River deltas, the fans and fingers of land created when the sediment carried by a river settles out at the river's mouth, grew by an average of 54 square kilometers per year between 1985 and 2015. A quarter of that growth was due to greater upstream erosion caused by deforestation "freeing soil from the grip of tree roots, allowing rivers to carry more of it downstream." More than half the global growth in deltas occurred in South, Southeast, and East Asia. North America, by contrast was the only continent to lose delta area, "partly due to damming along the Mississippi River."
Dutch researchers "examined 10,848 deltas to quantify humans’ impact. Three primary forces shape deltas: rivers delivering sediment; tides pushing or pulling sediment; and waves redistributing sediment along the coast. Humans exert a lot of control over how much sediment a river carries: While deforestation feeds the flow of soil, dams plug it up. ... In about 1,500 deltas, soil erosion due to deforestation increased sediment loads by more than 50 percent. ... Dams reduced sediment supply by more than 50 percent in 970 other deltas."
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