여울, yeoul: the place in a river where the bed comes up or the banks close in, and the water runs fast and makes a noise.
Water that has been going quietly for a mile arrives here and has something to say about it.
But a yeoul is a stretch, not the river. Past it the ground opens out, and the water slows and goes quiet and deep again, the way it was before and the way it will be.
You can hear one long before you reach it. That is how you know where it is. It is also how you know when you are past it.