Lakes, Rivers and Oceans

A river's ability to move sediment depends almost entirely on how fast it's moving. Fast water can lift and carry large, heavy particles like boulders and gravel. As the current slows, it loses that lifting power quickly, so it drops its heaviest load first and keeps carrying the lightest mud and clay the farthest. This is why a river leaving a steep canyon and spreading onto a flat plain dumps its boulders and coarse sand almost immediately, while fine silt rides on for miles before settling. The relationship isn't gentle, a small drop in speed costs the river a lot of its carrying power.

Inside a single bend, the river isn't moving at one uniform speed. Water on the outside of a curve has to cover more distance in the same time, so it moves faster there, cutting into the bank and carrying material away. Water on the inside of the curve is slower and sheds its load, building up a sandbar. Over years this pushes the whole bend sideways, eroding the outer edge while growing the inner one, which is how meanders slowly migrate across a floodplain.

Lakes show a similar story with heat instead of speed. Sunlight warms the surface in summer, and warm water is less dense than cold water, so it floats on top. Wind can stir that warm upper layer but can't punch through the sharp density boundary beneath it, called the thermocline, so the cold bottom water stays isolated until cooler weather weakens the contrast and lets the layers mix again.