Changing Planet

Ice Cover Affects Lake Levels in Surprising Ways

White Shoal Light, Lake Michigan. This lighthouse is home to one of five Great Lakes year-round evaporation-monitoring stations. Photo by Dick Moehl

The announcement last week that $300 million was included in the 2014 federal spending bill for the Great Lakes Restoration Initiative was followed this week with more good news about water levels.

The recent Arctic blast that gripped much of the nation will likely contribute to a healthy rise in Great Lakes water levels in 2014, according to a new study. But the processes responsible for that welcome outcome are not as simple and straightforward as you might think.

In a report released this week by the Great Lakes Integrated Sciences and Assessments Center (GLISA), a federally funded collaboration between the University of Michigan and Michigan State University, a team of American and Canadian scientists explains the relationship among evaporation, ice cover, and water temperature.

These interactions were among the topics of a 2012 post about the impacts of climate change and variability on Great Lakes water levels. In that post, I explained how these factors interact on Lake Superior, based on research results available at the time. John Lenters, the researcher involved in that effort is now at LimnoTech, an environmental consulting firm, and the lead investigator of the new study.

The most obvious climate-related change on the Great Lakes is that winter ice cover has declined by 71 percent over the last 40 years, on average. Summer water temperatures and annual evaporation are also on the rise as the climate warms. All of these are important pieces of the climate change puzzle. They are also important factors controlling water level fluctuations and the overall health of the Great Lakes ecosystem, including how chemicals mix and microorganisms sustain life in the lakes themselves.

The 11-page GLISA report “dispels misconceptions” about the impacts of ice cover on evaporation and points to the need for an expanded monitoring program.

I have already debunked five of the most commonly mentioned myths about why water levels reached record lows on the Great Lakes last year, so I won’t take up that topic again here. This new research provides a good opportunity to further examine why the answer to the question “Where did the water go?” is that it “simply evaporated.”

Evaporation plays a crucial role in Great Lakes water levels

One of the biggest misconceptions addressed in this latest report is about the underappreciated importance of evaporation. This is understandable given that evaporation is invisible, for the most part, unless you happen to catch sight of the eerie mist rising from the lake on a chilly morning.

Lake-effect snow is another visible sign of evaporation, and there’s been plenty of it this year, too. When cold winds blow over the relatively warm lake surface in winter, moisture is pulled out and then deposited as snow on land downwind of the lake. But most of this falls within the watershed and is returned to the lake after the snow melts away.

The eastern shore of Lake Superior is one of a few places where lake-effect snow falls outside of the basin, but the amount is relatively small. The end result is that lake-effect snow counteracts some of evaporation’s negative impact on the overall water budget for the Great Lakes Basin.

Photo: lake-effect snow clouds on Lake Superior. Credit: John D. Lenters.
Lake-effect snow clouds on Lake Superior near Grand Marais, Michigan on November 10, 2011. Photo by John Lenters

At first glance, these evaporation events just don’t seem to have as much impact as seeing water gush over Niagara Falls.

But a figure in the report shows the amount of water attributed to various components of the monthly water balance for Lake Superior: precipitation from rain and snow, runoff from rivers and streams, as well as water lost through evaporation and diversions.

And a convenient scale comparing these average monthly flow rates to the “number of Niagara Falls” puts evaporation in perspective. Not only does it show “the strong seasonal variation in evaporation,” but it also displays the alarming fact that December’s average rate of water loss due to evaporation is equivalent to the rate of water flowing over two Niagara Falls!

In reference to the figure, the report states:

“Lake Superior, for example, loses almost three feet of water every year through the St. Marys River (Lenters, 2004). And roughly two feet of water is also lost every year just through evaporation. That is a total of five feet of water lost annually from the surface of Lake Superior due solely to natural processes. Relatively little water is gained or lost through direct human intervention (e.g. less than 1 inch per year flows into Lake Superior through the Long Lac diversion).”

That’s a lot of water loss attributed directly to Mother Nature. But Lenters is quick to point out that “this does not necessarily mean that nature is not changing due to human causes.”

Figure: Lake Superior Water Balance. Source: GLISA 2014.
Four components of the monthly Lake Superior water balance, beginning with the month of June, which is the typical start of the “evaporation season.” Each component is shown as a flux of water in units of inches per month (left; spread out over the surface area of Lake Superior), as well as in equivalent “number of Niagara Falls” (right). Note, in particular, the strong seasonal variation in evaporation. Credit: GLISA 2014

Most evaporation occurs in the late fall and early winter

Another misconception is that most evaporation occurs during the heat of summer. In fact, the highest rates actually occur during the colder autumn and winter seasons.

As described in the earlier post, I had a hard time wrapping my brain around the concept because it seemed counter-intuitive.

Like most people, I had thought that evaporation was strongest in the summer, when the lakes were warmest. I envisioned water escaping like steam from a pot of boiling water on the stove or from a huge evaporating pan during summer months. This is not so, according to researchers studying evaporation.

“Highest evaporation occurs when the lake is cooling,” Lenters explained.

Evaporation is now being observed year-round through the bi-national team’s network of five monitoring stations. The high-tech devices are mounted on or near remote lighthouses on the Great Lakes. Their research shows that most evaporation occurs on the Great Lakes during the late fall and early winter. It turns out that evaporation is not directly driven by warm air temperatures, but by warm water temperatures, the report states.

“More specifically, high evaporation requires three factors: 1) a large temperature difference between water and air (i.e. warm water and cold air), 2) low relative humidity, and 3) high wind speeds. If all three ingredients are present, as often occurs in the fall and winter, evaporation rates from the Great Lakes can get as high as 0.4–0.6 inches per day.”

To put this number in perspective, an evaporation rate of half-an-inch per day across the entire Great Lakes is nearly 20 times the flow rate of Niagara Falls, the report says.

One of the other figures provided in the report helps explain the delayed effects of ice cover. It shows cumulative evaporation for Lake Superior based on direct meteorological measurements at Stannard Rock lighthouse for high- and low-ice winters between 2008 and 2012. The first thing that jumps out is how much higher the total evaporation was in the 2010/2011 season – roughly 10 inches greater than the other three years – an event that was also preceded by a low-ice winter, and then followed by a warm summer.

With the exception of this extreme evaporation event, the figure shows how similar the pattern of evaporation is from one year to the next, regardless of whether it was a low- or high-ice winter. It also shows how little evaporation occurs until the “evaporation season” begins, which is usually around the end of July.

Figure: Lake Superior cumulative evaporation. Source: GLISA 2104.
Four years of cumulative evaporation from Lake Superior, using direct meteorological measurements at Stannard Rock lighthouse (Spence et al. 2011). Each annual curve begins at the date of ice breakup and continues through the remainder of the evaporation season. Note, in particular, the much higher total evaporation during the 2010/11 season – roughly 10 inches greater than the other three years. This high-evaporation year resulted primarily from an early onset of the evaporation season during the particularly warm summer of 2010 (highlighted in orange). Source: GLISA 2014

Lenters explained this concept with a hypothetical example. “Even if we magically removed all of the ice from the lake in March, at the height of the winter ice season in a high-ice year, it would likely have a limited effect on evaporation,” he told me. Winter ice cover does prevent water vapor from escaping into the air, but evaporation only happens under the right conditions, as described above. These conditions are not typically found between March and June.

Understanding the interaction between ice cover and evaporation requires a two-way explanation

Another common misconception is that ice cover is good for lake levels because it “caps” evaporation. It’s true that ice cover – as a whole – is generally good for lake levels, but not necessarily because it prevents evaporation at the time the ice is present. What’s more important is that ice leaves the lake cooler the following summer and delays the next year’s “evaporation season,” according to the study.

In a prepared statement about the report, it notes that the previous “simplistic view of winter ice as a mere ‘cap’ on Great Lakes evaporation is giving way to a more nuanced conception, one that considers the complex interplay among evaporation, ice cover and water temperature at different times of the year.”

“The relationship between ice cover and evaporation is a two-way street,” Lenters said. “It is true that high ice cover affects evaporation, mainly by delaying the onset of evaporation until late summer. But the reverse is also true. For more ice to form on the lakes, the water needs to cool more rapidly, and the most effective way for a water body to cool is to evaporate.”

It is this two-way interaction between ice cover and evaporation that is key to understanding the effects of climate change on Great Lakes water levels. “In the long-term, summer rates of evaporation are picking up, and this is partly due to reductions in ice cover during the wintertime,” Lenters said.

An expanded program to monitor evaporation would improve Great Lakes water level forecasting and help to better understand the long-term impacts of climate change. A warming climate has negatively affected the Great Lakes, from declining water levels to its role in toxic algae blooms and the health of fisheries and the ecosystem as a whole.

Learning more about the important role of evaporation would be a wise investment, especially given the implications of a warming climate for the region’s economy and the huge investments in Great Lakes restoration efforts currently underway.

The cold winter has brought more ice than we’ve seen on the Great Lakes in 20 years. This is a good thing for water levels and the health of the Great Lakes overall.

Lisa Borre is a lake conservationist, freelance writer, and avid sailor. With her husband, she co-founded LakeNet, a world lakes network, and co-wrote a sailing guide called “The Black Sea” based on their voyage around the sea in 2010. A native of the Great Lakes region, she served as coordinator of the Lake Champlain Basin Program in the 1990s. She is now an active member of the Global Lake Ecological Observatory Network.

Lisa Borre is a lake conservationist, writer and avid sailor. A native of the Great Lakes region, she served as coordinator of the Lake Champlain Basin Program in the 1990s and co-founded LakeNet, a world lakes network that was active from 1998-2008. She is now a Senior Research Specialist at the Cary Institute of Ecosystem Studies and an active member of the Global Lake Ecological Observatory Network (GLEON). She is also on the board of directors of the North American Lake Management Society (NALMS), the advisory council of the Lake Champlain Committee, and an associate investigator with the SAFER Project: Sensing the Americas' Freshwater Ecosystem Risk from Climate Change. She writes about global lake topics for this blog and speaks to local, regional and international groups about the impacts of climate change on lakes and the need to work together to sustainably manage lakes and their watersheds. With her husband, she co-wrote The Black Sea, a sailing guide based on their voyage there in 2010.
  • Betsy

    Again, the author has taken complex information for a lay person and explained with easy to read charts and text. With this information I will be watching the Lake MI levels this summer having first hand information of the ice cover and winter temperatures. Interesting read … now I must put another log on the fire.

    • Thanks for following my posts and for your eye-witness accounts of what’s happening on Lake Michigan.

  • Dwayne LaGrou

    It is very good thing that the Great Lakes are finally getting the support they need so badly. Most people don’t understand just how much the Great Lakes impact the rest of the country, And not just the states to. The east either. The Great Lakes have a way of tempering the weather in many locations. Not to mention the number of people that rely on them to make a living. I have been a member of a number of people that are volunteers, That make daily weather reports to the National Weather Service, And I have seen the effects of Global Warming in my reports over the last 15 years or so. I just hope that the support does not come up short. There are a number of things that are threatening the lakes, From pollution to invasive species like the Zebra Mussels and the jumping Carp that are only being held back by an electric fence of sorts. Please support the Great Lakes and help to rebuild the health of the whole Great Lakes Eco-system.

  • OB1

    Excellent report…Betsy, charming as it is, don’t put that sequestered ghg emitting fibrous 1/4 organic cylinder on the fire – it’s part of the problem! Yeah, I know, I do too….

    • Ha! Thanks for the comment and humorous description of a log. I think that Betsy was just trying to point out how cold it has been in Michigan this winter. I’m glad to know that you would do the same to keep warm.

  • Steve Engelhardt

    Thanks for the report and information. I had heard on NPR that Lake Michigan is 60% covered and that would cut down on evaporation.

    • Thanks for reading, Steve. I also heard reports from other national news outlets last week that incorrectly stated that more than 60 percent of Lake Michigan was covered in ice. The “more than 60 percent” figure was for all of the Great Lakes, including Lake Erie, which at the time had more than 90 percent ice cover. As of yesterday (1/29/14), 44.9% of Lake Michigan was covered in ice, according to the NOAA Great Lakes Environmental Research Laboratory (GLERL). If you’re interested in more information about ice cover, including current and historical ice cover information, the GLERL website is very informative:

      I didn’t hear the NPR report you mentioned, but perhaps it was made before this latest study was released with a more nuanced explanation about the interaction between ice cover and evaporation.

  • Dwayne LaGrou

    Thanks Lisa, that article was very interesting. It goes to show just how interdependent the Great Lakes and many of the smaller inland lakes are. By the way if you would like to see my weather report history, My station is Lapeer 2 west.
    Thank You again for an eye opening article.

    • Thanks, Dwayne. I’m glad you found the other UW study interesting.

  • Richard Earle

    I would like to know if anyone can explain the rapid drop in the level of Lake Saint Claire in the fall of the year. The lake level can drop 10 inches in a couple of weeks.
    Also, does the Phragmighty plant have an effect on the lake level and overall health of the lake?

    • According to data on NOAA GLERL’s Great Lakes Water Level Dashboard, Lake St. Clair went through a normal seasonal decline from its peak in July (574.47 feet) to December (573.46 feet) last year. You can find more lake level data available online. In general, this is the time of year you will see a decline in lake levels because of high rates of evaporation. It’s also possible that the two week period you mentioned coincided with a strong wind event. If so, a seiche might cause a rapid change in water level, in addition to fall decline.

      Regarding your second question, phragmites does not have a measurable effect on lake levels, but it is a concern for the overall health of the lake. You can learn more about this highly invasive plant here:

    • Richard, I received an email from James Lewis at the USACE Detroit District in response to your question about the rapid drop in water levels. He confirmed my previous explanation and added: “In general Lake St. Clair can fluctuate quite a bit more than the other lakes because of its smaller size.” Here’s a link to the Great Lakes Water Level information available on their website:

  • Dave

    Great article Lisa…very interesting. When you refer to the “number of Niagra Falls”, does that mean the amount of water that flows over the falls per month?

    • Thanks for your comment, Dave. And yes, the “Niagara Falls” unit is equivalent to the amount of water that flows over the falls per month. John Lenters explains it as a “volumetric flow rate” similar to cubic feet per second or gallons per day. He says that even the “inches” on the left side of the figure are a flow rate, since it’s technically “inches per month” calculated over the surface area of the lake, or a volume per unit time. He chose to express it as the “Number of Niagara Falls” because most people can picture the flow rate of the Niagara Falls, rather than the total volume of water accumulated over a whole month.

  • John Collins

    From observations of lower Lake Huron for the last 20 years from my apartment, I am aware of the gradual extending of the shipping season so that many ships no longer stop for the winter. They call on the Coast Guard icebreakers to get them through (at tax payers expense). When a path is opened for them, and there is a northwest wind, snow streamers are generated and highways well inland from the Lake Huron shore in Ontario are snowbound. The increase in open water in winter is definitely caused in part, by shipping activities.

    • John, Thanks for sharing your observations.

  • Mark

    Lisa, thanks for an excellent article. Living in Chicago, I am especially interested in the levels of lakes Michigan and Huron. I have read that dredging of the head of the St. Clair river some time ago has increased the flow which has lowered the levels. Do you have a Niagara Falls equivalent for that. Is it an important factor.

    • Thanks for reading, Mark. In a previous post, I describe how past dredging and erosion in the St. Clair River resulted in a 10-15 inch lowering of water levels on Lakes Michigan and Huron: Climate Change and Variability Drive Low Water Levels on the Great Lakes.

      I asked John Lenters if this could be compared to the Niagara Falls flow rate. He explained that the St. Clair River dredging is a complicated issue, and it’s hard to pinpoint a specific timescale to express this as a flow rate. “No one argues that it had a permanent lowering impact on Lake Michigan-Huron, so it would be misleading to try and relate it to Niagara River flow,” he said via email. He suggested this publication for further reading:

      Lenters did calculate a Niagara Falls equivalent for the Chicago diversion. The Niagara Falls flow rate (1,834 cubic meters per second) is about 20 times larger than the Chicago diversion (90.6 cubic meters per second), or expressed another way, the Chicago diversion is 1/20th the flow rate of Niagara Falls, or 0.05 the “number of Niagara Falls.”

  • Nicholas


    I think the Ice is outstanding. It truely great to go very fare out on the Ice and Fish.
    I feel safer.
    I’m hope-in the ice sticks around longer, but than again I want a my plants to grow better.
    I just hope this colder water level this year kills off the Russian fresh water life invasion.

  • Becky

    I find it very interesting that, although the greatest evaporation occurs in the cold season, the greatest trends in evaporation are actually found in the warm season months (found with data analysis over Lake Michigan).

    I also wonder if there is increased year-to-year variability. Decreased ice cover –> more evaporation in the summer –> more evaporation aides in increasing next winter ice cover –> decreased evaporation in the summer. Seems like you would cycle back and forth between the two regimes (of course ignoring other external components that will impact both seasons). Do you think this increased variability will help improve lake level forecasts, or make it more difficult to forecast?

    • Thanks for your comments, Becky. Once again, I consulted John Lenters, an expert on Great Lakes evaporation who works at LimnoTech, an environmental consulting firm. He says that both questions you raise are ones researchers have definitely thought about. Here’s more of what he had to say:

      1) Yes, the strongest trends in evaporation seem to be occurring in summer. We’ve found the same thing for Lake Superior. Interestingly, though, these trends appear to be partly due to what’s going on in the winter, such as reductions in ice cover. The effects linger well into summer and end up influencing the start to the next season’s evaporation. So changes in winter climate appear to be affecting summer evaporation.

      2) I haven’t seen evidence for increased interannual variability. But I have wondered about the potential for the “multi-year” connections that Becky refers to … e.g., whether one year’s ice cover (or water temperature) can influence the next. Deep lakes like Lake Superior definitely have a lot of “thermal memory,” but we’ve mostly seen this effect on 1-6 month timescales. Whether or not the effects linger a full year or longer is still an area of active research.

      Thanks also to John for the helpful information!

  • Larry

    Thanks for your info – helps a lot….

  • Tom Sheley

    Very interesting three factor formula on what (and when) that contributes to the lakes evaporation. Possibly I can feel a little more educated when I talk to people about my observations.

    Niagara Falls equivalent is a neat term. For the novice it would be nice to include how that volume unit is determined. Right now I can only suspect it is accurately refined by measuring the volume of water that goes through the Hydroelectric station verses guesses of what goes over the falls, or is via a calculation on volume at a point down stream of the falls over a given river speed?

    Thanks Again

    • Tom, Thanks for your comment. Regarding “Niagara Falls equivalent,” here’s a more technical description from John Lenters of how this volume is determined in rivers:

      “River discharge is usually estimated through what is known as a “stage-discharge relationship.” Scientists measure the stage, or water level (in feet or meters), and then convert it to a flow rate, or discharge (in cfs or m3/s) by means of this stage-discharge relationship. Every river has a different stage-discharge relationship, due to varying river slope, cross-sectional area, and other characteristics. Discharge is more difficult to measure than stage, because it requires knowing the depth and speed of the river at many points across the stream bed, and across a range of different stages. So once a stage-discharge relationship is established for a river, it is typically only updated on a periodic basis.”

  • Phil Modjeski

    Do we have any idea what affect sublimation of lake ice will have on this record coverage year?

    • Phil, I asked John Lenters, one of the authors of the study mentioned in this post, about sublimation. Here’s what he had to say: “Sublimation generally occurs at rates much slower than open-water evaporation, so the effect on the spatial extent of ice cover will likely be minimal. The greater effect right now, as we’re getting closer to summer, is the higher sun angle and longer days. This increasing amount of solar radiation is readily absorbed by the dark patches of open water, as well as the ice cover itself (which darkens as it thaws). Windy days aid the thawing of ice by mixing it with the warming water.”

  • Michael Atkin

    I have enjoyed reading the report and all the comments. I just wanted to point out that even a partial ice cover for example Georgian Bay on Lake Huron prevents the lake effect snow from travelling further inland and out of the watershed.

  • Brian

    Interesting article Lisa,
    I have spent almost every day for the last 45 years at the waters edge of Severn Sound. I have watched what I consider to be the normal yearly fluctuations of inches to a couple of feet one way or another. The 4 foot drop we have been dealing with for the last few years has made it very difficult and expensive to operate our family business.

    If ice cover keeps water temperature lower, thereby slowing evaporation, then it also holds true that the lower water levels allow the sun to warm the lower volume of water faster in the summer months, thereby accelerating evaporation.
    We have noticed this in Severn Sound where the average depth is only 12 feet at the best of times. Now it is 8 to 10 feet and the water temp in the summer is reaching 75 to 80 degrees F. This situation also accelerates weed growth which chokes our water flow and stains the water from rotting weeds.

    It seems only common sense to me that if we could reduce the flow out through the St Clair it would help raise the levels slightly in Michigan and Huron. The resulting larger volume of water would take longer to heat and result in less evaporation.

    I compare these 3 lakes as a bathtub. If what is going out the drain is more than what is coming in from the tap then the water level is going to drop.

    thanks for the articles and research you are providing!

    • Thanks for the comment, Brian. The water levels are expected to continue to recover, so this should help the situation in Severn Sound. You make an interesting point about how the shallower water in bays is an added factor for warming water temperatures and weed growth. And you’re right about the effect of reducing flows through the St. Clair River, but this solution was considered too costly in a recent assessment by the IJC, especially given the possibility that lake levels could reach high levels again in the future, as some climate models predict. Time will tell.

  • MIcheal Macdonald

    Thank you for the great report! As a child growing up near Detroit, my family always told me what a precious natural resource there was in these Great Lakes. This year has been very encouraging with regards to the rising water levels.

  • Nirjala Koirala

    Thanks Lisa for this wonderful article. I was wondering as if warm lake water temperature are more responsible and lake -effect snow can be considered as good example of evaporation happening above the lake surface, so in this situation would it be good to link this phenomena with climate indices like ENSO and Elnino to prove that the heavy snow fall occuring during winter in around lake is actually , lake -effect snow.

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