Seepage lakes in the Northwoods (those fed primarily by rain with no stream in or out) are especially high right now. Most of us can remember when those same lakes were so low that people’s piers no longer reached the water.
So, why are the lakes so high? And what causes these wide fluctuations in lake levels over the years? For an answer, I turned to Carl Watras, Ph.D., a researcher at the University of Wisconsin Center for Limnology’s Trout Lake Research Station.
He explained that historically the lake levels rise and fall on roughly a 13-year cycle. To understand why, it’s essential to appreciate that lakes, the atmosphere and the groundwater are an interconnected system. With that, here’s the explanation.
Watras and colleagues started studying the lake cycles in 2012. They first looked at historic data on the levels of Buffalo Lake and Crystal Lake in Vilas County. These lakes’ patterns of high and low levels matched closely. Next they found by studying groundwater wells that groundwater levels have followed a similar pattern, as have the levels in Lake Michigan-Huron and Lake Superior.
“The next question was, why is this happening across the entire region?” Watras said.
To get an answer, they analyzed the difference year-to-year between precipitation and evaporation — the basic components of the water cycle.
“We saw a really nice correlation between this difference (P-E) and the change in lake level (stage) in any year,” Watras said. But another factor was also at play: groundwater outflows. That’s essentially the amount of groundwater in our part of Wisconsin that exits to the Mississippi River.
The bottom line: when the difference between precipitation and evaporation (the net amount of water entering the system) is greater than the amount of groundwater flowing out, lake levels rise. When that difference is less than the groundwater flowing out, the lake levels fall.
But why has this been happening on a 13-year cycle?
“That’s where it gets a little complicated,” Watras said. He and colleagues turned for help to atmospheric scientists at UW-Madison. They traced the wet-dry cycles to a pattern that exists in the central Pacific Ocean: “You’ve heard of El Niño? That is in the Pacific much farther south, and it has its own periodicity. But there’s another farther north that seems to be affecting us.”
It doesn’t have a name: “It’s just called a circum-global teleconnection.” To make a complex story simple, changes in water temperature in the Pacific are associated with air flow across North America into the Atlantic Ocean that correlates with our lake level changes.
“When we’re set up with high pressure to our southeast and low pressure to our northwest, the flow of moisture from the Gulf of Mexico to the Lake Superior region accelerates, and we get more rain and less evaporation,” Watras said. And lake levels rise. When those conditions reverse, lake levels fall.
Is climate change exacerbating these cycles?
“We don’t know yet,” Watras said. “The climate has changed — that’s not in question. The question is: how much of an impact will that have on our water levels? Since 1998, we have seen a change in the 13-year cycle. It’s now closer to 20 years, and the recent low is lower (2013) and the recent high is higher (this year). But it may be decades before we have a definitive answer.”
Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected]