An eddy is a reverse piece of current in a river. It will look like a whirlpool compared to the main flow of water. it can often occur downstream of an object such as a rock or towards the edges of rivers. An eddy can also be formed when a large volume of fast water is injected into a river such as when a sluice gate has been opened or a fast stream enters the river. In all my years of fly fishing, I confess I have never had much success fly fishing on eddies.
They can have a washing machine like effect on the fly line meaning it’s can be difficult present a fly in a natural fashion. The other point to remember is that as fish will always face up the current which in an eddy can mean they are facing backward in comparison to the main flow of the river. Another point to consider is that fish like to have a predicable line of current where they can intercept and pick of food in the current. In some eddies complex vortexes and swirling current means it’s not an easy place for fish to pick up their dinner compared to a nice straight piece of current flow. However where the current isn’t too complex, eddies can hold dead flies, bugs and nymphs in making easy pickings for trout and these can be good fish holding spots. On my local river, the back eddies seem to make great nursery areas for trout parr who feed on the countless midges trapped in the swirling surface film. I have had far more success fishing on the seam of the current of an eddy, where the straight flowing current meets the swirling water of the eddy.