If you've been to the water's edge at Lake Michigan and Lake Huron lately, you've noticed the water level is way down. In fact, the U.S. Army Corps of Engineers says that as of Dec. 5 the lake level is tied with its lowest level ever for this date, recorded in 1964. What's causing the two Great Lakes surrounding the Lower Peninsula to drop so quickly? The lake level can be understood in your own bathtub. Fill up your tub halfway. Then open the tub stopper and let water run out. If you are putting water into the tub faster than it's draining, the water level rises. So the simple answer to why the lake levels are going down is that more water is leaving the lakes than coming in.
Weather's part in the lowering lake levels
There are many inflows and outflows into Lake Michigan and Lake Huron controlled by man. Those are complex to understand. Many recent studies say those controls affect lake levels in a small way when compared to weather effects on the lake. So let's just look at nature's role in our shrinking lakes. The weather part of the equation is pretty simple. Rainfall and snow melt into the drainage basin of Lake Michigan-Lake Huron bring the lake up. Evaporation off of the surface of the lakes takes water away. In our current cycle it looks like both aspects can be blamed for lower water levels.
Evaporation has been greatly increasing
I obtained modeled evaporation amounts since 1954 from the Great Lakes Environmental Research Laboratory. Modeled means the data doesn't actually come from instruments, but rather a formula they use to derive evaporation. It may not be a factual number, but since all years were modeled the same way it's all relative. The graph shows evaporation in the 1960s and 1970s was on average around 24 inches a year. In the late 1990s the trend was visibly for more evaporation off of the lake surface, increasing to nearly 30 inches a year. Since 2000, we are losing around 31-32 inches of liquid due to evaporation! To see a larger image of the graph, click here. To put it in perspective, each year now loses eight inches more of water level than back in the 1960s.
Why so much evaporation?
Evaporation is caused by warmer temperatures, less ice cover in winter, and wind. I don't think that wind has changed dramatically. Temperatures, however, have been above normal in general for the last few years, and ice cover on the Great Lakes has been low. Ice on top of the Great Lakes slows down evaporation in the winter. So the warmer temperatures and less ice are leading to eight inches more evaporation yearly.
Less water from the sky hurts lake levels
The other variable in the lake level equation is precipitation and snow melt that runs off into the lake. It's probably hard for you to guess if we've been drier than normal or wetter than normal in Michigan. We had the drought conditions this summer. We also had a whole bunch of rain at times. Areas around Bay City and Saginaw received 8 to 10 inches of rain in two days back in August. Well, it has been drier than normal! The U.S. Army Corps of Engineers estimates rainfall amounts on the Lake Michigan and Lake Huron drainage basin. In November we had .89" of rainfall on the basin, which is 1.89" below normal. For the past 12 months the Lake Michigan and Lake Huron watershed has received 28.06" of rain. That's 4.38" below normal.
It all adds up to less water
Now it's starting to become easier to see how our water levels could have dropped so much this past year. The evaporation amounts are way up, and the run off into the lakes is way down. Both of those combined add up to a lot of water not making it into our lakes.
Lake Michigan and Lake Huron will continue to drop
There is a seasonal high and low pattern to our Great Lakes. Sometime in February or early March is when the lakes typically reach their lowest point, then rise to their peak around July 1st. So the Army Corps of Engineers predicts that Lake Michigan and Lake Huron will drop another three inches before February. Click here to see larger image.
Does the lack of precipitation combined with more evaporation sound like they are the cause of low lake levels, or do you think other factors play in our lowering lake levels?