Whew! It’s a good thing they told us this..otherwise I’d probably just stand there!
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They put up those signs after a flash flood about 20 years ago that claimed a few lives there in Colorado. While the sign itself looks “duh”-like, the posting of these signs accompanied a campaign to make the public aware of the roaring sound that accompanies a flash flood. This awareness is supposed to make you climb to safety BEFORE you see the water rushing at you, and hence the signs warn you to listen for this sound since they are located in areas that are prone to flash flooding.