A world-wide system for classifying brooks, streams, and small rivers is proposed, using the following criteria: width, flow, current speed, substrate, summer temperatures, winter temperatures, turbidity, total dissolved organic matter, total dissolved inorganic matter, water hardness, dissolved oxygen, rooted aquatic plants, streamside vegetation. Some of these criteria have much greater biological implications than others. It is contended that widely separated lotic habitats that are similar in the above features have biotas consisting of ecologically similar and parallel clusters of species. There are, nevertheless, many kinds of “atypical” polluted and unpolluted lotic habitats that cannot be classified with respect to these criteria.