Standing waves are a somewhat rare phenomenon in rivers. It is a type of rapid that cycles water in a circle, similar to a wave you would see at the beach. This is what allows the surfer to stay in the rapid and surf it!
@dillonk i don't know ahahha i am still with @lillyann and @cheerfulcallie with this one!! but i guess only people who are daring enough would do this! well for me i don't think i will be able to do it but i would love to watch someone surfing like that!