EARLY in this century, Thienemann1 and Naumann2 described the physical conditions in deep freshwater lakes in the temperate zone. They found a yearly cycle of events which depended on the thermal stratification of the water. Their findings, and the conclusions they drew from them, can be summarized briefly, as follows. In winter and early spring, the temperature and density of the water mass is almost uniform from the surface of the lake to the bottom, so that the viscosity of the water is the only hindrance to mixing by wind. In late spring and early summer this state of affairs changes because the surface layers become warmed up by increased radiation from the sun and conduction from a warmer atmosphere, and by the inflow of warmer rivers. The result of this warming is that, in summer, a warm upper layer of less dense water, the epilimnion, comes to lie over a cold deeper water mass, the hypolimnion. The two are separated by a region of very steep vertical temperature gradient, the thermocline, with the result that mixing by wind is impossible.