How are submarine canyons carved?
A submarine canyon is a deep, steep-sided valley cutting a continental shelf or slope. Some connect with river mouths, while others have no obvious modern river connection. They should not all be imagined as ordinary rivers flowing underwater.
Dense sediment flows
Water containing abundant suspended sediment can form a flow denser than surrounding seawater. Such a turbidity current moves downslope, eroding the bed and transporting material. Submarine landslides also contribute. Rivers during low sea levels may have initiated some routes, but terrestrial river erosion alone cannot explain the full depth of every canyon.
Routes into the deep ocean
Canyons can carry sand, mud and organic matter from coastal areas into deep water, with submarine fans developing at their outlets. Powerful sediment flows can affect seafloor cables and infrastructure, even when the hazard is not apparent at the surface. Sonar mapping and sediment studies are used to investigate them. Present activity is assessed from recent deposits and observed flows, not canyon size alone.