A wide variety of underwater factors can cause an eddy. But whatever it is that suddenly deflects the flow, it results in passing particles of food slowing down in the current so that fish are able to take them more easily. The illustrated swim on the upper Kennet affords a useful eddy that neither boils nor spins. In time of high water and flooding, the effect of eddies on passing food is exaggerated. But fish avoid being tossed and twirled in turbulent water, so that roach, perch, chub, bream and barbel position themselves close-by, where the eddy begins to attain an equilibrium. ‘Reading’ the exact spot requires a lot of practice. But there are eddies almost devoid of fish. In the swim on the Dorset Stour, for example, the undercut lay-by seems to promise big chub, but the boiling holes at this elbow are in fact, far too turbulent to contain fish.

An eddy is a natural dustbin as well as a larder. To avoid this vortex of rubbish, fish keep out of the thick of an eddy. However, floating rafts of rubbish can provide an excellent indicator of where the fish will be. The point where bits of the raft break away are likely hotspots, for the same is probably happening under the water to food particles which, swept along by the current, adhere to the rubbish.

Fishing lay-bys may be too complex and problem-prone for them to become your favourite kind of swim. But they certainly provide a challenge, and can be looked on as preparation for the even bigger challenge of a weirpool—a wild mass of eddies with its own distinctive character and fish population.