Most islands in the delta are thought to originate from tree saplings growing on termite mounds. As the trees grow, water and solutes are drawn up from the ground into plants and released through transpiration. This causes the water table to drop over time, which draws in water from the surrounding floodplains to replace it, and so keeps the area replenished with fresh water.
Salts concentrate under the islands creating zones of hyper-saline soils in which few organisms can survive, but as the concentrations of salts under the islands increases, the salts become sufficiently dense and heavy that they sink to the bottom, effectively ensuring a layer of fresh water above.
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