Rotational flow around a vertical axis. Circular current of water, sometimes quite strong, diverging from the main current. (It usually forms where the flow passes some obstruction or at the inside of river bends, and often forms backwater pools or pocket water in riffles.) A circular water movement that develops between the main flow and the bank. (An eddy usually forms downstream of a structure or an abrupt change in the shape of the bank. Eddies are often the cause of bank erosion.)