M ost lagoons are born when a river valley floods during a period of rising sea levels. As sediments fill a lagoon, the amount of water exchanged between the ocean and the estuary is reduced. Without tidal influence, the lagoon is no longer able to support estuarine habitats. O pposing the process of sedimentation is lagoon scouring. Scouring is the re-suspension of bottom sediments into the water column.
Water carrying the suspended sediment flows out of the lagoon with the ebb tide and into the ocean.
The better the scouring action within a lagoon, the longer its life span. Storms, with their high winds that churn the water, and rains which boost the speed of creek flow, help to scour lagoons and remove sediments to the ocean. T he life span of a most lagoons depends on the balance between the processes of sedimentation and scouring. Because sedimentation is usually stronger than scouring, it is the fate of most lagoons to become upland habitat with in a few hundred to a few thousand years. Bolinas Lagoon, on the other hand, has maintained a healthy estuarine ecosystem for many thousands of years.