Adventure and Recreational Rafting is not without risk, sadly there are fatalities and injuries occurring every year on various rivers, no matter how difficult.
Rivers are a dynamic, the only constant is change.
Always check out river conditions and bring the correct equipment for such conditions. High flows mean faster water, more unfamiliar hazards and differences than you may be used to on the river, and there will likely be many more dangerous strainers. Longer swims are likely in faster moving water and may allow hypothermic results to set in more quickly. With helmets, it's recommended you always use them if you have them, but always on Class IV or higher. Think about a boat flip throwing yourself and other participants against a rock and then slamming into you.