The river is meandering across the valley and erodes laterally.
Most water is directed towards the outside bend, there is also reduced friction and increased velocity.
The fast flowing water erodes the outside bends using hydraulic action, when the sheer force of the water gets into small cracks and breaks down the rock, corrosion, when the river water dissolves minerals from the rocks and washes them away and corrasion, when the river banks are eroded by the load hitting against them.
There is less water on the inside bend, an increase in friction and a decrease in velocity. As the river has less energy, it deposits material so its course is changing.
Over time, continual erosion and deposition narrows the neck of the meander.
Often during a flood the river will cut through the neck of the meander.
The river continues on its straighter path and the meander is abandoned.
The fastest current will now be flowing in the centre of the river channel and deposition is more likely to occur beside the banks.
New deposition seals off the ends and the cut-off becomes an ox bow lake that will eventually dry up, except during periods of very heavy rainfall.