Richard F. answered this one for you.
The water in rivers runs downhill, but in most places the slope of the hill is so gentle you can't see it. If the river is running through soft soil or sand, or even soft rocks, it wears them down until the slope is nearly flat. Sometimes, though, the water runs down a steeper slope through hard rocks. It can take millions of years to wear the rocks away. In the meanwhile, the water rushes down, over and between the rocks. These are what we call rapids. Rivers that run down from hills or mountains, or that run through rocky country, usually have rapids.% % filename IO.DOT
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