Keynote 6 - Challenges in deltas under pressure

James Syvitski

Tuesday 30 june 2015

10:25 - 10:55h at Mississippi (level 1)

Themes: Keynote

Parallel session: Keynotes 5 & 6

Perhaps the greatest challenge facing deltas is the human occupation of an otherwise transient environment. Many modern river deltas began to form 6,500 to 8,500 years ago, as sea level stabilized. Other deltas such as the Po, Ebro, Rhone, and Yellow deltas owe their existence to increased flux of sediment that was liberated from uplands due to human disturbance — deforestation, agriculture practices, mining, and urbanization in general. While deltas were rapidly growing and shorelines were shifting seawards, delta dwellers were mostly concerned with the ephemeral nature of water routes distributed across the delta. Distributary channels would shift every few years to decades. So while deltas offered lush vegetation, diverse forests, wildlife and fisheries, and rich organic muddy soils, movement through these ultra-flat environments remained difficult. Additionally, (sub) tropical infections, particularly from disease-carrying insects, kept human populations low. And while annual river-floods were greatly needed for nourishing agricultural soils, the Nile Delta being the type example, floodwater inundation also destroyed habitats and livelihoods. Some cultures adapted to these episodic floodwaters, such as in the Ganga delta, where growing population centers were often found upstream and off of the delta. To control floods and reduce risk, human-fortified levees surrounding the large branches of the river became popular. In the case of the Mississippi Delta, embankments were made with the full knowledge that future land might be lost to the sea. Constraining floodwaters through levees also forced the sediment-laden water to enter and exit the delta largely intact, with most sediment being deposited at mouth bars, as shallow marine deposits, within channels and along shorelines. Where delta swamplands were drained for agricultural purposes, soil and subsoil peats would rapidly oxidize and the land surface would compact.