Rivers go through three stages as they flow from their source to the sea - youthful, mature, and old. In the youthful stage, the fast-moving river erodes the landscape, creating V-shaped valleys, interlocking spurs, and waterfalls. During the mature stage, the river transports eroded material and deposits it, forming wider valleys, meanders, and flood plains. In the slow-moving old stage, deposition creates features like ox-bow lakes, levees, and deltas at the river's mouth. Rivers have long been important to people for transportation, water, and more recently, power generation through hydroelectric dams.