A Giant's Bathtub
When we think of water movement in Lake Champlain, we tend to think primarily of the lake's surface -- of steep-sided chop in squalls and gentle winds rippling calm surfaces. But it is well beneath the surface that most of the lake's water movement takes place. There, a giant wave that takes four days to roll back and forth through the lake and can reach heights of nearly 100 feet dominates throughout the summer and fall. Despite its massive size, this wave tips no boats nor smashes no lakeshore cottages.
The wave is called the internal seiche of the lake. And while it has few obvious effects on people using the lake, it is critical to aquatic life and water quality. For example, the seiche determines at what depth one should fish for salmon and where pollution from Burlington, Plattsburgh and other sources ends up.
The internal seiche owes its existence to calm, sunny spring and summer days. At such times, the sun warms the surface of the lake, but without wind to stir the waters, the heat can't reach the depths quickly. During just a few days of calm weather, the water near the surface may become a few degrees warmer than the deeper water, setting up a boundary that drives the internal seiche for the rest of the summer. When temperatures cool in the fall the seiche will fade, producing a subsurface calming that mirrors the ice covered surface calming of winter.
Because water's density decreases as it warms up above 39 degrees Fahrenheit, warmer waters tend to float on top of denser, cold water. The effect is much like oil and vinegar in a jar of salad dressing; the lighter oil floats on the surface and vigorous shaking is required to mix the two even temporarily.
Once a temperature and density barrier (called the thermocline) is established in Lake Champlain, wind will do the rest. A few days of Lake Champlain's prevailing south winds will pile surface waters up at the north end of the lake, gradually pushing the thermocline to greater and greater depths, and driving the cold waters below the thermocline south. Conversely, as warm waters move out of the southern portion of the lake and colder water sneaks in below the thermocline, the position of the thermocline in the water rises. Once the wind stops, the lake is primed for action: the cold water that has piled up at the south end of the lake will rush north, while the warm water piled up at the north end rushes south, gradually leveling the thermocline, and then overshooting to make it slant in the other direction.
The process resembles the waves created by a child in a bathtub; water piled up at one end of the tub (or the lake) will run toward the other end, overshoot, pile up at the other end, and then slosh back toward the first end. Given an occasional push by a child's hand or a south wind, the process can continue almost indefinitely.
One way that the internal seiche differs from a child's sloshing, however, is that its effect can hardly be perceived at the surface. On the surface, daily shifts in the wind dominate waves and currents, but beneath the surface, the seiche is the dominant transport mechanism in the lake.
Anglers may go out one day and find salmon, which prefer cold water, at a depth of only 25 feet. A few days later, temperatures at the same location and depth may be ten degrees warmer, and the salmon dozens of feet deeper. The difference is that the seiche will have rolled through, driving the thermocline, and the cooler water below it, deeper into the water.
Typically in Lake Champlain the seiche takes about four days to oscillate from one end of the Main Lake back down to the other, with smaller and independent sieche systems taking hold in Malletts Bay and the Inland Sea. Depending on its magnitude and variations in the depth of the lake, the vagaries of the seiche can produce situations where water temperatures at a depth of 100 feet are only a few degrees cooler than the surface water temperatures, or it can bring 50 degree water to the surface in mid-summer.
Essentially, the thermocline divides the lake biologically in two, with separate pools of nutrients and oxygen in the surface and deep waters. This puts nutrients in the deep waters out of reach, thus limiting the growth of algae. The internal seiche counteracts this effect to some extent, as when it brings deep, nutrient rich waters to the surface. In part because of its large internal seiche, Lake Champlain's thermocline is a less effective barrier to transport than that of many lakes. For example, mixing between the deep and shallow waters of Lake Champlain takes place at a rate almost four times that of Lake George.
The oscillations of the seiche also move water rapidly around the lake. This means that even though the lake drains from its northern end, and the prevailing winds are from the south, pollutants can still move south from such urban centers as Plattsburgh and Burlington. The seich acts as a great mixer.
Only in autumn, as the lake gradually cools, do the temperature differences that drive the establishment of the thermocline and give the seiche its power gradually fade. The surface waters will eventually cool to the point where the thermocline vanishes and the internal seiche goes into hiding until the following spring.