Lake Superior Water Levels: The Role of Precipitation

August 17th, 2016 by Roy W. Spencer, Ph. D.

The last couple years have seen exceptional erosion along portions of the south shore of Lake Superior, especially where the ground is very sandy. The following photo was recently taken west of Whitefish Point in the eastern Upper Peninsula of Michigan, of a cabin built in the 1950s:

Erosion on the south shore of Lake Superior (Ben Musielak, Ben Musielak Photography, Paradise, MI).

Erosion on the south shore of Lake Superior (Ben Musielak, Ben Musielak Photography, Paradise, MI).

While water levels have been on a slow, irregular decline for decades, there was a sudden rebound during 2013-2014 to near-record high levels:


What caused this rapid rise? In fact, what controls the water level of Lake Superior on a year-to-year basis?

The hydrology depends upon many factors, both natural and human. Precipitation over the lake and the drainage basin feeding the lake, evaporation, and outflow through the rapids on the St. Marys River at Sault Ste Marie are the primary natural processes.

But locks built in the Sault in the 1800s also altered the natural lake levels, as well as contruction of a canal which feeds a hydroelectric power plant. Subsequent dredging of the shipping channels has also altered the flow. (Here’s an amazing high-resolution 1905 photo during a celebration of the locks, it still looks the same today…except for fences to keep idiots from falling in, and an observation deck).

Gates at the head of the power canal are raised and lowered to provide some human control over lake levels, and are raised to allow more water to flow out when lake levels are high. Many factors are weighed in deciding to adjust the flow out of Lake Superior, including the water level in the rest of the Great Lakes downstream, as well as Canadian concerns. The governing body for these decisions is the International Joint Commission (IJC).

For example, there has been much debate over water levels on Lake Erie, which have also been running high. If you let more water out of (much larger) Lake Superior, then the coastal interests along Lake Erie (which I suspect are much larger in number and politically more powerful) are going to be very concerned. The total population living along the lower lakes is about 15-20 times that living along Lake Superior, so you can see why interests along Superior aren’t the only ones deciding how much extra water will be allowed to flow out of the lake.

How Much Control Do We Have Over Lake Superior Levels?

Given that the Sault locks are there to stay, just how much control could we have over the water level in Lake Superior, if we wanted to?

The answer, it turns out, is not very much.

The river discharge out of Lake Superior through the St. Marys River has been running around 100,000 cu. ft./sec in the last couple of years. If you assume that we could change that rate at will by, say, 10%, and divide that into the number of square ft. covered by Lake Superior (~884,000,000,000), you will find that such a change in river discharge would only change the lake water level by 4 inches in one year. The above graph shows that much larger changes occur on the lake than this. Obviously, natural influences on water level are much larger than human influences.

I used to live on the lower St. Marys River, and can attest that when the lake level is high, so is the level of the lower river. I specifically remember the summer of 1973 (see above graph) when water on some days went up on our lawn and came close to our front door. Then, years after I moved away, low lake levels led to the shoreline going out about 100 ft. further from the house than normal.

The reason I mention this is that, when lake levels are high, there is an increase in river discharge out of the lake. That is the direction of causation: high lake levels => increased outflow. It is not: decreased outflow => high lake levels.

In other words, human manipulation of water flow out of Lake Superior is not the cause of high water levels…although we have some small amount of control to mitigate changes in lake levels, after they have occurred.

The Primary Control Knob: Precipitation

While I’m not a lake hydrology expert, I suspect that the balance between precipitation and evaporation is the governing factor in Lake Superior water level.

If we look at the average yearly precipitation departures from normal from the National Centers for Environmental Information website for the Upper Peninsula and northeast Minnesota averaged together, we find that the Lake Superior water level rises and falls depending upon excesses and deficits in precipitation:


In fact, during the most recent rapid rise in water levels, a 2-year excess in precipitation of 11 inches led to a 22 inch rise in lake levels! This is pretty spectacular.

Why would the lake rise more than the precipitation? Because the drainage basin for Lake Superior is 1.55 times larger than the lake itself, and so some of the excess water that falls on the surrounding land flows into the lake, potentially more than doubling the lake level rise due to a precipitation increase:

How well this quantitatively explains things, I don’t know. I’m ignoring changes in evaporation, which would have to be estimated through some modeling assumptions.

The bottom line is that Mother Nature is largely in control of water levels on Lake Superior. Humans can help mitigate it somewhat by adjusting flow through the St. Marys River, and I believe that is already done, but I suspect that coastal interests along the lake simply have to live with the changes, which we have very little control over.

I have sent a few questions to the experts at the Great Lakes Environmental Research Lab (GLERL), and will update this post if I find out any additional information.

29 Responses to “Lake Superior Water Levels: The Role of Precipitation”

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  1. dam1953 says:

    I once read that the apparent drop in the Great Lakes water level was due to post Ice Age rebound. The water level wasn’t going down, the shore line was instead going up. Could this be a contributing factor to the erosion you are witnessing?

    • Ric Werme says:

      That would require that the outflow goes up relative the point of interest. While in general, the area north is going up, and the south is less so (or actually going down as the surface pivots, the effect has to be somewhere close to unmeasurable on the scale of the Great Lakes. Heck, it’s hard to measure in the oceans!

      • lewis says:

        The idea that a rise in the landmass due to rebound would affect the lake level is humorless.

        The lake sits atop the same land, so any change due to rebound or whatever, would push the lake up or down the same amount.

        But, then again, the lake level may not be associated with the land around and under it. Shoot, it might even freeze with geese in it and when they fly away, they’ll take the lake with them.

    • David Gray says:

      I heard the same thing a year or two ago on a TV show (Discovery Channel?), which was talking about the Great Lakes in general. Specifically, the average lake level was not really dropping, the land was rebounding. However, the elevation chart above shows the actual sea level amount to be dropping, which is not a relative measure. This could be part of a longer period variance, or a general trend that would end with a tiny lake in a few hundred thousand years.

  2. mpainter says:

    One thing is inevitable: Sandy banks will erode. Water level is a factor and wind, too, which generates waves. I suspect it is the waves smacking against the base of the bluff that is the real problem in the photo.
    But actually, the real problem is people who build where they shouldn’t, poor devils.

  3. Brian D says:

    As a resident of the North Shore of Lake Superior, I remember well the start of the rise in lake levels. Starting in the Spring of 2012, we had most of our season’s snow in March and April that year. The last snow storm was on May 1st. Then it warmed and quickly melted all that snow. May and June had lots of rain. 6+” in May and 10″ in June. St. Louis River, that empties into Duluth Harbor, had a record crest at 16′. The next two winters were cold and snowy. High ice on the Lake kept the evaporation down, and the snows were top 5 all time for the 2 seasons. The next 2 Springs were also above normal rains. High precip and low evap caused the rapid rise on Lake Superior, and the lower lakes.

    My brother -in-law commercial fishes the lake and he has had to reinforce his boat house with cement walls because of the higher water.

  4. Typical says:


    The idea that lake Superior water levels have been slowly decreasing is a hoax perpetrated on us by Al Gore and the liberal left. If you look at your graph, it is clear that lake levels were lower in 1926 and a linear fit from 1998 to the present clearly shows rising levels.

  5. Ric Werme says:

    The relationship between autumnal evaporation rates, leading to cooling and ice cover in the winter, leading to reduced evaporation rates all seems a lot more complex than I would have guessed.

    However, a good layer of ice and insulating snow certainly squashes a lot of wintertime evaporation.

  6. jerry l krause says:

    Hi Roy,

    I believe, but do not know, that the flowage of most rivers and streams, are now regularly monitored. I do not know when this practice started or how these data files can be accessed. But I know that a cyclic rise and fall of small lakes in the upper Midwest occurs just as is known to have occurred in Lake Superior. And in many of these cases I am sure no human cause to the observed oscillation can be identified.

    But it would be good to actually have quantitative data to support what seems qualitatively obvious.

    Have a good day, Jerry

  7. doctor no says:

    Another complicating factor is runoff – this can vary with the nature of the rainfall i.e.whether it is sudden or spread over many days.
    The 6 years of decreases 1921 to 1926 dip deserves attention – what makes this so different to the rest of the data.

  8. Mickey says:

    The resolution on the 1905 picture is amazing.

  9. argus says:

    Neat post, interesting stuff. Sometimes all the climate change arguing in the news, comments, etc makes me tune out. Life right now doesn’t revolve around whether Earth is hotter/colder 100 years in the future.

  10. dave says:

    Of course, the “resolution” we are “seeing” here, comes to us courtesy of – and restricted by -the digital flows of the internet, the makers of our monitors, and some trompe l’oeil!

  11. ren says:

    Another wave of severe storms in Louisiana.
    See for water vapor over the USA.

    • mpainter says:

      Extraordinary wet season in the middle of August in Texas, where & when a summer shower is a rare and blessed event. I expect to see some precipitation records out of this.

  12. Svend Ferdinandsen says:

    It is like floods. Ten inches of rain raises only the level overall ten inches. But if some of this water collects at low points it gets much worse at the low points, but the higher points have no problems.

    The graph of water level versus precipitation says it all.

  13. Tom Curtis says:

    True that.

  14. Over the 95 years of Lake Superior elevations represented in this post, isostatic adjustment of the crust at a rate of approximately 1 cm/year would increase the elevation of the land by about 1 meter.

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