Why do some rivers change course suddenly after floods?

During a large flood, a river carries much more water and sediment than usual, so its banks and bed become highly unstable. Water may breach a low point on the floodplain and carve a new, shorter path. If the new route is more efficient, it can capture the main flow and leave the old channel dry or reduced. This process, called avulsion, is common in flat deltas and alluvial plains. It can dramatically reshape landscapes, redirect farmland, and alter navigation routes.