The term 'swirl in a river' can be represented by the crossword solution 'EDDY', which is a four-letter word. An EDDY is a swirling body of water that forms in a river or stream due to the presence of an obstacle or change in the current. Eddies can form on the downstream side of rocks, islands, and other obstructions, and can also form when two currents collide. Eddies are often characterized by a circular motion, and can be seen in many rivers and streams.