Life on land and in the river are perhaps most closely linked in flood plains—wide, flat regions that extend on either side of a river. Rivers form flood plains over thousands of years. The process begins where a river flows into a low valley and a single channel may braid into multiple channels. During a flood, the multiple channels overflow their banks. As the water spreads out across the land, it slows, and any soil or gravel being carried along drops out as sediment. Over time, the build-up of sediment levels the landscape, creating the extensive flat surface of a flood plain. In just this way, the Brahmaputra, Ganges, and Meghna rivers, which flood every rainy season, have created the broad plains that nearly cover what is now Bangladesh in south Asia.
Many flood plains are underwater at least once every year. There, the only plants that thrive may be grasses or shrubs. But some flood plains are rarely flooded at intervals of less than hundreds of years. Flooding that infrequent permits the growth of forests that cover large areas and contain large, old trees.