How are ox-bow lakes formed?

As a river flows across a valley over time it will meander in order to find the path of least resistance through patches of soft and hard rock. As the river meanders it erodes the outside of the bend (the softer side) and deposits sediment on the inside of the bend (the hard side). Continued erosion on the outside of bends over time narrows the land between bends (the 'neck'). In times of flood the river will go through the 'neck' and create a new more direct path. As this is now the path of least resistance the meander becomes redundant and despotion will occur, eventually leading the meander to be cut off from the river and forming an ox-bow lake.