What term describes the low-lying swampy plain built up by a river's load at its mouth?

The term that describes the low-lying swampy plain built up by a river's load at its mouth is "deltas." Deltas form when a river reaches a standing body of water, such as a lake, sea, or ocean, and begins to deposit the sediment it has carried from upstream. As this sediment accumulates over time, it creates a triangular or fan-shaped area known as a delta, characterized by rich, fertile soil and complex ecosystems.

Deltas are typically found at the mouths of rivers where they slow down and spread out, allowing for sediment deposition. This process leads to the formation of various channels and wetlands that contribute to biodiversity and often serve as critical habitats for wildlife. The rich nutrients in deltaic soils make them ideal for agriculture, contributing to the livelihoods of many communities.

Other terms such as estuaries, basins, and floodplains refer to different geographical features. Estuaries are areas where freshwater from rivers meets and mixes with saltwater from the sea, basins are depressions that collect water and sediment but do not necessarily have the same dynamic characteristics as delta environments, and floodplains are the flat areas adjacent to rivers that are subject to periodic flooding but do not form at the river's mouth.