Deltas are named after a Greek letter because the Nile's looked like one
Greek writers saw that the land where the Nile meets the sea formed a triangle, like the capital letter delta. Herodotus used the name fourteen times for Egypt, but applying it to other rivers came later. In English, delta as a general landform only appears in the late 18th century, in Edward Gibbon.
A delta builds up where a sediment-laden river enters slower or still water: an ocean, a lake, a reservoir, occasionally another river too weak to carry the load away. Freed from its banks, the flow spreads out and slows, loses its carrying power and drops its sediment as alluvium. Deltas matter enormously to people as farmland and population centres, protect coasts, affect drinking water, and over geological time lock away carbon.
Aeschylus called the Nile's mouth the triangular Nilotic land without using delta. Polybius compared the ground between the Rhône and the Isère to the Nile's, calling both islands. According to Strabo, Onesicritus, who marched with Alexander the Great into India, called the Indus mouth a delta, and Arrian later drew the same comparison.
A single channel pushes a lobe into the water, such as the bird's-foot shapes of the Mississippi and Ural deltas. As it lengthens, its slope eases, sediment settles in the bed, and the channel rises above its floodplain until a flood breaks the levees and the river takes a shorter, steeper path. Repeated switches, along with mouth bars that split channels in two as at the Wax Lake Delta, build a distributary network and smooth the outline into a fan. The Mississippi has shifted between seven channels over the last 5,000 years.
Most big deltas, including those of the Nile, Amazon, Ganges and Yangtze, sit on passive continental margins, where gentle, broad terrain lets sediment pile up; steep, tectonically active coasts trap coarser debris near its source. Geologists classify deltas by whether rivers, waves or tides dominate. River-dominated ones behave differently depending on water density, and a Gilbert delta, named after Grove Karl Gilbert, forms from coarse material, as when a mountain river spills into a freshwater lake.
Source: River delta
