It is erosion by flowing water, especially when the river is in spate, that can widen the river's banks.
bank erosion
a bank erosion is when the river side takes the dirt and soil from the sides of the ground we stand on were we what to jump in at
Daniel H. Stern has written: 'Effects of bank stabilization on the physical and chemical characteristics of streams and small rivers' -- subject(s): Erosion, Rivers, Stream channelization, Streambank planting, Turbidity
water runnoff
Horizontal erosion is also known as lateral erosion. It is the wearing away of earth from a footpath or river bank.
The four types or mechanisms of erosion are rill erosion, gully erosion, stream bank erosion, and sheet erosion. In sheet erosion a thin layer of the surface is removed. In rill erosion, small rivulets of running water gather together and cut small channels. In gully erosion, rills enlarge to form bigger channels too large to be removed by normal tillage. Finally, streambank erosion is the washing away of soil from banks of streams and rivers. Erosion is a natural, continuous process, but agricultural practices can speed erosion, resulting in lost soil, lost soil fertility, air pollution, and water pollution. Wind and water are the main agents that move soil.
Please rephrase - the question is unclear.
Hold up a bank
erosion on a bank of a pond
It changes because the force of the water and because the bank is loose and dry from the erosion. The bank will move which ever way the water is flowing.
Lateral or horizontal erosion means erosion taken place on the sides of a footpath or river bank. The opposite if lateral/horizontal erosion is vertical erosion, where erosion is taken place under the river bed or the bottom of a footpath.
Rivers may slowly meander along a level valley floor. Erosion may happen at certain points along the river bank, while silt and sand may be deposited at other points. This action tends to form bends in the river.