The loop curve in a river or stream is called a meander. These twists and turns form over time and can completely change the shape and direction of the river.
The flowing water of a river or stream erodes sediments and rocks from the banks of the outer curves of each bend. The sediments are carried farther down stream and deposited on the inner curve of a meander, gradually allowing the meanders to grow larger and larger. Water flows fastest on the outside curve of a meander, causing faster erosion. The inside of each bend is more shallow with slower water flow. Unless the conditions are right for a sudden and drastic change in the river's form, meander bends take years to develop.