If you’re at the cottage in spring, you can see all kinds of changes happening. Migrating birds arrive, leaves and flowers emerge, frogs start to peep… and the lakes flip.
That last one may come as a bit of a surprise, but nearly all lakes do it. To see it happening, you just have to look for the signs.
Lake flipping, or lake turnover, happens twice a year – spring and fall.
As every swimmer knows, in summer the lakes are much warmer near the surface.But you don’t need to dive down very far before you hit much colder water.
In fact, the lake in summer is divided into three distinct zones – a shallow area of warm water near the surface, a thin transitional area, and the colder region that makes up the entire lower part of the lake. Once you get into the colder region, the hyperlimnion, the temperature doesn’t change all that much the deeper you go.
What isn’t immediately apparent is that colder water is also denser. Water is at its most dense at 4 degrees C, which means that both warm water and ice (or near-ice) will float on denser, colder water.
In fall, as the air gets colder, so does the top layer of water. The autumn winds stir the lake and the three layers we had in summer get mixed into one layer – the water at the surface is roughly the same temperature and density as the water further down. That’s the lake turnover of fall.
As winter comes on, the lake has two layers – solid ice and near-ice at the surface, and cold water below. There is no region of warm water.
So what happens when spring comes and that ice melts? The ice melt begins to warm, until for a brief period the entire lake is somewhere around 4 degrees. Winds once again stir the lake, and some of that deep, dense water from the lower levels gets brought up to the surface.
Eventually the sun starts to warm the upper level of the lake, and it settles into its usual summer pattern of three layers once again.
If you have a Secchi disc, you can see this happening. A Secchi disc is a fairly simple device that’s used to measure water clarity – it’s basically a black and white disc that’s lowered into the water, to measure how far you can see into the depths (instructions to make your own can be found here).
As the lake turns over in spring, the water from the bottom brings sediment toward the surface, and for a short time the water clarity declines. Measure the water clarity daily beginning as soon as the ice goes out, and you’ll see when the turnover happens. The lake water on ice-out will be fairly clear; it will get murkier as the turnover happens, and then become clearer again as summer approaches.
You can also see it with a fish finder. In winter, the fish tend to be at the bottom of the lake, where the oxygen levels in the water are higher. As the lake warms, plants begin to grow near the surface – starting with algae. We may not see that happening at first, but the fish can tell: the algae photosynthesize, increasing the oxygen levels near the surface. Fish need oxygen, so they begin moving up the water column.
This is why spring is a fantastic time to catch lake trout: as a cold water species, they’re usually found deep in the lake in summer and winter, but in spring they come near the surface where it’s easier to catch them without having to use special deep-water trolling gear. When you start to see fish near the surface on the fish finder, you know the lake is turning. When the fish start to go deeper, summer conditions are arriving.