When the sediment deposits build to a height that severs the sea's influence, the lagoon dies. Many factors affect sedimentation. For example, because fresh water is lighter than seawater, creek sediments don't mix with the denser, saltier water of the lagoon but 'float' towards its center. The faster the creek flows, the further the sediments will be carried away from the creek mouth. During times of heavy rains creek sediments may even be carried directly out to the ocean, with no chance to settle within lagoon boundaries.
Slower creek flows encourage settlement within the lagoon.
When creek flow is low, sediments settle near creek mouths, often at the far reaches of the lagoon. Increased sediments accelerate plant growth, which perpetuates the problem by holding on to bottom materials with their root systems. When the masses of continents interact, their shapes evolve. Mountains and valleys are formed. During earthquakes, sections of the earth may drop many feet.