The back eddy is a slow reverse flow of current, usually located on the side of the pool opposite of the main current. This can range from three feet o twelve feet in diameter and it can be from a foot to three feet deep. During periods of high water this becomes a very important feeding area because the trout can locate here to avoid the powerful currents. I once caught many large trout here during a high spring run off. There was a great hatch of blue quill mayflies but due to the cold air and cold stream many of the nymphs were having trouble popping their wing pads and many of the duns were having trouble drying their wings to fly off the surface. Consequently the back eddies were covered with dozens of these flies. Many large trout held beneath these flotilla of mayflies and sucked them in at their leisure. An effective way to fish this situation is to cast a dry Blue Quill dry fly size 16 out in front of each feeding trout.