A river delta is a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or standing water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot transport away the supplied sediment.
CRATER bowl-shaped cavity in the ground or on the surface of a planet or the moon, typically one caused by an explosion or the impact of a meteorite or other celestial body.
VOLCANO mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapor, and gas are being or have been erupted from the earth's crust.