To the casual observer, Lake Champlain might seem pretty calm right now. But lake scientists know that it is kicking up a storm. It's undergoing turnover and seiche (sounds like saysh) as we speak.
What, exactly, are these phenomena? Breck Bowden, director of the Lake Champlain Sea Grant program, explains.
What's turnover?
Turnover is "one of the most unusual and least-known properties of water," Bowden says. It starts with the lake's stratification; the warmer water sits on top, and the cooler water sinks to the bottom.
But as the season progresses, Bowden says, something interesting happens. "As we get cooler temperatures during the fall, those surface waters begin to cool down, and at some point they reach a temperature that's more or less the same temperature as the bottom waters. And at that point there's no longer a density difference."
That's when turnover happens.
"At 39 degrees Farenheight, water undergoes a change that no other substance that we know of, really, undergoes," Bowden says. "As the water begins to get cooler, it actually begins to get less dense. And so as a consequence, the cooler water, as it approaches the freezing point, begins to float, and finally at the freezing point ... it becomes considerably less dense, and we have the ice floating on the surface of the lake."
This isn't just fun party trivia. It's critical to life in the lake:
"If it weren't for that peculiarity in water [turnover] which we see in no other natural substance, lakes would freeze from the bottom, or they'd freeze solid, and life on Earth would be completely different." - Breck Bowden, Lake Champlain Sea Grant program director
"If it weren't for that peculiarity in water which we see in no other natural substance, lakes would freeze from the bottom, or they'd freeze solid, and life on Earth would be completely different."
Bowden says that in the winter, the water at the bottom of the lake doesn't get much colder than 39 degrees. "And that's plenty warm enough for a lot of organisms to at least hang out until the next year when they can access other parts of the lake again."
So remember that in a few months, when you're shivering in your living room. At least you're not at the bottom of the lake.
What is seiche?
For this explanation, Bowden asks you to imagine a cake: "If you imagine the lake as being this long, narrow cake that's kind of thick, and it's got two layers two it; the upper layer is the warmer water we've been talking about and the lower layer is the colder water, and there's a layer of icing between there that's this very rapid transition ... of the warmer water going to the colder water."
Got it? Bowden says that layer of transition and temperature (which scientists refer to as a "thermocline" or a temperature gradient), is flat.
"Now, let's start blowing water down the length of [the] lake, or down [the] cake ... What's going to happen is that the air begins to drag the water with it. And if the wind blows hard enough and long enough, it will actually pile some water up at the downwind end of the lake. And that's what we call a surface seiche."
When the wind stops blowing, Bowden says, the water will rock back and forth from the higher level at one end of the lake to the lower end at the other end of the lake.
"If the wind blows hard enough and long enough, it will actually pile some water up at the downwind end of the lake. And that's what we call a surface seiche."
The degree to which the water is piling up is very small, according to Bowden. It may be only a few inches, "and the rocking back and forth is on a period of about four hours or so."
"But something that we don't see is that as that water's being blown to the far end of the lake, it's actually piling up below, too. So now what I'd like to imagine is my layer cake with that middle icing layer tilted, so that I basically have two wedges that are interlocking. I have the surface water that's warmer at the down end side of the lake and the colder water that's thicker at the other," Bowden says.
This is where something called internal seiche happens. Really, Bowden explains it best:
"When the wind stops blowing that now begins back rock back and forth too, but there's two very different characteristics. One is that the excursion of that layer moving up and down is now on the order of tens of feet, upwards of 50 feet. And the water is moving at quite high velocities -- many feet per hour -- so that we can transport large quantities of water and large quantities of material that is dissolved in the water up and down the lake, as this so-called internal seiche is going on."
Plenty to think about next time you look at -- or float on -- the surface of Champlain, no?
