Eddies are a sections of the river that actually move upstream.
Eddies are created when the main current flows around an obstruction and as the main flow of water continues downstream, some water backs up on its self to refill the space left behind the obstacle. This refill creates a circulating flow of water that travels back upstream and then back out into the main flow. Because of this generally the edges of a eddy can be faster flowing and less stable the middle of an eddy, which can often be almost still. The point where the eddy meets the main river flow is called the eddy line. This is often an unstable turbulent section of the river and may be made up of whirlpools and other interesting types of river flows.
Eddies can be typically be found located behind surface rocks or any obstacle that disrupts the surface flow. This can even be behind a hole!
Eddies are great places for kayakers to stop and take a break as they travel downstream. They also provide great places to scout rapids, and you can play lots of games by working to catch as many eddies as you can on your way down stream.
The ability to eddie hop (move from one eddy to the next) your way down a rapid provides a great amount of control to your descent of a rapid. Generally when we prepare to run a rapid we look at finding a route that will allow us to kayak safely between eddies. It like a dot to dot. Linking up the eddies as we make our way downstream.