Any part of the river that is at all rough and produces the white foam mentioned by Peachy is called white water.
Usually you navigate white water in a rubber raft. You can go singly or in a larger raft with more people (considered safer).
Each set of rapids (fast moving water) is rated 1 through 6. 1 is very small waves, very easy. It gets harder until you get to 6, which is considered almost suicidal, even for a good rafter.
The best I've ever done is a rating 4 rapids and that was with a group. We got dumped over in the water on one of the rating 4 rapids. It's pretty scary. You learn that the one thing you never do is to stand up, even in shallow water. The current is so fast, it can tip you over and if you catch your foot, you will drown.
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