Riverboat hazard is a term used to describe the dangers that can be encountered while navigating a river. Two of the most common hazards are snags and shoals. A SNAG is an underwater obstruction, such as a tree trunk or large rock, that can cause a boat to become stuck or damaged. Shoals are shallow areas of water that can cause a boat to run aground. Both of these hazards can be difficult to spot and can cause significant damage to a riverboat if not avoided.