Even when the lake seems dead calm, there is still a lot of movement going on. Storming along at nearly 30 miles per hour, is a surface seiche. And below that, an internal wave the length of the lake is rolling.
When you blow on a coffee or bowl of soup that is too hot, you see miniature waves build up against the far side of the container. Now take this effect and amplify it to fit a windstorm on Lake Champlain. This is how a seiche forms.
As wind blows across the surface of the water, it creates friction that drags the water with the wind. Across a lake as large as Champlain, a significant amount of water will build up at one end of the lake. When the wind stops, however, gravity takes over and the water is free to rush back. As this oscillation continues, a single standing wave is formed. On Lake Champlain this wave takes approximately 4 hours to make its trip back and forth.
Any good fisherman knows the importance of taking into account temperature and depth. The deeper the water, the colder the temperature. This change is not gradual, however. There are very distinct boundaries where it drops. These temperature layers are called thermoclines and are created by differences in the water density at varying temperatures. Cold water is denser, so it sinks. Warm water is less dense, so it sits on top of the cold.
As the seiche flows along the lake, it pushes the thermocline with it, changing the depth of the temperature boundaries. This means two things: one, the thermocline, and therefore temperatures, are always moving. And two, the variable bottom structure of Lake Champlain causes a secondary seiche to occur. As the surface seiche moves, the thermocline will run into shelves, reefs, and other objects that restrict its flow through the lake. This disturbance in the flow causes a wave within the layer of the thermocline that was obstructed. Due to the differences in water density, the denser water along the bottom pushes up against the warmer water, resulting in colder water near the surface. Gravity, of course, will push the heavier water back down, creating the wave. In Lake Champlain, the secondary seiche, which can get from 10-20 meters tall, takes approximately 4 days to complete its end-to-end journey.
What does this all mean for fisherman? Seiches can cause drastic changes in water temperatures in a very quick time. Temperature-sensitive fish are likely not going to be staying put through the fluctuations. A temperature gauge is essential to find out where the thermocline is, and where the fish are.
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Originally posted September 25, 2012 by Eric_LaMontagne on LCI’s ‘So You think You Know Champlain?’