Slosh, Slosh

August 2008

On a gorgeous July day the lake called me incessantly. The air was hot and still. Over the weekend the water temperature had neared 70 degrees Fahrenheit. Expecting warm bathwater, I jumped into Kingsland Bay, only to find the water markedly colder than it had been just a day earlier. The swim was refreshing, but abbreviated. I had observed a hidden effect of the internal seiche, an example of what can happen to water when wind pushing on it overwhelms some of gravity’s pulling effect.

Wind pushes water. The energy of the wind is transferred to the water, and waves rise. Waves of different sizes move at different speeds, so they start both to augment and to interfere with one another. Large waves can rise too high and then topple, creating whitecaps. Eventually, the waves expend their energy by crashing against the shore. With the smallest waves (less than three-quarters of an inch in length), the attraction of water molecules to one another pulls the surface back to a placid state. With larger waves, gravity provides the calming force.

When the wind is sufficiently strong, it pushes water to one end of the lake, as waves pile atop one another. A strong, constant wind of thirty miles per hour blowing along the long axis of the lake would create a pile less than a foot high—not exactly a towering tsunami. However, once the wind relaxes, that one foot of extra water sloshes back toward the other end. Sloshing continues, back and forth, until gravity pulls the water flat once again—a process that takes about four hours. The back-and-forth sloshing, known as a surface seiche, can reverse currents through some of the more constricted areas of the lake, like the Gut, the area between Grand Isle and North Hero Island.

In order for the waves that induce the surface seiche to be created on the lake, there must be a difference in density between a moving medium and an otherwise stagnant medium. Wind—air in motion—represents the moving medium. Water, denser and more viscous, represents the stagnant medium. Currents produced by the surface seiche rarely influence water more than fifteen feet deep. By that depth, the density and inertia of the water have absorbed the excess energy of the wind push. However, the thermocline provides another area in the lake where media of different densities lie in contact.

A swimmer diving through the water column will come across an area where the temperature drops quickly—the thermocline. A temperature difference also indicates a density difference, with the warm top layer being slightly less dense than the cold deeper layer. These differences become accentuated as warming continues through spring and summer. The thermocline develops most clearly in the Main Lake and Malletts Bay. The Inland Sea hosts a thermocline, but it is less distinct than in deeper segments. Wind thoroughly mixes shallower lake sections like Missisquoi Bay and the South Lake, so they often retain a uniform top-to-bottom temperature.

The piling of waves on one end of the lake that creates the surface seiche also generates downward pressure on the thermocline—the start of an explanation for the quick change in water temperature observed that day at Kingsland Bay. Due to the relatively small difference in density between the layers above and below the thermocline, just a small downward pressure creates a very large wave. Imagine how much a water bed mattress sags beneath you when you flop upon it, compared to a denser wooden board.

Once induced, this large underwater wave, an internal seiche, sloshes back and forth from north to south (with a bit of east-west rocking) throughout the summer. Though the conditions that established the internal seiche may have disappeared, the currents once set in motion continue. It takes four to six days for a wave to complete the trip. The thermocline rises at one end of the lake on day one and at the other end on day three. In between it flattens. At Kingsland Bay I had experienced the rise of the thermocline to near the water surface. By the next day, the swim would once again resemble bathwater as the thermocline fell back.

The internal seiche represents perhaps the greatest mixing force in the lake. It can displace the thermocline as much as 145 feet. Middlebury College researcher Tom Manley has suggested that over 25 percent of the water in the Main Lake shifts with the passage of an internal seiche wave. It can create currents in excess of one mile per hour—one hundred times faster than the average north-south current. While largely restricted to the Main Lake, it has been detected reaching south beyond the Crown Point Bridge. The internal seiche causes substantial exchange of energy and materials throughout the lake, all created by the simple push of wind against water and the transfer of that force to lower depths.