In the upper course, the river channel is narrow and steep as it cuts downwards through the landscape. In the middle course, the river widens and deepens its channel through lateral erosion, forming meander loops that can become cut off to create oxbow lakes. In the lower course, the river slows and widens its floodplain, where deposition builds up rich alluvial soil and landforms like natural levees and deltas can form at the river's mouth.