A hydraulic in a river is formed when water falls over a ledge, rock, log, or other object. When the water falls off this object, it picks up speed and when it hits the
…water below, it rolls back on itself, creating what paddlers call a "hole." These are easily identified on a river by the white, foamy water that can be found just below a large ledge or rock. Not all hydraulics are dangerous, but some are. Hydraulics can be very sticky, which means that a kayak, raft, or person that falls into a hydraulic can become stuck in that hole. Most hydraulics are not very serious, unless you are on a Class IV-V river where they become very powerful, capable of holding a raft full of people. It's best to avoid them, or to keep forward momentum when going into a hole or hydraulic. Dangerous conditions occur around man-made concrete lower water dams because they form "perfect" hydraulics not usually found on a natural river. The clean, straight lines of a concrete dam allow water to spill over in a perfect line, and this creates a very powerful hydraulic that can trap a boat, even one equipped with an engine. If you are caught in a small hydraulic, paddling your raft will be difficult because the water is aerated (full of air pockets and bubbles) and your paddle will not find much purchase to pull against. If you find yourself swimming in a hydraulic, it is best to try to swim out. Sometimes you can swim out of the sides of a hydraulic if there isn't a lot of water feeding back upstream into the hole. If you are swimming and nothing is working, try swimming towards the bottom of the river and head downstream. The current is often pushing away from the hole at the bottom. Some hydraulics will require outside assistance to escape, such as a rope thrown from the bank. -DF (MORE) 
