August 27, 2019
By Adam Bechle, Coastal Resilience Outreach Specialist, Wisconsin Sea Grant
The water levels on all of the Great Lakes remain very high, with every lake above or within a few inches of its record high monthly mean for August. Looking back at the month of July 2019, Lakes Superior, Erie, and Ontario all broke their record high monthly mean for July. Lake Michigan-Huron just barely missed setting a new July record monthly mean by three inches. There is a good chance that we’ve seen the highest water levels for the year, as the “seasonal decline” in water levels has begun.
Here are five things to know about water levels on Lake Michigan for August 2019.
What are the current water levels on Lake Michigan?
The water level of Lake Michigan as of July 23, 2019 is at an elevation of 581.73 feet above sea level (from the International Great Lakes Datum). For the most current daily water level, click here. To put this level into perspective, here are some statistics for Lake Michigan relative to the period of water level record measured from 1918 to 2018: (statistics from U.S. Army Corps of Engineers Weekly Water Level Update)
|Compared to…||Current Water Levels are…|
|One month ago||3 inches lower|
|One year ago||15 inches higher|
|Average August (from 1918 to 2018)||30 inches higher|
|Record August monthly mean (set in 1986)||3 inches below|
|Record high for any month (set in October 1986)||7 inches below|
What is the outlook for future water levels?
The US Army Corps of Engineers projects Lake Michigan to remain near record high levels for the foreseeable future (see six-month forecast issued for July 2019 below). Water levels are expected to begin their seasonal decline in August and September, with a projected drop of about 3 inches by September 23rd, 2019. This decrease in water levels is expected throughout the fall and winter as precipitation and runoff into the lakes typically decreases and evaporation from the lakes increase. In an average year, water levels vary seasonally by about one foot from a peak in summer to a low in winter, though every year is different. Water levels would be expected to begin a seasonal rise again next spring when runoff and precipitation increase.
Why are lake levels so high?
In short, the water supply into the Great Lakes has been quite high. This year, the Great Lakes as a whole has had above average precipitation this spring and summer. In the month of July, precipitation directly onto the lake was actually 27% below average. However, runoff from the rivers and streams that drain into Lake Michigan was above average, contributing to a rise in water levels in July.
But precipitation and runoff are not the only contribution to lake level change, as water also leaves the lake through evaporation (click here to read more on Great Lakes evaporation). To account for this, we turn to Net Basin Supply, which is calculated as
Net Basin Supply = Precipitation + Runoff – Evaporation
In general, when Net Basin Supply is positive, more water enters the lake than leaves, yielding a rise in lake levels. A five-year history of the measured Net Basin Supply is shown below, where blue indicates a positive Net Basin Supply and red is a negative Net Basin Supply. As a whole, the Great Lakes has seen a lot more blue than red in the last five years, indicating a consistently positive Net Basin Supply which has driven the lakes to rise. The Net Basin Supply of Lake Michigan specifically has been above-average for ten consecutive months, with big spikes in April and May of 2019. The result is our jump in lake levels this year to near-record levels.
How do high lake levels affect the coast?
High water levels present many challenges that are currently being felt throughout the Great Lakes, including:
- High water levels allow erosive waves to reach higher elevations on the shore where they batter shoreline infrastructure and eat away at the base of bluffs and dunes
- In low-lying coastal areas, high water levels can also make coastal flooding in low-lying coastal areas more likely
- In ports and harbors, high water can cause operational and safety issues if a large elevation difference exists between vessel and dockage
See our Coastal Hazards page for details about these impacts.
Our blog post Resources for Great Lakes Coastal Property Owners: Where do I start? Resilience Resources has links to many resources to help understand coastal hazards, weigh the risks coastal hazards pose to a property, understand options for addressing these hazards, and get started on implementing actions if necessary. One featured resource includes Adapting to a Changing Coast:Options and Resources for Lake Michigan Property Owners (pictured right)
Great Lakes Water Budgets from the University of Michigan gives a bit more information into what makes the lakes go up and down