Alex George Lipp , Gareth G Roberts 

Large rivers play crucial roles in determining loci of civilisation, natural resources and biodiversity. The positions of their mouths control nutrient and sediment supply to oceans. The paths that rivers take across the Earth’s surface varies considerably with scale. For example, at large scales big North American rivers (e.g. Mississippi, Colorado, Columbia) have simple flow paths that can be described by a few changes in direction. However, at smaller scales, in headwaters or meanders for example, their paths can change rapidly. We map the scales at which river planforms are set so that their positions can be compared to driving processes at appropriate scales (e.g. lithology, mantle convection, biota). To do so, we develop a spectral methodology to map azimuths as a function of distance and scale (wavenumber). The resultant maps of azimuth in distance-wavenumber space are compared to independent environmental variables (e.g. lithology, dynamic topography, crust and lithospheric thickness) across the scales of interest, here 1–10 3 km. The results show that the low directions of large rivers in Western North America are set at large scales, O(10 3 ) m, and follow patterns of mantle convection. Planforms of major rivers, and as a result loci of civilisation, natural resources and chemical efflux to the oceans, are therefore principally driven by evolution of the solid Earth.