Fly fishing eddies: 1
Another useful area of knowledge in fly fishing is the behaviour of water. So let’s first take a look at the eddy.
Eddy is a term used in the physical science of fluid dynamics to describe the swirling and reverse currents associated with moving fluids mediums such as air and water as it passes over and around an obstructing object and back-filling the void space behind or downstream from the object. It does this with a characteristic swirling current running counter to the main current's forward direction.
In a river environment, eddies (also spelled "eddys") are fairly common and generally occur in the space downstream from or behind large exposed rocks, boulders, and partially submersed obstructions like fallen tree limbs and other debris, either natural or man-made.
You can observe eddies in river environments as the gently swirling, slow moving areas between between point bars and main channels. They can also be created near severely undercut banks where localised reverse currents tend to create an "eddy-like" effect at the water's surface. Reading the water, not just the eddies, is a great fly fishing skill worth developing.