Final answer:
Meanders are formed and migrate due to the widening of the river channel, the slowing of the current, and the deposition of sediment on the inner bend.
Step-by-step explanation:
Meanders are formed and migrate due to the combination of the widening of the river channel, the slowing of the current, and the deposition of sediment on the inner bend. As the river flows away from its source, the channel gradually widens and the current slows. This slow-moving water, caused by a decrease in gradient and an increase in volume from tributaries, leads to more sedimentation. The sediment is deposited on the inner bend of the meander, resulting in the formation and migration of meanders over time.