Lake Michigan's hidden waters spread rapidly

Lake Michigan, the only one of the Great Lakes entirely within the United States, stretches over 500 kilometers and borders Wisconsin, Michigan, Illinois, and Indiana. At first glance, its surface may appear tranquil, but beneath lies a dynamic and rapidly spreading water system.

In 2013, researchers from Purdue University and NOAA's Great Lakes Environmental Research Laboratory conducted an experiment to understand how quickly a water patch disperses once it enters the open lake. They released a fluorescent dye and monitored its spread over 21 hours. The results showed the dispersion rate rapidly increased from approximately 1.5 to 4.2 square meters per second.

The increase in rate was attributed to near-surface shear associated with internal movements called Poincaré waves. These waves, created by the stratified nature of Lake Michigan with warmer water near the surface and colder water underneath, stretched the dye plume and helped it spread across a larger area.

Further experiments with floating drifters revealed that the lake's behavior can change rapidly. Initially, the drifters showed slower movement under calm conditions. However, when stronger winds arrived, the dispersal became faster and more irregular. This demonstrates that conditions such as stratification, waves, and wind can significantly influence how materials move across the lake.

Understanding how quickly substances disperse in Lake Michigan is crucial for modeling the impact of accidental spills, predicting the movement of agricultural nutrients, and studying heat redistribution due to climate changes. The 2013 dye experiment shows that even the open, seemingly peaceful interior of Lake Michigan can experience surprisingly vigorous mixing, a reminder of the hidden currents beneath its calm surface.