The large Tamanrasset River flowed from the Atlas Mountains and Hoggar westward towards the Atlantic and entered it in the Bay of Arguin in Mauretania. It once formed the 12th largest watershed in the world and left a submarine canyon and riverine sediments. Together with other rivers it formed estuaries and mangroves in the Bay of Arguin. Other rivers in the same area also formed submarine canyons, and sediment patterns in marine sediment cores and the occurrence of submarine landslides in the area have been related to the activity of these rivers.