RIVERS ARE ONE OF THE MAJOR FORCES that shape the landscape. Near its source, a river is steep. It erodes downward, carving out V-shaped valleys and deep gorges. Waterfalls and rapids are formed where the river flows from hard rock to softer, more easily eroded rock. Farther downstream, meanders may form and there is greater sideways erosion, resulting in a broad river valley. The river
sometimes erodes through the neck of a meander to form an oxbow lake. Sediment deposited on the valley floor by meandering rivers and during floods helps to create a floodplain. Floods may also deposit sediment on the banks of the river to form levees. As a river spills into the sea or a lake, it deposits large amounts of sediment, and may form a delta. A delta is an area of sand bars, swamps, and lagoons through which the river flows in several channels called distributaries—the Mississippi
delta, for example. Often, a rise in sea level may have flooded the river mouth to form a broad estuary, a tidal section where seawater mixes with fresh water.
