Any time you're near or in a river or stream, you should be aware of the risk of flash floods. Flash floods claim a number of lives every year. They can strike when you least expect them to, in a matter of seconds.
A river or stream has a normal current, that goes through it all the time. When it starts to rain heavily upstream, a whole lot of water is dumped into the river in very little time. The effect is a sort of tidal wave that moves downstream, taking anything it might find in its path: logs, branches, rocks, garbage, whatever. Get caught by the wave, and it's very likely you're not going to get out.
Always look at the water flow when you're crossing or entering a river. Watch out for water that looks muddy or is carrying a lot of floating debris. If you see dark clouds upstream, especially in the mountain ranges, that's a warning sign that something might come down eventually.
When crossing, use a safety rope whenever possible, and measure the depth with a stick as you're crossing. If you're bathing in the river, be careful of any sudden increases you notice in the force of the water, or sudden changes in color from clear to muddy. If you start to see muddy water, or the river starts to swell, get out fast. You might have only a few minutes before the big wave hits.
You should wait at least an hour after the first warning signs before re-entering the river. Remember the swell doesn't always move at the same speed, and rain upstream may calm down for a while, before really coming down.