Grain Size in Landscapes

Earth's terrestrial topography evolves in response to the interaction of tectonics, climate, and lithology. Recent discoveries suggest that the grain size of sediments produced on hillslopes and transported through river networks is key to understanding these interactions. Hillslope grain size varies systematically with erosion rate and residence time, the degree of chemical and physical weathering, and the fracture density and susceptibility to weathering of rock. Variations in initial grain size strongly influence the spatial evolution of grain size distributions as particles mix and wear during downstream transport through channel networks. In rivers, the size and flux of the coarse fraction of the sediment load control the rate of incision into bedrock and thus govern channel slope and ultimately the relief of actively eroding landscapes. These relationships suggest that a primary way that tectonics, climate, and lithology influence landscape evolution is through their controls on sediment grain size. ▪ Recent research reveals the central role of sediment grain size in controlling bedrock river morphodynamics, linking grain size to channel slope and topographic relief. ▪ Tectonics, climate, and lithology govern the size of sediments produced on hillslopes; hence, grain size mediates their influence on landscape evolution. ▪ Feedbacks linking sediment grain size, topography, weathering, erosion, and sediment transport provide new opportunities for advances in Earth surface science.