DELTAS ARE “AT RISK” ANTHROPOCENE LANDSCAPES

Many coastal deltas formed 6,500 to 8,500 years ago, as Holocene sea level stabilized. Other deltas owe their more recent existence to the increased flux of sediment liberated from uplands due to deforestation, agriculture practices, mining, and urbanization (e.g. Ebro, Po deltas). Distributary channels would shift every few decades, and near-annual flood inundation would nourish soils (e.g. Nile Delta). Some cultures adapted to these episodic floodwaters by keeping their growing population centers upstream and off of the delta (e.g. Ganga delta). Other cultures constrained floodwaters through levees, and forced the sediment-laden water to exit the delta largely intact, with sediment being deposited at mouth bars, as shallow marine deposits, within the leveed channels, and along shorelines. Swamplands were drained for agricultural purposes, and thus peats rapidly oxidized.

During 20^th-century dam emplacement, billions of tonnes of sediment were sequestered in the upstream reservoirs— the flux of sediment delivered to deltas was greatly reduced. In some rivers (e.g. Colorado, Ebro, Nile) little to no sediment is now delivered. Where distributary channels are not embanked, sediment-free floodwaters can no longer aggrade the land surface.

With 600 million people now living on or near deltaic environments, urbanization of the landscape is common. Delta subsidence has followed, attributed primarily to: 1) groundwater mining for potable water, and aquaculture and aquaculture needs (e.g. Yellow, Mekong, Yangtze, Chao Phraya deltas), 2) petroleum mining (e.g. Po, Mississippi, Niger deltas), and 3) peat oxidation (e.g. Rhine delta). Deltas are sinking at rates of 10s to 100s of mm/y. Additionally; sea level is rising at rates now exceeding 3 mm/y due to human-caused climate warming. Consequently, shorelines are rapidly receding, and great swaths of land are being given up to the marine environment. Elsewhere and to accommodate land surfaces below sea level, coastal barriers encircle shorelines, and soils are continuously pumped to limit saturation. In essence, deltas are presently Anthropocene landscapes, and they are ever more at risk.