This is fun fun fun, a cooling escape from the heat of a Utah midsummer, a wonderful challenge, and an experience you will not forget. My daughter-in-law, who is afraid of water and doesn't swim, was naturally nervous about going on this; 2 hours later she was trying to figure out how she could arrange to have her wedding in the river (she opted for SLC and just brought her wedding party here to enjoy the honeymoon with her). Read about it first, carry lots of water (you don't want to drink the river), have appropriate shoes and some sort of hiking stick (we use old ski poles). Plan for several hours/all day in the river. Get out if it starts to rain. Float down the river on your back to come back, it is the easiest way and you will get hundreds of funny looks but it is a riot. It is VERY slippery and rocky, there are hidden petroglyphs, take a waterproof camera and assume you will get completely wet. The best combination of water, rock and desert that I have ever seen. Your older kids/teens will love this. (Date of my visit actually is multiple visits from 1997 through 2010, usually in July. Don't try this if the water level is high, it is too dangerous.)
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