River mouth landforms are typically composed of deltas, which are formed when a river meets an ocean or lake and deposits sediment. Deltas are characterized by their triangular shape, with the apex pointing upstream. They are also known for their intricate networks of distributaries, which are smaller rivers that branch off from the main river and carry sediment to the delta's edges. Deltas are important ecosystems, providing habitats for many species of fish, birds, and other wildlife.