Lake Sagamore, with maximum depths of around 18 feet, is a persistently stratified lake. Every summer, a layer of warm, well-aerated, sunlit water forms on top of the cooler and thus heavier water at depth. The two layers mix somewhat during storms, when wind and rain move things around, but the disturbance never reaches the depths and the layers soon reform. During the winter, the lake is also stratified, but this time with the coldest water at the top, not the bottom. In the spring and again in the fall, as you might therefore expect, the cold and the warm trade places. This occurs because water is at its densest at 39 degrees F, seven degrees above freezing. It expands like a normal substance as it warms from this point – but it also expands as it cools. When it freezes it expands even more (which is why ice floats). Thus, as the sun warms frigid Lake Sagamore in the spring, the surface water gets heavier, and sinks to the bottom! This brings bottom water to the top, where it warms up in its turn, and sinks. This goes on for several weeks, and is called the “spring overturn.” Eventually the whole lake gets to 39 degrees, and overturn stops. Since the surface layer can’t sink any more, it just keeps heating up and getting even less sinkable — and the two layers are formed for the rest of the season. Actually, the bottom water also warms up, but more slowly, so that in late summer the bottom water may be up to 65 degrees while the surface layer comes close to 80. In the fall, the water on top cools down, and a “fall turnover” proceeds until the whole lake is once more at the same magic temperature of 39 degrees. From this point on, the top layer rapidly gets colder and less dense, while the bottom layer cools more slowly, in a mirror image of the summer stratification.

The depth to which sunlight warms the lake depends on the clarity of the water, which in turn depends on nutrients, because turbidity is not due to suspended dirt and mud but to the green clouds of one-celled algae (mainly diatoms) that float in the upper few feet of water. Lake Sagamore is not overly nutritious, so that algae are usually not a nuisance. During overturns, however, nutrients that have been out of reach all season are brought into the surface zone and the result is an “algal bloom,” which only dissipates after the water stratifies. In early summer the thermocline between warm and cold lies at a depth of around four feet, but the warm layer thickens over the summer and the thermocline sinks to around nine feet by end of August. (Swimmers who find their feet swishing in cold water as they tread may also be hovering over one of the springs in the lake floor.)