methodology to map flow‐directions as a function of distance and scale. This methodology allows short‐wavelength features (e.g., meanders) to be filtered from river flow‐paths. With short‐wavelength structure removed, the flow‐directions of rivers in Western USA correlate with long‐wavelength gravity anomalies suggesting control by subplate support. This relationship is replicated by an ensemble of landscape evolution models. These results combined suggest that drainage at large scales, O(103) km, is set by subplate support. Plain Language Summary: Rivers play important roles in developing civilization, biodiversity, and the supply of sediment and nutrients to the oceans. Their position on Earth's surface depends on geologic, climatic, hydraulic, and biologic processes, which operate at a large range of spatial and temporal scales. Rivers obviously flow from high to low topography; however, their paths vary at a range of scales. They can be determined by, for example, rock fractures, biota, and antecedence (preexisting drainage patterns). It is generally unclear what processes determine the flow directions of rivers across the scales of interest. Here, a methodology is developed to map the scale‐dependent shapes of rivers, which can be compared to independent observations at appropriate scales. This spectral approach is used to quantify how rivers' flow directions change as a function of scale. We show that it can be used to remove small‐scale variations in flow directions (e.g., meanders) leaving only the larger‐scale changes in flow direction of western North American rivers. The flow direction of these rivers at large scales run broadly parallel to the line‐of‐descent of Earth's gravity field and away from areas where the lithosphere is thin. This result is consistent with western North American drainage patterns being principally controlled by mantle convection. Key Points: River flow directions are determined as a function of position and scale by wavelet transformsSpectra show importance of long wavelength (>1,000 km) processes for determining flow directionsLandscape modeling indicates river planforms can be predicted using proxies for subplate support [ABSTRACT FROM AUTHOR]