What is the triangular shaped area of land at the mouth of ariver?

The triangular-shaped area of land at the mouth of a river is called a delta. Deltas form from the deposition of sediment carried by the river as it slows down and meets a standing body of water, such as an ocean or lake. This accumulation of sediments creates fertile land that is often rich in biodiversity and is important for agriculture. Famous examples include the Nile Delta and the Mississippi Delta.