Final answer:
The formation of river meanders starts due to differences in roughness on the river's bottom, leading to varying water velocities and resulting in sedimentation patterns that shape the meanders.
Step-by-step explanation:
The process of meander formation begins when a difference in roughness on the bottom causes water to move faster on one side of the channel than on the other. This is because the channel is not uniform, and any difference in the roughness of the bottom can affect the velocity of the water flowing over it. As the river flows, it picks up sedimentation, which tends to be deposited where the water moves more slowly. Over time, this leads to the river's characteristic meandering shape, as the outer bends are scoured by faster-moving water while sediments are deposited on the inner bends.