How do sedimentation and subsidence affect the depth of a lagoon?

Sedimentation and subsidence act as competing forces that determine the depth and longevity of a lagoon.

The Role of Sedimentation

Lagoons are typically zones of active sedimentation, where fine materials transported by rivers or derived from reefs accumulate in quiet waters. This process is often accelerated by flocculation and the presence of vegetation, causing the lagoon floor to build up over time. In many cases, this continuous deposition allows a lagoon to maintain its depth or gradually become shallower.123

The Role of Subsidence

Subsidence involves the sinking of the land surface, which effectively increases the depth of the lagoon basin. Whether a lagoon maintains its depth depends on the balance between these two factors. For example, in Texas lagoons, high rates of sedimentation have historically outpaced subsidence, leading to a net increase in the elevation of the lagoon floor. Conversely, if subsidence or a relative rise in sea level occurs too rapidly, the lagoon may deepen significantly or, in the case of barrier islands, be drowned entirely.13