Delta

Abandoned delta lobes experience rapid subsidence primarily due to sediment compaction. As water depth increases accordingly, enhanced wave and current attack contribute to rapid erosion of old delta deposits—a process called delta retreat—and the loss of vast tracts of ecologically important wetlands and barrier islands. Modern flood controls, such as channelization and levee construction, sharply reduce avulsion and delta switching. As a result, little or no new sediment is contributed to the delta plain outside of the large channels. Consequently, compaction, subsidence and erosion continue unabated. This enhanced effect results in the loss of up to 15,000 acres of wetlands per year in inactive areas of the Mississippi River delta plain. Global warming could accelerate this effect by triggering higher rates of global sea level rise and resulting in more rapid increases in water depth. Increased hurricane incidence in the 1990s and beyond also takes a toll.

Construction of dams upstream impacts deltas too. Dams not only trap water, they trap sediment as well. This sediment load would normally contribute to delta progradation, or at least help stabilize delta lobes. Construction of dams instead results in significant delta retreat. For example, construction of Egypt's Aswan High Dam on the Nile River in 1964 lead to rapid erosion of the delta plain with loss of both wetlands and agricultural lands.