All summer, the sun heats the surface of rivers and streams while the deeper water stays relatively cold. When fall comes, the surface water chills, reaching a temperature of about 4°C. At this temperature, the surface water is at maximum density, which means it is heavier than the water at the bottom. So the surface water sinks to the bottom while the deeper water rises to the top. This natural mixing allows among other things for the water to re-oxygenate and better disperse minerals. Remember this phenomenon when, on a windless day, you still can see waves and movement in the river!