Why do rivers meander instead of flowing in a straight line?

A river usually starts to bend when tiny differences in the landscape or in flow create a slight curve. Water moves faster on the outside of the bend, so it erodes that bank, while slower water on the inside drops sand and silt. Over years to thousands of years, this process makes the bend wider and more pronounced. Meanders are especially common on gentle slopes and in river valleys with soft, easily eroded sediment. Eventually, a meander can be cut off, leaving behind an oxbow lake.