A geographic feature typically defined as a low-lying coastal area is called a delta. Deltas form at the mouth of rivers where they meet larger bodies of water, such as seas or oceans. This occurs when the sediment carried by the river is deposited, creating landforms that can support ecosystems and human activity. Deltas are often characterized by their rich biodiversity, fertile lands, and high agricultural productivity. Notable examples include the Mississippi Delta in the United States and the Nile Delta in Egypt. These regions are critical for both environmental and economic aspects.