Why do river deltas often have rich wildlife and fertile soil?

When a river reaches a lake or sea, its flow slows and drops mud, silt, and organic matter. This creates a patchwork of channels, marshes, and islands that are ideal for wildlife. The sediments also refresh the soil with nutrients, which is why many deltas are important farming regions. Deltas such as the Ganges-Brahmaputra and Nile support dense human populations as well as birds, fish, and amphibians. Their productivity depends on regular sediment supply from the river.