I hope this doesn't seem insensitive, but under controlled conditions with a life-guard nearby, it would be useful to reconstruct someone of similar age/size falling into the river at that exact point wearing the exact same clothing.
Does the volunteer immediately get into serious trouble? Are they able to surface, if so, how quickly? If they surface without panicking too much, how well can they tread water, can they hold onto the side, for how long? If they are unable to surface, or tread water, where do they end up in 30 seconds? What happens to their clothing - does it fall off, at all? Etc. I suppose it's pointless because they'd never be able to do it...