Chicago parks officials ordered a swimming ban on all beaches Wednesday because of a possible seiche, or rapid water rise, along the Lake Michigan shore.
The action came after the National Weather Service said a storm front had caused a rapid drop in water levels on the east side of Lake Michigan. Officials said the drop indicated that a seiche was moving across the lake and would likely result in a sudden rise of .6 metres or more on the west side shoreline.
The weather service said that this is a "very dangerous event for waders" as water levels may fluctuate rapidly, "taking unsuspecting waders out to more open waters."
Chicago's Office of Emergency Management and Communications issued a release warning residents to stay away from the edge of the water and to avoid beaches and marina areas. Evacuation of docks, piers and breakwaters was also advised.
City departments were also warned of the potential for flash flooding along the shoreline.
Late Wednesday evening, officials said the water level appeared to have swelled about .3 metres before receding. However, the weather service warned that water fluctuations and dangerous rip currents were still possible.
A seiche — described as a "standing wave" in an enclosed body of water — is created when strong, sustained winds cause large amounts of water to move from one part of a lake to another.