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Deltas are areas of low-lying land formed by the accumulation of soil and sediment that is deposited at the mouths of rivers, typically where they meet a larger body of water, like an ocean or lake. The process occurs when the river slows as it enters a standing body of water, causing it to lose energy and drop its sediment load. Over time, this can create a network of small islands and waterways, which can support diverse ecosystems.

Estuaries, while also related to river mouths, specifically refer to the transitional areas where freshwater from rivers mixes with saltwater from the sea, rather than being defined primarily by sediment deposition. Marshes are wetlands that may flood periodically and can be found in low-lying areas, but they are not exclusively formed by river sediment. Bays are coastal bodies of water partially enclosed by land and are not directly tied to sediment deposition by rivers. Therefore, the defining characteristics of deltas make them the correct answer.