Even without chilly mornings, I know winter is coming. Loons are losing their black-and-white checkerboard colors and fading to gray. Skeins of snow geese are threading south across the October sky. Maples and birches are giving their last shows, a few flames of yellow and red against the mauve of bare hillsides.

Watching Vermont's land and inhabitants make ready for the long winter ahead is fascinating. Much less visible, but every bit as interesting, is the transformation that's taking place in Lake Champlain.

It's easy to think of the lake as a big bathtub with a surface that wrinkles in strong northerly or southerly breezes. The reality is far more complex.

During the summer, the deep cold water in the lake's bottom sloshes back and forth in a giant underwater wave. This deep, cold part of the lake remains at about 43 degrees all summer, a leftover from winter's chill. Cool lakewater hunkers below the warm surface water much like vinegar lies under oil. There's even a sharp boundary between the two layers, just like in your salad dressing bottle. The underwater wave occurs at this temperature boundary.

It works like this: when high winds push the warm surface water layer to one end of the lake, that pile of warm water actually pushes down on and displaces the cold layer underneath, sending it sloshing down to the other end of the lake. Even when the wind stops, the sloshing doesn't halt immediately, but continues, slower and smaller, over days.

The wave, which scientists call a seiche (rhymes with mesh), can take as long as four days to roar from one end of the lake to the other and back again. Because it's a wave detectable only in water temperatures, no surfer will ever ride it -- even though it can be 100 feet high or more.

Anglers know all about the lake's layering and how the water sloshes around. Many cold-water fish, like salmon, hover in the cool water at the boundary line. The sloshing lake and its giant wave explains why these fish can be caught at 25 feet one day and 75 feet another. The layering will persist all summer. But as the fall days grow shorter and air temperatures cool, cool, the water sinks, sending small tendrils snaking down toward the lake's low-lying 43 degree water. When fall's chill gives way to winter's freezes, the top layers of the lake reach 39 degrees, the temperature at which water is densest.

Then something funny happens.

The cool water that's been sloshing around the bottom of the lake during the summer is dense, but it isn't quite as dense as water at 39 degrees. So the lake water literally turns topsy turvy – a process that biologists call fall turnover. Lake Champlain's length and huge volume -- 6.2 trillion gallons -- means fall turnover doesn't actually happen until December.

That turnover mixes all sorts of things into the lake water, including nutrients and pollutants that might have been locked in the deepest parts of the lake all summer. The uniform temperature also makes a difference for smelt, the tiny fish that are food for larger game fish. In summer, smelt spend their days at the very top of the cold layer of water. They rise up at night to feed. But after the lake turns over and there's no layering, smelt will be spread throughout the lake. The fall turnover also eliminates the boundary layer where the wave once formed.

So, as honking geese remind you of the season to come, think also of how winter's cold will mix Lake Champlain's waters like a big milkshake, then still its invisible, 100-foot waves.