Diversion dams are also called low-head dams. They are a HUGE risk in river kayaking. They are hard to see coming, and are frequently lethal. The water flows over the top of the dam, then forms a recirculating hydraulic or hole at the bottom. The drop may be relatively small, but in fast current, the hydraulic is very strong.

In most naturally-occurring hydraulics you can work your way to one side or the other, or find a weak spot in the flow, and flush out. With manmade low-head dams, the hydraulic doesn't have any weak spots and extends all the way across the river - usually with steep concrete banks on each side. There's no way to get out the sides, and no weak spots.

This is also one situation where PFDs can make things worse. The water in a hydraulic is usually flowing out at the bottom of the river. If you are in a lifejacket, it can keep you too high in the water to get caught in the outflow. You get carried to the front of the hydraulic, forced underwater, carried a little bit downstream, then float to the top where the water carries you back to the front of the hydraulic.

Condolences to their friends and family.