Volume 188: 2012 Annual Summary

Background

The U.S. Army Corps of Engineers (USACE) 

tracks the water levels of each of the Great Lakes. 

This report highlights hydrologic conditions of 

the Great Lakes basin in 2012 and summarizes

recent water level behavior lake by lake.

Official records of the Great Lakes water levels 

are monthly averages and not daily averages. The 

period of record used for each of the lakes 

includes the years 1918 to 2011 and this data has 

been coordinated between both the United States 

and Canada. All 2012 water levels are considered 

provisional and will be officially coordinated in 

the spring of 2013. The elevations used are 

referenced to the 1985 International Great Lakes 

Datum. The water level of each lake is averaged 

from a network of individual gages around each 

lake. Also of note is that Lake Michigan and 

Lake Huron are hydraulically treated as one lake 

due to their connection at the Straits of Mackinac.

The water levels of the Great Lakes behave 

differently than most inland waters because of 

their huge surface area and volume. A single 

storm is very unlikely to cause a significant 

change to the water levels. Rather it takes 

multiple months, seasons, or even years of 

persistent wet/dry conditions to really impact the 

water levels.

The primary factors which determine water level 

changes are precipitation falling on the lake 

surface, runoff draining to the lake, evaporation

from the lake surface, diversions into or out of the 

lake, and connecting channel inflows and 

outflows. The Net Basin Supply (NBS) is an 

important quantity for understanding the amount 

of water which arrives to the lake. USACE uses 

the residual method to compute NBS, shown 

below in the summarizing equations.

WL: Water Level Change

I: Connecting Channel Inflow

O: Connecting Channel Outflow

D: Diversion into(+) or out(-) of lake

P: Precipitation falling on lake

R: Runoff draining to lake

E: Evaporation from lake surface

Water balance for each lake:

WL = I + D + P + R – E – O

Residual Method Net Basin Supply:

NBS = WL – I – D + O

Altogether, NBS represents the combined effects 

of precipitation, runoff, and evaporation. NBS is 

far and away the main driver of water levels, and 

is discussed in more detail below.

2012 Overview

Overall, the year 2012 was a very warm and dry 

year for the Great Lakes basin causing the water 

levels to decline. There was very little snow pack 

last winter (2011 – 2012) and this led to below 

average spring runoff for all of the lakes. On top 

of that, precipitation during the spring and 

summer of 2012 was very low. Lastly, because 

of the warm temperatures all year, evaporation 

was very high during the summer and fall of 

2012. This accumulation of multiple dry seasons 

has severely lowered the water levels.

Great Lakes

Update
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Page 2

All of the Great Lakes are currently below their 

levels of a year ago and also below their longterm averages. Lake Superior’s January water 

level was 1 inch below its level of a year ago 

while all of the other lakes were 17 to 22 inches 

below levels of one year ago.

Lake Michigan-Huron set new record low water 

levels during the months of December 2012 and 

January 2013, lower than any other December 

and January in the entire period of record. 

January’s level not only broke the previous 

January record, but it also broke the all-time 

record low for Lake Michigan-Huron. Water 

levels on Superior and Michigan-Huron have 

been below long-term averages for over 13 years, 

the longest consecutive period on record for each 

lake. Michigan-Huron setting new recorded 

minimums continues to highlight the extreme 

conditions which have been experienced in the

basin.

Lake Superior

The 2012 water levels for Lake Superior were 

below long-term average (LTA) for the entire 

year, extending a consecutive 13 year streak of 

below average levels. This is the longest stretch 

of below average levels in Lake Superior’s 

recorded history back to 1918. Lake Superior 

was below chart datum for nearly the entire year 

of 2012, being above datum only during the 

months of July and August.

Water supplies to Lake Superior were very dry 

for the year 2012. As mentioned earlier, the Net 

Basin Supply (NBS) is a helpful way to 

understand the total influence of precipitation, 

runoff, and evaporation. Figure 1 below shows 

the NBS for each calendar month. Dots in the 

figure represent the NBS for each year going 

back to 1900, giving an idea of the historical 

distribution of NBS. The horizontal line shows 

the average, and the arrow indicates the estimated 

2012 value.

Figure 1: Lake Superior Net Basin Supplies by Month
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The month of September 2012 for Lake Superior

clearly stands out as extreme. Figure 1 shows 

that the NBS was the driest it has ever been in 

September since 1900. In fact, the NBS for 

September was negative two inches, meaning that 

evaporation was much larger than precipitation 

and runoff combined. This contributed to Lake 

Superior’s October water level (600.79 ft) being 

only one inch above the record low water level

for October set in 1925 (600.72 ft).

Overall, the total NBS for 2012 was well below 

average. During the winter months of January 

and February, Lake Superior received much less 

snow than normal and experienced higher than 

average temperatures. Very dry conditions 

through the summer and early fall caused the 

water level to begin declining much earlier than it 

typically does. Altogether for the year, 

precipitation and runoff were both below average 

and evaporation was well above average.

Lake Michigan-Huron

Lake Michigan-Huron set a new record low water 

level in December 2012 of 576.15 ft, breaking the 

previous record low set in 1964 of 576.18 ft. The 

preliminary January 2013 water level of 576.02 ft 

is below both the previous January minimum of 

576.12 ft set in 1965 and the all-time record low 

of 576.05 ft set in March 1964. As mentioned 

above, the official period of record for water 

levels dates back to 1918.

The preliminary 2012 water levels for Lake 

Michigan-Huron were below LTA for the entire 

year, extending a consecutive 13 year streak of 

below average levels. This is the longest stretch 

of below average levels in Lake MichiganHuron’s recorded history. Lake Michigan-Huron 

was above chart datum only during the months of 

April through August in 2012.

Due to the very dry conditions of 2012, 

Michigan-Huron’s water level declined 17 inches 

from December 2011 to December 2012. A 

change in the water level of any of the lakes is 

caused by an offset between inflows and 

outflows. The 2012 water balance of MichiganHuron compared to historical averages can be 

seen in Figure 2. A primary factor causing the 

Figure 2: Lake Michigan-Huron Annual Water Balance
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water level to decline was the extreme 

evaporation in 2012. Evaporation is discussed in 

more detail later in this report, but it was 

12 inches above average in 2012 for MichiganHuron. Altogether, the water level plummeted 

because the outflows from the lake were much 

larger than inflows.

Figure 3 shows the month by month NBS for 

Lake Michigan-Huron. Two months of 2012 

clearly stand out as extreme; April and 

September. Michigan-Huron’s NBS is typically 

the highest in the month of April due to the 

melting of snow pack accumulated during the 

winter. The figure shows that the NBS in 2012 

was the driest it has ever been for the month of 

April, due primarily to the extremely small snow 

pack last winter. The NBS in September 2012 

was also the driest it has ever been. In fact, the 

NBS was negative three inches, meaning that 

evaporation was much larger than precipitation 

and runoff combined.

For the six consecutive months from April to 

September the NBS values for Lake MichiganHuron were continually below average; indicating 

extremely dry conditions during these months 

with very low precipitation and runoff and very 

high evaporation. Overall, 2012 was the second 

driest year on record (back to 1900); 1958 

remains the driest year on record.

Lake Michigan-Huron experienced a very slight 

seasonal rise in 2012, rising only 4 inches from 

February to June 2012 compared to the typical 

10 inches of rise. From June to December 2012, 

the lake declined 18 inches. The lake is expected 

to continue its seasonal decline into March.

Figure 3: Lake Michigan-Huron Net Basin Supplies by Month
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Lake St. Clair

From January 2012 to January 2013, the water 

level of Lake St. Clair declined 20 inches. 

Typically the water level rises from February to 

July each year, but this was not the pattern of 

2012. Instead, the monthly water levels have 

never once increased since December of 2011, as 

shown in Figure 4.

The preliminary water levels of Lake St. Clair 

started off above LTA in 2012. In April, the 

water level dropped below LTA and has ranged 2 

to 15 inches below LTA since then. Even though 

preliminary estimates of NBS in 2012 were not as 

low as the other lakes, Lake St. Clair’s water 

level still declined, primarily because low water 

levels on Michigan-Huron caused the flow 

through the St. Clair River into Lake St. Clair to 

be below average.

Figure 4: Lake St. Clair Water Levels

Figure 5: Lake Erie Net Basin Supplies by Month
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Lake Erie

Lake Erie’s water level declined 22 inches from 

January 2012 to January 2013. This is actually 

the largest drop over the past year among the 

Great Lakes. Normally, Lake Erie experiences a 

seasonal rise in March through May. In 2012, 

however, Lake Erie did not experience a seasonal 

rise, and was in decline for 10 consecutive 

months, from December 2011 to October 2012. 

The lake rose slightly in November 2012, but has 

continued to decline since then. Lake Erie began 

2012 above LTA, but since May it has ranged 1 

to 9 inches below LTA.

Figure 5 shows the net basin supplies to Lake 

Erie for each calendar month, comparing 2012 

with each year in the historical record back to 

1900. Overall, it was a very dry year for Lake 

Erie as well. The NBS was at or below average 

for 8 out of 12 months. The NBS was actually 

negative from June through October, meaning 

that evaporation was greater than the combined 

precipitation plus runoff for each of those 

months.

Lake Ontario

Lake Ontario’s water level dropped 17 inches 

from January 2012 to January 2013. In February 

2012, the water level rose to 15 inches above 

LTA. From February to December, Lake Ontario 

declined 28 inches. Lake Ontario’s preliminary 

January 2013 water level was 7 inches below 

LTA.

The explanation for the drop in Lake Ontario 

water level can be seen in Figure 6 showing the 

NBS for each month. Water supplies were near 

their driest ever for April, July, and August due to 

very low snow pack last winter and very low 

precipitation all year. The NBS was at or below 

average for 8 out of 12 months. Only 11 other 

years since 1900 have been drier than 2012 for 

the Lake Ontario basin.

Figure 6: Lake Ontario Net Basin Supplies by Month
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Evaporation

The water lost due to evaporation was a huge 

factor in the declining water levels of 2012. The 

warm air temperatures during the year caused 

well above average water temperatures. Though 

evaporation can happen at any time of the year, it 

is typically at its peak in the late summer to early 

winter when cool, dry air blows across a warm 

water surface.

Evaporation during 2012 was extremely high. To 

put it in perspective, evaporation from just the 

surface of Lake Michigan-Huron alone was 

greater than the water flowing over the Niagara 

Falls during the months of September and 

October 2012. Over four inches of water has left 

Lake Michigan-Huron each month since August 

2012 through evaporation.

Figure 7 below shows the total evaporation per 

year, averaged by decade. Estimates of 

evaporation come from the National Oceanic and 

Atmospheric Administration (NOAA) Advanced 

Hydrologic Prediction System developed by the 

Great Lakes Environmental Research Laboratory. 

Evaporation has clearly increased over the last 

few decades across the Great Lakes basin.

Figure 7: Great Lakes Evaporation by Decade
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More Information

Future update articles will be included in the 

February and August Monthly Bulletins

highlighting topics and explanations relevant to 

Great Lakes water levels. February’s Monthly 

Bulletin will typically include an annual summary 

from the year before and each August Monthly 

Bulletin will typically include a summary of the 

first six months of the year.

The Monthly Bulletin can be received either by 

postal mail or electronically. To receive it by 

email, you can visit our website 

(http://www.lre.usace.army.mil/glhh), navigate to 

the “Water Levels Forecasts” section, and choose 

to subscribe to the water level forecast(s) of your 

choice. To receive the Monthly Bulletin by postal 

mail, please email hhpm@usace.army.mil or call 

1-888-694-8313 and select option 1. If you wish 

to remove yourself from the postal mail 

subscription list or change your address please 

email us or call us with the contact information 

just mentioned.

In addition to the Monthly Bulletin, the Detroit 

District issues the Weekly Great Lakes Water 

Level Update and the Weekly Great Lakes 

Connecting Channels Water Levels and Depths. 

Both products are updated each Thursday and can 

be located here:

http://www.lre.usace.army.mil/greatlakes/hh/grea

tlakeswaterlevels/waterlevelforecasts/

In the near future, you may notice changes to our 

website as we migrate to a new website. We will 

include a note along with the Monthly Bulletin for 

assistance in navigating the new site when it is 

complete. We appreciate your patience through 

this transition.

The Detroit District welcomes comments on all of

our forecast products. Please email questions and

comments to hhpm@usace.army.mil. To contact

the District by phone call toll free 1-888-694-

8313 and select option 1. 

The Detroit District’s mailing address is:

Department of the Army

Detroit District Corps of Engineers

ATTN: CELRE-HH-W

477 Michigan Avenue

Detroit, MI 48226