What is the term for the flat, low-lying area adjacent to a river that may become inundated during flooding?

The term used to describe the flat, low-lying area adjacent to a river that can become inundated during flooding is "floodplain." Floodplains are formed by the deposition of sediments carried by the river during floods and play a crucial role in the river's ecology and hydrology. These areas are significant for various reasons; they often support diverse vegetation and wildlife, serve as important agricultural lands due to their fertile soils, and act as natural buffers that can absorb excess water during flood events, helping to mitigate damage upstream and downstream.

In contrast, the other terms do have specific meanings that do not apply in this context. The water table refers to the level below which the ground is saturated with water and is not directly related to flood dynamics. An estuary is a coastal area where freshwater from rivers meets and mixes with saltwater from the ocean, which is a different geographical and ecological context. A basin typically refers to a depression in the Earth's surface that can collect water, but it does not necessarily relate directly to the area beside a river that is subject to flooding. Thus, floodplain is the most accurate term for the area in question.