The rising and falling of sea level

The rising and falling of sea level has caused many coastal canyons to be alternately exposed on land and then submerged beneath the sea as submarine canyons. Many of these canyons began forming during times of low sea level when rivers cut downward and outward to a distant, now submerged shoreline. When sea level rose again and drowned the canyons, the continued to grow through submarine erosion processes, such as turbidity currents, debris flows, and landslides. Some of these canyons are gigantic features, and many have very steep, even overhanging, walls.
One of the best studied submarine canyon systems lies just offshore from the Scripps Institution of Oceanography in San Diego, California (H 11.34). A broad sandy beach diminishes and stops because the beach sand pours down into the La Jolla submarine canyon. The beach sand loss to deep water averages 340,000 m3/yr; it is a permanent loss.