Volume 200: Frequently Asked Questions about Water Levels

Current Conditions 

All of the Great Lakes are above their long term 

average monthly mean water levels. Figure 1 shows 

water levels on each of the Great Lakes from 1918 to 

2017, which is the period of record for which monthly 

mean water levels have been coordinated between U.S. 

and Canadian federal agencies. During this period, all 

of the Great Lakes have experienced multiple periods 

of high and low water levels. From December of 2017 

to March 2018, Lake Superior water levels were the 

second highest on record (higher levels were seen in 

1985 and 1986), but after very dry conditions in March 

and April, a dramatic decline in level brought the April 

monthly mean water level to the 9th highest on record. 

The Lake Michigan-Huron water level in April was the 

16th highest April level on record, and the highest April 

water level since 1998. These high water levels stand 

out, in part because they follow an extended period of 

low water levels that occurred from the late 1990’s to 

the early 2010’s. In this Great Lakes Water Level 

Update, frequently asked questions about Great Lakes 

water levels, especially questions related to the 

currently high water levels, are addressed. 

Figure 1. Historical Great Lakes water levels. This image can be downloaded as a pdf with higher 

resolution from www.lre.usace.army.mil/Missions/Great-Lakes-Information/Great-Lakes-WaterLevels/Historical-Data

Great Lakes

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How are water levels determined? 

Lakewide average water levels of the Great Lakes 

are determined by averaging data from a network 

of gages operated and maintained by the U.S. 

National Oceanic and Atmospheric 

Administration (NOAA) and the Canadian 

Hydrographic Service (CHS), shown in Figure 2. 

Monthly mean water levels and beginning-ofmonth water levels are computed by the Detroit 

District’s Great Lakes Hydraulics and Hydrology 

Office from data provided by these gages, and 

these levels are routinely coordinated with Canada. 

Figure 2. Great Lakes Water Level Gages. 

What is the International Great Lakes Datum 

of 1985 (IGLD 85)? 

The International Great Lakes Datum of 1985 is 

the common height reference system by which 

Great Lakes water levels are measured. The IGLD 

1985 has its zero base at Rimouski, Quebec near 

the mouth of the St. Lawrence River 

(approximately sea level). The IGLD must be 

adjusted every 25 to 30 years, because of the 

movement of the earth’s crust. The last adjustment 

was conducted in the 1990’s, with 1985 as the 

central year of the period during which water level 

information was collected for the datum revision 

(1982-1988). Work is currently underway by 

federal agencies in the U.S. and Canada to conduct 

the next revision to the IGLD. 

What is chart datum? 

Chart Datum, also commonly referred to as Low 

Water Datum, is a vertical plane of reference used 

to measure water depth. There is a distinct Chart 

Datum for each lake which is determined from 

historical water level records. These datum planes 

have fixed elevations relative to the International 

Great Lakes Datum of 1985 (IGLD 1985). Water 

levels are referenced to chart datum in order to 

agree with navigation charts. According to the 

Canadian Hydrographic Service, “Chart datum is 

selected so that the water level will seldom fall 

below it and only rarely will there be less depth 

available than what is portrayed on the chart.” 

Why is Lake Michigan-Huron considered to be 

one lake? 

Geographically speaking, Lake Michigan-Huron 

is often referred to as two lakes: Lake Michigan 

and Lake Huron. However, hydrologically 

speaking, these two bodies of water are referred to 

as one lake: Lake Michigan-Huron. This is 

because Lake Huron and Lake Michigan are 

hydraulically connected via the Straits of 

Mackinac, and as a result, they rise and fall 

together as one lake. For example, precipitation 

over Lake Michigan causes both Lake Michigan 

and Lake Huron to rise. 

What causes water levels to change? 

Water levels on the Great Lakes can change at 

different timescales, depending on different 

natural drivers. Large fluctuations in lake levels 

from one location on a lake to another can occur 

on the order of hours to days, and are the result of 

meteorological conditions. For example, a seiche 

occurs when there is a rapid change in atmospheric 

pressure that causes water to be pushed from one 

side of a lake to the other (Figure 3). This occurs 
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frequently on Lake Erie, owing to its shallow depth 

and southwest-to-southeast alignment. While most 

seiche events are relatively small in magnitude, in 

extreme cases the water levels at Toledo and 

Buffalo can differ by more than 10 feet. 

Figure 3. A seiche is an atmospherically driven 

fluctuation in lake levels across a lake, and 

occurs on the order of hours to days. 

Monthly and seasonal fluctuations are the result of 

changes in the amount of water flowing into and 

out from the lakes. While human influences have 

a small impact on these inflows and outflows (see 

FAQs related to Lake Superior regulation and 

Great Lakes Diversions in this article), these 

inflows and outflows are primarily determined by 

regional climatic conditions, which influence the 

Net Basin Supply (NBS) of water to each lake 

(Figure 4). NBS represents the net influence of 

precipitation over the lake, runoff from a lake’s 

watershed into the lake, and evaporation from the 

lake’s surface. 

Figure 4. Inflows and outflows driving monthly 

to seasonal changes on the Great Lakes. 

Why is there a seasonal cycle to water levels? 

The water levels of the Great Lakes follow a 

seasonal cycle in which water levels rise in the 

spring and early summer, and then decline in the 

late summer and fall. The processes that drive the 

seasonal cycle are shown in Figure 5. During the 

winter, snow falls and accumulates in snowpack, 

limiting the amount of precipitation that becomes 

runoff into the lakes. Then, in the spring, as the 

snow melts and rainfall becomes runoff, water 

levels increase. During the summer, the lakes 

warm, and in the fall, when cold air comes into the 

region and passes over the lake, evaporation from 

the lake surface increases. 

Figure 5. Seasonal cycle of water levels. 

What is the influence of aquifer recharge on 

water levels? 

Water that recharges aquifers does not typically 

directly factor into the computation of the water 

budget of the Great Lakes. It is, however, 

indirectly accounted for in models that simulate 

the translation of precipitation to runoff, and the 

climatic conditions that lead to a high groundwater 

table would also lead to higher lake levels. It is 

common practice to assume that groundwater 

movement into the lakes is negligible, relative to 

other components of the water cycle. 

Why are the water levels high? 

Over the past several years, annual NBS has, more 

often than not, been above average on all lakes. On 

Lake Superior and Michigan-Huron, the extremely 

high 2013, 2014, and 2017 NBS were especially 

noticeable, and were accompanied by a recordWind

Set-up Storm Water Level

Undisturbed Water Level

Wind
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setting 2-year rise in 2013-2014. This dramatic rise 

in water levels came on the heels of a decade of 

mostly below average NBS that resulted in an 

extended period of low water levels and 

culminated in a record low water level on Lake 

Michigan-Huron in January 2013. On Lake 

Ontario, record high NBS during the spring of 

2017 contributed to a rapid rise, resulting in record 

high water levels in May, June, and July of 2017. 

Figure 6 shows the annual NBS over the past 

decade for each lake, as well as water levels for the 

same period. 

Figure 6. Difference from long term average 

annual NBS in cubic feet per year (top panels) 

and monthly mean water levels in feet over the 

past decade (bottom panels). 

Have water level trends changed in recent 

years, relative to the past? 

Great Lakes water levels vary significantly from 

year to year and can swing from above to below 

average water levels within a few years (see Figure 

1). While it is difficult to find long term trends in 

lake levels using the coordinated period of record 

(1918-2017), the past 5 years have changed the 

water level story dramatically from one of 

persistently low levels, especially on Lakes 

Superior and Michigan-Huron to a story of high 

water levels. In fact, Lakes Superior and 

Michigan-Huron both experienced a record setting 

2-year rise in 2013 and 2014. Over the long term, 

however, there is no indication that this recent 

trend will continue. 

How do high water levels influence shoreline 

erosion? 

Variation in lake levels, whether short or long 

term, have little effect on the creation of waves, the 

primary erosion agent. Most waves are generated 

far offshore in deep water where such relatively 

small water level variations are insignificant. As 

long as the long-term meteorological and 

hydrographic factors that determine wave energy 

remain the same, the long-term erosion rate would 

remain essentially unchanged. The lake level 

does, however, have an effect on where wave 

energy is dissipated on the beach profile, and thus 

may affect bluff recession rates over short time 

periods. 

The lake level is only one of many factors 

influencing coastal erosion and recession. To date, 

the relative importance of lake level compared to 

the other influencing factors has not been fully 

quantified. Observations suggest that along much 

of the coast, storm duration and return frequency, 

and sediment supply have much more influence on 

coastal erosion and recession than higher lake 

levels do. 

How do diversions into or out from the Great 

Lakes impact water levels? 

There are five locations around the Great Lakes 

where water is diverted using hydraulic structures. 

These include the Long Lac Diversion, the Ogoki 
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Diversion, the Chicago Diversion, the Welland 

Canal Diversion, and the New York State Barge 

Canal Diversion. These diversions have a small 

influence on Great Lakes water levels, relative to 

the climatically driven NBS on each lake. 

Figure 7. Great Lakes diversions. 

The Chicago diversion is the only structural 

diversion that moves water outside the Great Lakes 

basin, and annual flow in the Chicago Diversion is 

limited to 91 m3

/s (3,200 cfs) by a Supreme Court 

decree. Relative to the average annual NBS 

(112,700 cfs), average annual inflows from the St. 

Marys River (74,900 cfs), and outflows from the 

St. Clair River (183,300 cfs), this 3,200 cfs 

diversion is very small and has very little influence 

on Great Lakes water levels. The Long Lac and 

Ogoki diversions collectively add roughly 5,300 

cfs into Lake Superior, which is small, relative to 

the average annual NBS (roughly 71,300 cfs) and 

average annual outflow through the St. Marys 

River (74,900 cfs). The Welland Canal diversion 

transfers an average of about 8,000 cfs from Lake 

Erie to Lake Ontario, which is small, relative to the 

Niagara River, which averages roughly 204,100 

cfs annually, and NBS of Lake Erie, which 

averages 21,900 cfs annually. The New York State 

Barge Canal Diversion presently diverts water 

from the Niagara River watershed and returns it to 

Lake Ontario, and its flows are accounted for 

within the Niagara River flows. 

Where are flows in the Great Lakes regulated? 

Regulation of connecting channel flows in the 

Great Lakes occurs at three locations: the St. 

Marys River, the Niagara River, and the St. 

Lawrence River. Oversight of regulation at these 

three control structures is provided by the 

International Joint Commission. Unlike regulation 

at the St. Marys and St. Lawrence Rivers, 

regulation at the Niagara River has no influence on 

the total outflow of Lake Erie, and therefore no 

influence on the level of Lake Erie or Lake 

Ontario. Regulation of Niagara River flows is 

determined by the 1950 Niagara Treaty, which 

specifies the minimum amount of water that must 

flow over the Niagara Falls at different times. 

How is Lake Superior outflow regulated? 

Regulation of Lake Superior outflows is the 

responsibility of the International Lake Superior 

Board of Control, under the authority of the 

International Joint Commission. Plan 2012 is the 

current regulation plan for Lake Superior that 

provides a set of rules used to determine Lake 

Superior outflows each month. These rules begin 

with a “pre-project” flow relationship, which 

determines the flow that would occur if the 

infrastructure that controls flow in present times 

did not exist. This relationship is based on flows 

prior to 1887. Then, a balancing principle is 

applied. This balancing principle adjusts flow 

based on the level of each lake relative to seasonal 

targets based on average conditions. These 

adjusted flows are subject to physical and 

operational limits. Finally, the flow is allocated 

among municipal and industrial uses, navigation 

(via the locks), hydropower, and the St. Marys 

rapids. The end result of the operation of Plan 2012 

is a determination of the Lake Superior outflow 

and the gate setting. Note, however, that because 

of the lack of ability to control or predict NBS, the 
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ability to regulate Lake Superior outflow does not 

mean that full control of Lake Superior (or Lake 

Michigan-Huron) water levels is possible. For 

more information, see the resources available on 

the International Lake Superior Board of Control 

website: 

http://www.ijc.org/en_/ilsbc/International_Lake_

Superior_Board_of_Control. 

Figure 8. St. Marys River, looking downstream. 

Operation of Plan 2012 results in a 

determination of the coming month’s Lake 

Superior outflow and flow through the St. 

Marys Rapids (via the Compensating Works). 

To what degree does regulation impact Lake 

Superior water levels? 

The ability to control Lake Superior’s outflow 

does not mean that full control of lake levels is 

possible. This is because major factors that affect 

the water supply to the Great Lakes are over-lake 

precipitation, runoff, and evaporation (together 

known as net basin supply). These components of 

net basin supply can vary dramatically from month 

to month and cannot be controlled, nor can they be 

accurately predicted in the long term. The NBS, St. 

Marys River discharge, and inflow from the Long 

Lac and Ogoki Diversions, which collectively 

comprise the Lake Superior water budget, are 

shown in terms of their influence over Lake 

Superior water levels in Figure 9. The month-tomonth and interannual variability in NBS shown in 

Figure 9 is the main factor in limiting the ability of 

regulation to control Lake Superior water levels. 

Figure 9. NBS, inflow (Long Lac and Ogoki 

Diversion, or LLO), and outflow (St. Marys 

River) for each year of the historical record, in 

terms of inches over Lake Superior. Note that 

outflows are shown as having a negative effect 

on Lake Levels, so higher St. Marys River 

discharge is plotted lower on the y-axis.

Why isn’t more water being sent through the 

St. Marys River? 

The International Lake Superior Board of Control 

utilizes Lake Superior Regulation Plan 2012 to 

determine the amount of water to release from 

Lake Superior and balance the impacts to Lake 

Superior, Lake Michigan-Huron, and the St. Marys 

River. As described above and in Figure 9, the 

ability to control water levels via regulation is 

small, relative to natural influences on lake levels. 

How would adding or removing an inch of 

water over Lake Superior via the St. Marys 

River change the water level on Lake 

Michigan-Huron? 

Lake Michigan-Huron’s surface area is about 43% 

larger than Lake Superior (45,300 square miles, 

compared with 31,700 square miles, shown 

overlapping in Figure 10). Accordingly, if the 

volume of water contained in an inch of water over 
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Lake Superior’s surface was added or removed 

from Lake Superior, it would result in a 0.7 inch 

rise or fall on Lake Michigan-Huron (if there were 

no outflow through the St. Clair River). 

Figure 10. Lake Superior’s surface area is 

roughly 67% of Lake Michigan-Huron’s 

surface area. 

 

What was the variability in lake levels prior to 

regulation? 

Regulation of Lake Superior began with the Order 

of Approval of 1914, with full control of the 

current 16-gate structure being achieved in 1921. 

Regulation has had little impact on the variability 

of water levels of Lake Superior, as can be seen in 

Figure 11. Looking back prior to the 1920s, water 

levels are determined using a single master gage 

on each lake (Figure 2 shows the location of these 

gages). Monthly mean water levels from 1860 to 

present are shown in Figure 11. Prior to 1921, the 

highest monthly mean water level occurred in 

1876 (603.58 ft) and the lowest monthly mean 

water level occurred in 1866 and 1911 (600.07 ft). 

Subsequent to 1921, the highest water level 

occurred in 1985 (603.38 feet) and the lowest 

water level occurred in 1926 (599.48 ft). 

Figure 11. Monthly mean water levels on Lake 

Superior. The vertical line is drawn at 1921, 

when full regulation of the St. Marys River 

began. 

What water level forecast products are 

available? 

The Detroit District Office of Great Lakes 

Hydraulics and Hydrology publishes several 

forecast products that are available online at 

https://www.lre.usace.army.mil/Missions/GreatLakes-Information/Great-Lakes-WaterLevels/Water-Level-Forecast. The Monthly 

Bulletin of Great Lakes Water Levels provides a 

forecast of monthly mean water levels 6-months 

out, and is updated at the start of each month. This 

forecast is produced through a coordinated process 

between the Detroit District and Environment and 

Climate Change Canada. In addition to the 

Monthly Bulletin, the District also publishes the 

Weekly Great Lakes Water Level Update, which is 

updated each week and provides a forecast of 

water levels 30 days out; and the Connecting 

Channels forecast, which is also updated weekly, 

and provides a forecast of depths in the St. Marys, 

St. Clair, Detroit, and St. Lawrence shipping 

channels for the next four weeks. In addition to 

these forecasts, the Detroit District also publishes 

a Great Lakes Water Level Outlook (also found on 

the forecast website listed above). Although this 

Water Level Outlook is not a forecast, it provides 

an indication of what the Great Lakes water levels 

could be under alternative scenarios of net basin 

supply conditions over the coming 12 months. 
Vol. 200 Great Lakes Update June 2018 

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This scenario-based product is updated on a 

monthly basis. The National Oceanic and 

Atmospheric Administration provides nearer term 

forecasts of water levels at specific locations on 

the order of hours to days. 

Why do water level forecasts change from 

month to month? 

Each month, forecasts of water levels for the next 

6 months are released through the Monthly 

Bulletin of Great Lakes Water Levels. As a new 

month begins, the actual starting level may be 

different from the water level that had been 

forecast during the previous month. Accordingly, 

the “most probable” forecast, as well as the 

forecast range is adjusted to reflect the actual 

starting levels. In some cases, this can result in a 

significant change in the forecast from on month 

to the next. See, for example, the difference in the 

forecast range between the forecast published in 

April of this year, compared with May (Figure 12). 

In this case, the NBS in April was very small, 

despite climatic outlooks that did not indicate 

particularly wet or dry conditions in April. 

Figure 12. Lake Superior water level forecasts 

from April (top) and May (bottom) of 2018. 

More Information 

Like this article, future Great Lakes Updates will 

be included in various editions of the Monthly 

Bulletin. Topics will rotate between Great Lakes 

hydrology and lake levels and other less technical 

pieces relevant to the Great Lakes as a whole. 

February’s Monthly Bulletin will typically include 

an annual summary from the prior year. All of our 

past Update Articles can be read online here: 

https://www.lre.usace.army.mil/Missions/GreatLakesInfor

mation/NewsandInformation/GreatLakesUpdateArticles.asp

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If you have an idea for an Update Article topic, we 

would love to hear it. Please send your ideas via 

email to hhpm@usace.army.mil. 

The Detroit District also has a Facebook page 

which can be found here: 

https://www.facebook.com/pages/Detroit-District-USArmy-Corps-of-Engineers/144354390916