What are deltas and estuaries? How does their use affect their future?
During the last glacial maximum, a larger fraction of the planet’s water was tied up in surface ice. Correspondingly, the fraction of water in the oceans was less, meaning that the sea level was much lower during that time. Rivers flowed and eroded the exposed land like they would any other river canyon, carving depressions into the terrain. As the glaciers receded and their water returned to the oceans, sea level rose to fill the river territory, forcing the mouths of light-sediment-loaded rivers upstream. This formed estuaries, areas where seawater has intruded into river channels. In some cases, the river was depositing enough sediment to fill up these canyons as the mouth retreated, forming an extrusion of sea-level sediment called a delta. Deltas and estuaries create fertile land and marine environments, respectively. Deltas, full of well-sorted nutrient-rich sediment, are some of the most fertile areas in the world. Marshes and wetlands form along the edges of estuaries, where sediments collect.
The most notable delta on the east coast is that of the Mississippi River. The Mississippi drainage basin is a huge area bounded by the Rocky Mountains to the west and the Appalachians to the east. This 3 million km2 area captures a large amount of precipitation, which picks up glacial till brought into the region by ancient glaciers. This is the sediment that is deposited to form the Mississippi Delta. Estuaries, on the other hand, are fed by much smaller drainage basins. The Appalachians follow the east coast fairly closely, limiting the drainage area available to rivers flowing out to the east. The Chesapeake Bay Estuary is fed by a basin no more than 175,000 square km; as a result, it has a much lower sediment load and it terminates in an estuary.
In a natural steady-state delta, the sediment deposition by the supplying river is equal to the subsidence processes (erosive forces from the ocean plus the crustal sinking due to the heavy load of the introduced sediment). Humans have erected dams for recreation, agricultural, and hydroelectric purposes along inland rivers. Damming a river reduces its speed, which causes it to drop significant portions of its sediment load. Additionally, pumping groundwater, oil and gas from belowground causes the compaction and oxygenation of the source rock, adding an additional pressure towards crustal sinking and sediment removal. The combination of these actions decreases sediment deposition and increases sediment removal, shifting the equilibrium point at the outlet strongly in favor of subsidence.
Sea level along the Atlantic-Gulf Coast is currently rising at a rate of 2.1 mm/year, while the area around the Mississippi delta experiences an apparent sea level rise of 9.4 mm/year. The difference between the two rates (7.3 mm/year) can be attributed to local anthropogenic and geologic factors specific to the Mississippi Delta outlined above. The rate of sea level rise is expected to increase in the near future to an estimated rate of 5.6 mm/year. Even this higher rate of sea level rise is less than the local subsidence processes in play at the Mississippi Delta.
Like the Mississippi Delta described above, the California Bay-Delta is also fighting a losing battle in the balance between soil deposition and subsidence. However, the California Bay-Delta has agriculture as the main culprit in subsidence. The agriculture practices on the delta are oxidizing the soil and exacerbating soil removal by wind erosion to rates of over 10mm/year. These farming islands in the delta are now below the level of the river, requiring pumping and levees to keep water out of the farmland. The farmland is based on a peat layer, and pumping water out from under it oxygenates the peat material, accelerating decomposition and compaction which further exacerbates the subsidence. Like the disaster in New Orleans, floods or earthquakes could cause the protective levees to fail catastrophically, but the potential to pollute the water supply diverted to Southern California with intruding seawater is the main hazard resulting from the failure.