Tremendous quantities of silt settles at the mouth of the river, forming a delta. At one-fifth the size of France, the Amazon delta is the largest in the world. Scientists believe, however, that it is not likely to get any larger. This delta is considered "mature" because it has not expanded beyond the coastline of the South American continent. Three reasons are proposed:
(1) The Earth's crust sinks slightly under the weight of the delta.
(2) The ocean current along the shores carry away deposits.
(3) The estuary banks are eroded by the tidal flood waves.