Rivers shape the land through erosion and deposition as they flow from their sources downstream. In the upper course near the source, the river erodes steep V-shaped valleys due to its high gradient and velocity. In the middle course, meanders and oxbox lakes form as the river erodes the outside of bends and deposits sediment inside bends. In the lower course near the mouth, floodplains and deltas form as the river's velocity decreases and it deposits large amounts of sediment.