Eddies are your friend! All good boaters catch eddies and use them to control their descent downstream. A good rule of thumb in paddling is to never leave an eddy until the coast is clear from above and you can see how to reach an eddy below. Eddies are formed by blockages in the river. A blockage can be a rock, penninsula, ledge, or even a man-made object. Water flows downstream around the object creating a depression in the surface of the water. Water directly behind the object is either still or moving slowly back upstream to fill in the small depression. When the water makes it back up the the object, it bounces off to the sides and flows back downstream. The calm water is what we call the eddy. The area between the downstream and upstream water flow is called the eddy line or fence. The top of the eddy line closest to the obstruction is very well defined. As you follow the eddy line downstream away from the boulder the dividing line is less well defined. When catching eddies, it is important to catch the eddy high, right below the rock where it is strongest and the eddy fence is the most narrow.

Here is a good example from the Lower Yough. Notice how relaxed we look. The slack water prevents us from drifting downstream. The eddy line is also very easy to see on our left side.