As a river flows towards its mouth, the countryside around the river often changes from hilly to flat. As it flows over the flat land the river becomes wider and slower. A wide slow river is called an 'old river'. An old river often floods across the land after there is lots of rain at the headwaters. An old river slowly builds up its banks on either side; the high banks are called levees. An old river often meanders (twists and turns), and sometimes, after a flood, it leaves lakes behind which are called ox-bows or billabongs. Old rivers are the most useful type of river for growing crops. Corn, rice, fruit, cotton, hay, tobacco and sugar are some of the crops that are grown near old rivers.