Lake Levels Overview

Sustained Assessment of the Great Lakes

Societal Impacts of Lake Level Fluctuations

Fluctuations in water levels have adverse effects on the Great Lakes region economically and environmentally:

  • Issues in shipping and navigation arise during periods of low water levels as ships can be too large or deep to travel in some areas.1 2 For every inch of water level decrease, cargo capacity on freighters decreases by several hundred tons and tens of thousands of dollars in daily shipping profits per ship.3
  • Ecosystems and fisheries in the lakes are affected by lake levels because of impacts on natural habitats such as loss of coastal wetlands that serve as breeding zones and changes in thermal conditions affecting the survival of certain fish and plankton.4 5
  • High wind and wave events during periods of high water levels can cause shoreline flooding and coastal erosion, while low water levels can increase the need for dredging in harbors and canals.6 7
  • Hydropower generation is also vulnerable to low water levels when the slower water movement to turn the turbines weakens the capacity to generate electricity.8 9
  • Recreational boating can be adversely affected by low water levels, particularly in already shallow areas.10

Hydrologic Budget

The water budget of each Great Lake is driven by the combination of inputs: precipitation, runoff, upstream inflows; outputs: evaporation, downstream outflows; and diversions. The magnitude of these drivers can be seen in Figure 1, denoted for each Great Lake.

Figure 1: Water budgets of the Great Lakes. Left: runoff, overlake precipitation, and overlake evaporation. Right: flow between the lakes and diversions. Source: NOAA GLERL.

Diversions represent a comparatively small component of the water balance, and the flows between lakes represent a large component of the balance. However, these drivers remain fairly constant over time, with most changes happening gradually over many years. The climate-driven water supply components of precipitation, evaporation, and runoff can change on much shorter time spans and therefore have a larger influence on water level shifts. Figure 2 represents the hydrologic inputs to and outputs from the Great Lakes basin that contribute to changes in lake levels.  The upper Great Lakes (Superior, Michigan-Huron) are more heavily influenced by runoff from their surrounding basins than the lower Great Lakes (Erie, Ontario) that are dominated by inflow from these upper lakes.

Figure 2: Water balance components that affect Great Lakes water levels.

Precipitation that falls over the lakes and evaporation from the lake surfaces directly affect water levels, and precipitation over-land and evapotranspiration from the land contribute through their effects on basin runoff into the lakes. Evaporation from the lake surface is mainly driven by large differences between the water temperature and air temperature, high wind speeds, and low relative humidity. This peaks in the fall when lake temperatures are still warm from the summer and air temperatures are cooler, creating a temperature gradient ideal for evaporation. Evapotranspiration over land consists of the combination of evaporation from bodies of water in a lake basin (that are not a Great Lakes) and transpiration of water from soil, plants, and trees. Runoff consists of overland flow from rivers and streams, which are partially fed by rainfall, snowmelt, and groundwater. In addition, groundwater enters and leaves a lake through the subsurface of the surrounding land, a process known as direct discharge. Runoff and groundwater are affected by physical factors including land use, vegetation, soil type and moisture, topography, drainage systems, and elevation.  Runoff typically peaks in the spring when the basin’s snowpack melts. These climate or weather processes and other components of the water budget can be modeled by the Total Water Supply equation, shown below.

 TWS = P + R – E +/- GW + I – O – CU +/- D

TWS = Total Water Supply
P = over-lake precipitation
R = runoff into the lake (includes over-land precipitation and evapotranspiration)
E = evaporation from the lake surface
GW = groundwater (direct discharge)
I = inflow from an upper lake
O = outflow from the lake
CU = consumptive use
D = diversions

The components of precipitation, runoff, and evaporation from the equation above make up a lake’s Net Basin Supply (NBS):

NBS = P + R – E 

Though the five Great Lakes are part of one water system, they each have a unique NBS. Because of the different properties of each of the lakes, including management and control systems, as well as differences in climatology over the area of the basin, the level of each lake is affected differently by the effects of a changing climate. For example, Lakes Superior and Ontario have anthropogenic control structures at their outlets (St. Marys River and St. Lawrence River, respectively). The regulation of Lake Ontario’s outflow generally ensures that Lake Ontario experiences less overall water level variability than the rest of the lakes, but it cannot prevent extremes (see more under the Diversions, regulation, and management section below).

Drivers of Change: the Role of Climate and Variability

Water levels in the Great Lakes oscillate on an annual cycle as a result of seasonal trends in NBS  components. During the spring, snow melt increases runoff, leading to increased water levels in the lakes. In the fall, the temperature difference between warm surface water and cold air causes higher evaporation rates, leading to a decrease in water levels. As the climate changes, the timing and magnitude of this annual cycle can shift. For example, if snowmelt occurs earlier in the season due to warmer air temperatures in the winter, it would affect the timing of the spring runoff peak and lead to an earlier rise in the lake levels.

The timing and amount of lake ice also influence the lake levels, as it impacts evaporation rates during the winter months. If the timing and amount of lake ice changes, with less ice forming later in the year, there are higher observed evaporation amounts, leading to lower lake levels.11 For more information on this subject, please see the Lake Ice Overview pageChanges in regional climate have affected the patterns of Great Lakes precipitation, runoff, evaporation, and lake ice, and will continue to drive changes in the future. The following observational trends affect the water supply of the Great Lakes and water levels:

  • Since 1951, there has been an 14% increase in region-total precipitation as well as a 35% increase in the amount of precipitation falling in the heaviest 1% of storms.12
  • Annual average air temperatures have increased by 2.3°F in the U.S. Great Lakes region since 1951,  with lake temperatures increasing even faster.13
  • Annual average ice cover on the Great Lakes underwent a shift from higher amounts prior to the 1990s to lower amounts in recent decades. There remains strong year-to-year variability, and high ice years are still possible.14 15
  • The timing of spring snowmelt is changing.  During the period of 1960-2000, observed snow depths in the late winter and early spring decreased, implying an earlier onset of spring thaw.16

Changes in lake levels are the result of these different competing physical processes. For example, warming temperatures enhance evaporation over the lakes and in the drainage basin, and can lead to more years with low lake ice cover. Increases in evapotranspiration coupled with reduced ice cover duration can subsequently lead to lower water levels.17 18 Warmer temperatures can also reduce snowpack and soil moisture contributing to weaker runoff and lower water levels.19 20 Conversely, increases in precipitation frequency and intensity could lead to rising water levels.21 22 Any water level changes will depend on how one or more of these processes will dominate another in the future. GLISA suggests using a scenario planning approach (described in Future Water Levels section below) to help manage the uncertainty associated with these competing effects.

Recent water level rise (2014-present)

The past several decades have shown examples of how shifts in these processes led to low and high lake levels. The 1980s and 1990s experienced relatively stable high-water levels across the Great Lakes basin. In 1998, there was a sharp decline in water levels on the upper Great Lakes, Lakes Superior, Michigan, and Huron, that persisted for approximately 17 years (Figure 4). This decline and subsequent period of low water levels has been attributed to warmer temperatures, low ice cover, increased evaporation, and decreased runoff.23 Recent years have been characterized by prolonged high water conditions, record highs, flooding, and shoreline erosion across the Great Lakes. This began in 2014, a year that coincided with a cold air outbreak, low temperatures, extensive ice cover, and high precipitation rates. 24 These increases continued over the next several years on all five lakes and water levels remain above average today (Figure 4). New monthly high water level records were first set on Lake Ontario in spring of 2017, coinciding with flooding conditions through the late spring and early summer. Lake Erie set a new monthly water level record in 2018, which was then broken by even higher water levels in 2019, the year that Lake Superior also broke its monthly high water level record. Most of the high water level records broken in recent years were previously set in the period of high water levels during the 1980s.25 During that time, all of the upper Great Lakes (Superior, Huron, Michigan, Erie) remained above average for much of the decade, with extreme high water levels occurring between 1985-1987. On most lakes, the rise that led to these high levels in the 1980s was significantly more gradual in comparison to the sharp rise that led to today’s high water levels, as seen in Figure 4.

Figure 4: Monthly Water Levels on the Great Lakes.  Figure source: modified from USACE.

Diversions, regulation, and management

Human decision making does play a role in the hydrological budget of the lakes, primarily through plans that regulate outflow from Lakes Ontario and Superior, and to a far lesser extent, diversions. Long Lac and Ogoki, the only 2 major diversions into the Great Lakes basin, were completed in the early 1940s to divert water from the Albany River systems in Northern Ontario into Lake Superior for hydropower and logging purposes.26  Both diversions increased the water supply into the Great Lakes basin (see Lake Superior Climatology). The Chicago diversion from Lake Michigan into the Mississippi River system is the only major diversion out of the Great Lakes Basin. The original Illinois-Michigan canal was opened for shipping in 1848 to flow from the Chicago River into Lake Michigan. The flow was reversed in 1900 to flow from Lake Michigan into the Chicago River and on to the Mississippi River basin. The process is estimated to have permanently lowered basin-wide water levels by about 2 inches, but this has been compensated by the Long Lac and Ogoki diversions into Lake Superior during the 1940s27 (see Lake Michigan Climatology). The Welland Canal was originally constructed in 1829 and today is a series of locks that allows for ships to travel between Lake Ontario and Lake Erie, which are naturally separated by a 230ft drop at Niagara Falls.28 This created an additional flow from Lake Erie to Lake Ontario, but is not a diversion of water into or out of the Great Lakes basin. These anthropogenic diversions (into and out of the lakes) along with water taken out of the lakes for drinking water, agriculture, industry, etc. are small in comparison to precipitation, evaporation, and runoff (Figure 1).

Lake Ontario’s outflow into the St. Lawrence River is regulated by anthropogenic control structures, most notably, the Moses-Saunders Dam built in 1952.29 Lake Ontario and Lake Superior each have their own board from the International Joint Commission Boards of Control to regulate outflow from the Moses-Saunders Dam and St. Marys River control structures, respectively. The allocation of flow to these facilities is determined monthly, based on the outflow specified by the regulation plan and the conditions given in the Order of Approval.30 The Niagara River Control Center focuses on the Chippawa-Grass Island Pool control structure above Niagara Falls, and supervises the annual installation and removal of an ice boom at the outlet of Lake Erie.31 These boards serve to establish regulations for flow rates at the control structures to keep the lake water levels in an approved range that helps protect the ecosystem, while also maintaining adequate flow for hydroelectric power generation, minimum depth for municipal water intakes and safe navigation, and protect against flooding along the Lake Ontario shoreline and downstream in the Province of Quebec (see Lake Ontario Climatology). The most recent Lake Ontario regulation plan, Plan 2014,32 allows for more natural variations in water levels on Lake Ontario to promote ecosystem health while moderating extreme high and low levels for navigation, recreational boating, and hydropower. However these structures are not able to fully control lake levels as major components such as precipitation, evaporation, and runoff can’t be controlled. This lack of full control of water levels was demonstrated by the record high water levels in the mid-2010’s which resulted in flooding around Lake Ontario in 2017 and 2019.33 34 Currently, such regulations cannot significantly influence short-term lake levels or long-term trends, only aid in alleviating extremes.35 For more information, please refer to the Lake Ontario Climatology).

Future Water Levels

There are a number of approaches to looking forward at future water levels, including physical climate and hydrological modeling and scenario planning. Global and regional climate models, when coupled with hydrologic lake routing models can produce projections for water levels through the end of the 21st century.36 37 38 However, the uncertainties associated with such models are very high. Many global climate models do not have a fine enough spatial resolution to adequately capture the Great Lakes hydrologic cycle and integral weather-scale processes. Climate model-driven projections are therefore not particularly useful for making explicit predictions, but are “perhaps most useful for offering guidance to frame analyses of future Great Lakes water level variability scenarios”.39 If you are interested in learning more about the state of Great Lakes water level modeling, please refer to the curated list of publications under the State of the Research page. Since the outcomes provided by models are highly uncertain, GLISA uses models as guidance in a scenario planning approach to manage uncertainty. The goal of scenario planning is to account for uncertainty by developing a framework to plan for potentially disastrous disruptions, rather than only focusing on specific, likely outcomes. When focusing only on likely events, actors discount other high-risk scenarios. Planning for multiple plausible futures, including extremes, can increase the robustness of management practices and preparedness for climate change impacts. GLISA is in the process of developing a set of broadly applicable lake level scenarios that integrate information about historical lake levels, hydrological budget, and drivers of change. The scenarios will capture a range of possible futures, be based on plausible changes to existing drivers, and explore possible changes to the timing and variability of lake levels as well as the longer term average. These will be published on our website in late-2021 with accompanying guidance for public use.

Useful Resources

  • Great Lakes Dashboard (NOAA GLERL)
    • Interactive tool for viewing water level observations on each of the Great Lakes
  • Lake Level Viewer (NOAA)
    • Interactive mapping tool to visualize changing lake levels for each of the five Great Lakes, including shoreline inundation
  • Water Level data (US Army Corps of Engineers)
    • Historical water level observations and records for all five Great Lakes
  • Fourth National Climate Assessment Midwest Chapter (USGCRP)
    • Report authored by many climate organizations (including GLISA) under the coordination of the U.S. Global Change Research Program (USGCRP) and mandated by congress to be released every four years.
  • Great Lakes Coastal Resilience Planning Guide (NOAA)
    • Resource to connect people with the tools and data needed to consider natural hazards and climate change in local planning efforts.  Includes case studies of how coastal communities are using science based information to address coastal hazards such as flooding, shore erosion, and lake-level fluctuations.
  • NOAA’s Key Great Lakes Water Level Products & Services
    • 2-page fact sheet with over 30 links to various NOAA products and service providers

On the State of the Research page, you can find a list of publications relevant to the topic of Great Lakes water level research, including modeling, historical analysis, and observational data, maintained by GLISA and updated regularly.


If you have questions, comments, or feedback on the Sustained Assessment of the Great Lakes, please contact Kim Channell (


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