Lake Vibes!

While surface waves are obvious to us all, what happens beneath the water surface is far less obvious and more mysterious. Because of its depth, West Okoboji is of particular interest to these researchers. Like all lakes, the upper surface of West Okoboji warms faster than lower layer during summer months, forming a layer of lighter, warmer water overlying a layer of cooler, denser water with a transition zone in between called a thermocline. But unlike many shallower lakes in Iowa, Okoboji's thermal layers stabilize and do not mix as the summer progresses no matter how hard the wind blows, ( at least until autumn, when falling air temperatures cool the surface waters, making it more dense and cause it to sink and mix with the now warmer water layer underneath)

Rehmann and his graduate students are interested in the interaction between the warm upper and cool lower water layers. On a calm day, not much is happening, but on a windy day the action starts when a seiche begins to form. A seiche (pronounced say-esh) is a standing wave. It forms when prolonged strong winds tilt the lake surface: it is higher on the down wind shore and lower on the upwind shore. When the wind dies down, the lake surface literally rocks back and forth up to days at a time. We can't detect it with our bare eyes, but if you have ever sloshed the water around in a bathtub you can imagine what a seiche is and imagine it taking place on a large scale on a lake.

The rocking action of the seiche sets up internal waves- or vibes as I like to call them -- between the upper and lower water layers, causing the interior lake motions that are the source of fascination to Rehmann and his research team.

Unlike surface waves- whose crest and energy move in the same direction- the crest and energy of internal waves travel perpendicular to one another. Internal waves are complex because their energy is not only bounding back and forth between shorelines, but their vertical movement is mediated by the different densities of warm and cool water in the upper and lower lake layers. These waves can cause mixing because they can generate currents and break on the sloping sides of the lake.

This summer Rehmann's and his team will use their underwater instruments to track the movements of these waves and follow the mixed fluid.

What does this have to do with water quality? "One application of this research," explains Danielle, "is to understand how internal motions within lakes dictate how nutrients and pollutants are transported from lake shorelines into its interior."

Lakeside faculty lectures continue throughout the summer, every Tuesday evening at 7:30 pm in the Waitt Community classroom. On Friday, June 13, the public is invited to an informal chat with the researchers over coffee and rolls at Lakeside's Conservation Conversation, at 8:00 am at the Waitt porch.