Force of Nature

Most swimmers in the Great Lakes probably know the sensation of being pushed off course on a windy day, finding themselves far down shore.

Now imagine if the same current flowing along shore suddenly headed away from shore. As water is funneled through a break or low point in a sand bar, it creates a rip current. Rip currents are powerful currents that flow offshore in narrow channels. If swimmers get caught in a rip current, they need to stay calm.

“Above all, don’t fight the current,” says Kinnunen. “Rip currents are powerful enough to tire a strong swimmer. The best thing to do is to swim parallel to shore to escape the current.”

Rip currents are typically associated with sandy beaches, which often have well-developed sand bars. When waves continually break over a sand bar, excess water builds up near shore. Eventually the water flows lakeward through a break or low point in the sand bar, forming a rip current.

Wind-driven waves are the most common way for water to build up near shore. Conditions can be particularly dangerous on windy days when waves are two- to three- feet high or during the aftermath of a storm, when strong longshore currents may be present. Other factors also influence the development and strength of rip currents.

“We have some unique conditions here in Great Lakes,” says University of Michigan Marine Engineer Guy Meadows, who has studied currents in the Great Lakes for 30 years. “The unique near shore dynamics of the Great Lakes can combine to create treacherous conditions.”

A steep offshore slope is one concern. Recent years of low water levels means that the offshore portion of the beach along the sandy coasts of the Great Lakes is steeper, explains Meadows. This allows larger waves to break closer to shore.

The configuration of the lake basins may also contribute to unstable conditions. Even though the lakes are huge, their basins are enclosed “like giant bathtubs,” says Meadows. A seiche, or standing wave, can occur following storms. Sometimes called a wind-driven tide, a seiche occurs when water is pushed to one side of the basin and eventually flows in the opposite direction, away from shore. Another phenomenon, explains Meadows, is a Kelvin wave—a long-period wave that travels around the perimeter of the basin, resulting in rapidly changing water depth near the shore.

Strong currents also typically exist near river mouths where water empties into a lake and near engineered structures such as piers and jetties. In areas where shoreline structures exist, the danger to swimmers is compounded. These structures interrupt the flow of water along shore, creating what Meadows refers to as permanent rip currents: “From our underwater survey work with the State, we can see rip channels around almost all of these structures all of the time.”