Undercuts represent an extreme hazard to boaters, since rescue is nearly impossible once someone becomes trapped under such a rock by the force of the current. Some, but NOT all, undercut rocks may also be recognized by the presence of a current coming out from the under the rock on the downstream side. Avoid undercuts, period. These come in two flavors, upstream and downstream. What a boat does when it gets plastered onto some obstruction by the current.
Rafts tend to fold around rocks, dumping the occupants into the river.
Luckily, most of the time, raft occupants will fall out to either side of the pinning obstruction and wash away. Wrapping and pinning situations are extremely dangerous, and call for quick, knowledgeable rescue. Embarassing in benign situations, and dangerous in difficult whitewater. All rights reserved.