- Prior Climate
- Lake Levels
- Storms, Storm Surge & Waves
- Sediment Transport & Erosion
The Great Lakes-St. Lawrence River Basin as we know it today was formed by glacial forces which retreated about 9,000 to 14,000 years ago. Research on the formulation of the region indicates that climate change has caused massive shifts in its hydrology and hydraulics and that Great Lakes water levels have varied since the last glacier retreated. This is also true of the flows of water between these five massive lakes and their combined outflow to the Atlantic Ocean. Warmer air and water temperatures, reduced ice cover, lower precipitation records and high evaporation off the lakes are all contributors to the low Great Lakes water levels measured in 2013.
The graph of Lake Michigan water levels, reconstructed through research conducted by the Indiana Geological Survey, shows that water levels have varied by over two meters (six to seven feet) since the glaciers retreated from the region.
Lake Michigan Paleo-geologic/Historic Water Level Plot from Indiana Geological Survey
Continued investment in Great Lakes natural processes and hydroclimatic research and monitoring infrastructure will allow scientists to collect the basin-wide precipitation and evaporation data needed to improve models. This continually improving knowledge will allow coastal community decision-makers to make smart investments over time.
During the periods of glaciation, giant sheets of ice flowed across the land, leveling mountains and carving out massive valleys. Where they encountered more resistant bedrock in the north, only the overlying layers were removed. To the south, the softer sandstones and shales were more affected. As the glaciers melted and began receding, their leading edges left behind high ridges, some of which can be seen today in the cliffs of Door County, Wisconsin, and the Bruce Peninsula in Ontario. Huge lakes formed between these ridges from the retreating ice fronts, and continually changed over time as the ice sheet moved northward.
Early drainage from these lakes flowed southward through the present Illinois River Valley toward the Mississippi River, through the Trent River Valley between present Lakes Huron and Erie and through the Lake Nippissing-Ottawa River Valley from Georgian Bay on Lake Huron downstream to the present Montreal, Quebec, area.
Post-glacial Rebound – Crustal Movement
Crustal movement, the rebounding of the earth’s crust from the removed weight of the glaciers, does not affect the amount of water in a lake, but rather affects water levels at different points around the lake. Crustal rebound varies across the Great Lakes Basin. The crust is rising the most — more than 21 inches per century – in the northern portion of the basin, where the glacial ice sheet was the thickest, heaviest and the last to retreat. There is little or no movement in the southern parts of the basin. As a result, the Great Lakes Basin is gradually tipping, a phenomenon most pronounced around Lake Superior.
Lake Level Reconstruction
The Indiana Geological Survey has collected samples of high organic materials on the tops of concentric beach ridges along Lake Michigan. This organic material is nature’s marker of a past high water event, that is, the debris line or “swash” zone along the shoreline. The concentric beach ridges are typically found in embayments, with nature working continuously to “straighten out” the shoreline. By carbon dating these organic materials and knowing their heights, a record of past high water levels has been generated.
Indiana Geological Survey
Basic hydrometeorologic data have been collected over the region since the mid-1800s, particularly for Great Lakes water levels, meteorological observations and tributary stream flow characteristics. Greater precision, spatial coverage and temporal detail have been added to the historic record as more resources and technologies have become available. Most research supports the belief that climate variability over the last 150 or so years is consistent with reconstructed prehistoric climate. Further historic information and reliable data sources are contained in this section of the Planning Guide.
Water levels in the Great Lakes are a complex balance between riverine input and output, precipitation and evaporation, wind, humidity, and extraction rates. To different degrees, both natural factors and human influences affect lake levels.
Screenshot of GLERL Water Level Dashboard
Exploring the Great Lakes Water Level Dashboard is one way to visualize water level change by individual lake over time. A comparison of future lake level projections via a variety of models is also available using the tool. Note that while some models project lake level declines over time, some project increases in water levels. The Great Lakes Lake Level Viewer procuded by NOAA's Office for Coastal Management is another way to visualize fluctuation in lake levels. This interactive web-map illustrates the scale of potential flooding or land exposure associated with a 6 foot increase or decease in lake elevation.
Lake levels are likely to continue to fluctuate within the historic range of variability. However, coastal managers and community planners should plan for both extremes to ensure viable, long-term investments in grey and green infrastructure.
Water, a renewable resource, is continually recycled and returned to the ecosystem through the hydrologic cycle. As weather systems move through, they deposit moisture in the form of rain, snow, hail or sleet. Water enters the system as precipitation directly on the lake surface, runoff from the surrounding land including snowmelt, groundwater, and inflow from upstream lakes.
Water leaves the system through evaporation from the land and water surface or through transpiration, a process where moisture is released from plants into the atmosphere. Water also leaves the system by groundwater outflow, consumptive uses, diversions, and outflows to downstream lakes or rivers. Evaporation from the lake surface is a major factor in the hydrologic cycle of the Great Lakes. Water evaporates from the lake surface when it comes in contact with dry air, forming water vapor. Some of this moisture returns in the form of rain or snow, completing the hydrologic cycle. Generally though, much of the evaporated water is removed from the system by prevailing wind patterns.
Hydrologic Cycle from USGS
Some water level fluctuations are not a function of changes in the amount of water in the lakes. These fluctuations, generally short in duration, are due to winds or changes in barometric pressure. Short-term fluctuations, lasting from a couple hours to several days, can be very dramatic. Fluctuations due to storms or ice jams are two examples.
The range of seasonal water level fluctuations on the Great Lakes averages about 12 to 18 inches from winter lows to summer highs. The lakes are generally at their lowest levels in the winter months due to more water leaving than entering the system. In the fall and early winter, when the air above the lakes is cold and dry and the lakes are relatively warm, evaporation from the lakes is greatest.
As snow cover melts in the spring, runoff to the lakes increases. Evaporation from the lakes is least in the spring and early summer when the air above the lakes is warm and moist and the lakes are cold. At times, condensation on the lake surface replaces evaporation. With more water entering the lakes than leaving, the water levels rise. The levels peak in the summer.
Long-term fluctuations occur over periods of years and have varied dramatically since water levels have been recorded for the Great Lakes. Continuous wet and cold years will cause water levels to rise. Conversely, consecutive warm and dry years will cause water levels to decline. Water levels have been measured on the Great Lakes since the 1840s. Older records may not be as accurate as current observations since measurements were only taken at a single gauge on each lake until 1918, and were not taken as frequently as they are today.
The Great Lakes system experienced extremely low levels in the late 1920s, mid-1930s and again in the mid-1960s. Extremely high water levels were experienced in the 1870s, early 1950s, early 1970s, mid-1980s and mid-1990s. Long-term fluctuations are shown on the Lake Michigan-Huron hydrograph.
Hydrograph – plot of water levels versus time
Lake Level Controls
Lake Superior and St. Marys River
The outflow from Lake Superior is controlled near the twin cities of Sault Ste. Marie, Ontario and Michigan. Lake Superior’s outflows are adjusted monthly, taking into consideration the water levels of Lakes Superior and Michigan-Huron. The objective is to help maintain the lake levels on both Lake Superior and Lake Michigan-Huron in relative balance compared to their long-term seasonal averages. Since the Compensating Works, a 16-gate control structure, was completed in 1921, Lake Superior outflows have been regulated by humans.
Lake Ontario and St. Lawrence Seaway and Power Project
The St. Lawrence River is a majestic and expansive river course which drains Lake Ontario. The criteria for regulating outflows explicitly recognize the needs of three major interest groups: riparian (shore) property owners, hydropower, and commercial navigation.
There are four key objectives of the Lake Ontario regulation plan:
- maintain the Lake Ontario level within a four-foot range during the navigation season
- maintain adequate depths in the International Section of the river for safe navigation
- maintain adequate flows for hydropower generation; and
- protect the lower St. Lawrence River below the control works from flooding
St. Clair River and Detroit River
The St. Clair, Lake St. Clair and Detroit River system is naturally regulated; flows in the St. Clair and Detroit rivers are limited by the size of their channel ways and the levels of Lake Huron upstream and Lake Erie downstream. The St. Clair River is an interconnecting channel between Lakes Huron and St. Clair. The Detroit River receives inflow from Lake St. Clair and discharges into the west end of Lake Erie.
Dredging in the St. Clair-Detroit system began in the 19th century and continued through the early 1960s to deepen navigation channels. Dredging has increased the flow capacity of these rivers and, as a result, has permanently lowered the levels of Lakes Michigan and Huron by nearly 15 inches. The effect on Lake Erie’s water level was temporary.
The Niagara River runs approximately 35 miles between Lakes Erie and Ontario. Hydropower plants take advantage of the abundant energy potential represented by the nearly 330-foot elevation difference between the lakes. A factor that affects lake levels is man-made construction in the connecting channels between the lakes and in the St. Lawrence River system. This construction includes fills, piers, marinas and other structures built into the river course beyond pre-existing shorelines. Development activities such as these can affect the outflow of a channelway. Although an individual construction project may not have a measurable consequence, continual development over time can have a significant cumulative impact.
There are five diversions on the Great Lakes: the Long Lac and Ogoki diversions into Lake Superior, the Lake Michigan diversion at Chicago, and the Welland Canal and New York State Barge Canal between Lake Erie and Lake Ontario. The Welland and New York State Barge Canal do not divert water into or out of the Great Lakes, but rather provide navigation channelways between two of the lakes. Man-made diversions play a minor role in the balancing of Great Lakes water levels when compared to natural forces. The cumulative impacts of all five diversions have raised water levels on Lake Superior by less than one inch, had no measurable effect on Lake Michigan-Huron, lowered Lake Erie by almost four inches, and raised Lake Ontario by less than one inch.
Storm winds cause rapid changes in water levels. Sustained high winds from one direction can push the water level up at one end of the lake and make the level drop by a corresponding amount at the opposite end. The temporary rise in water level is called a storm surge, storm set-up, or storm-induced rise. The drop in water level is a set-down. Storm surges and set-downs occur along all of the Great Lakes shorelines.
Wind Set-up from Living with the Lakes
Changes in barometric pressure can add to this effect. When the wind abruptly subsides or barometric pressure changes rapidly, the water level often will oscillate until it stabilizes again. This phenomenon is known as seiche (pronounced “sayshe”). The pendulum-like movements within seiches can continue for days after the forces that created them vanish. Lake Erie is most susceptible to storm surges and seiches due to its east-west orientation in an area of prevailing westerly winds and its generally shallow western end. One or more seiches following a storm may cause repeated flooding of low-lying land.
Trained design professionals take into account the various types of rapid water level changes that can occur at a particular site when designing shoreline structures. Storm surges typically rise one to two feet (0.3 – 0.6 meters) on the open coast, two to five feet (0.6 – 1.5 meters) in bays, and up to eight feet (2.4 meters) at the eastern end of Lake Erie near Buffalo—with a similar set-down at the western end of the lake.
Wave power is determined primarily by wind speed, wind duration, and the open water distance over which the wind is in contact with the water surface, known as fetch. As the wind blows across the surface of one of the Great Lakes, energy is transferred from the wind to the water surface. Most of this energy generates currents. The rest of the wind energy builds waves. The lakes respond to strong winds more quickly with waves and storm surges than with currents. Storm winds may last from less than an hour to up to three or more days. Storm wind conditions are least common in the summer months.
During a typical fall storm, wind speed can increase rapidly and the lake’s surface may within an hour go from flat and calm to rough with waves two feet (0.6 meters) high. Within eight hours, wave heights may approach 17 feet (5.2 meters) or higher. These deep-water waves move toward shore and form large breakers in the surf zone and in harbor entrances.
As large storm waves approach shallow water, they lose their power—first by partial spilling of the wave crests, followed by wave breaking, and finally in wave runup on the shore. The waves’ power can be released gradually in spilling breaking waves running over gradually shoaling lakebeds, or released suddenly in plunging breakers running over steeply shoaling lakebeds. Water depth limits the height of waves passing through shoal waters to approximately one-half to one times the water depth, depending on the lakebed slope and wave characteristics.
Waves Feeling the Lake Bottom from Living on the Coast
Rising lake levels and/or lakebed erosion create deeper water close to the water’s edge and allow more wave power to attack the shore. Falling lake levels have the opposite effect. Coastal property may be protected from damaging breaking waves by unseen offshore shoals and/or a gently sloping lakebed that causes most of the storm wave power to dissipate before it reaches shore. Where deep water is closer to shore and the unseen underwater portion of the beach has a steep slope, large waves may reach and damage the shore.
Sediment transport is the method by which dynamic coastline features such as beaches, spits, dunes and offshore bars are built and maintained. Beach-building materials are in many places prevented from entering the littoral transport system, resulting in diminished beaches and nearshore bars. Littoral transport is nearshore sediment transport that is driven by waves and currents. This transport occurs both parallel (longshore) and perpendicular (cross-shore) to the shoreline.
Beach-building materials are mostly sand, gravel, and stone that enter the littoral transport system from dunes, bluffs and lakebed erosion along the coastline, with additional material contributed by streams. Material may be blocked from entering the littoral system in many ways. Material from streams may be blocked by dams or removed from river channels and harbors by dredging. Littoral contributions may be blocked by shore protection structures.
In general, Great Lakes shorelines are retreating at various rates – sometimes slowly with little notice and sometimes as rapid, episodic events. Shores that have cohesive materials (clay, till and bedrock) have strong binding forces. Shores that have noncohesive materials (sand and gravel) have weak or no binding forces. Rock is the least erodible of these materials, while sand and gravel are the most erodible. One type of material may occur in a low bank, but several types typically occur in layers or mixtures within higher banks and bluffs.
The erosion of a coast occurs in response to storm waves, rising groundwater and instability in slope soils, surface water runoff, and other factors. Contributing factors include: soil composition, weathering of the slope face by freezing and thawing, vertical cracks in upper slope soil, steep slopes, lake levels, nearshore shoals and lakebed slope, storm wave energy and duration, amount of precipitation, shoreline ice cover, shoreline orientation, beach composition, width, and slope, presence or absence of shore protection, and types of shore protection used.
Erosion of the lakebed is a common occurrence along cohesive shorelines of the Great Lakes. Lakebed erosion or downcutting occurs on nearshore lakebeds where sand or gravel in a thin layer acts as an abrasive, wearing away the lakebed under nearly constant wave motion. Lakebed erosion rates tend to be highest close to shore where the waves break and cause turbulence.
Where lakebed erosion is occurring, any structure built to protect the toe of the bluff is subject to increasing wave energy and the undermining of its foundation as the water depth in front of the structure increases. During periods of low lake levels, the nearshore lakebed is subject to higher water velocities from wave motion, and the zone of wave breaking—where erosion is highest—occurs further offshore. When high water levels return, the water depth close to shore is greater than it was during the previous high water period, increasing wave impacts and erosion on the shore.
Erosion can be spectacular and threatening, with sudden slumping and sliding of massive blocks of soil, or it can be subtle, significant, and undetected. The erosion of bluffs along the coast can be quite unpredictable. A bluff edge may not have moved significantly in 40 years yet may lose five to 50 feet (1.5 to 15 meters) or more in the span of a week. Bluff slumping can be triggered by wave or current erosion in the lower parts of the slope and the lakebed. Mechanisms that trigger landslides on bluff slopes include intense rainfall or rapid snowmelt that quickly seeps into the bluff, causes a rapid rise in groundwater levels, adds to soil loads, and weakens soil strength.
As the climate changes, concurrent changes in the frequency and intensity of storms and major precipitation events, as well as changes in the frequency and severity of freeze-thaw cycles, may bring soil conditions that will alter slope stability in ways that were not experienced by property owners in years past.
Water arrives on the land as either surface water runoff or as groundwater. Some of this water originates in the coastal zone, while other surface water and groundwater travels from throughout the coastal watershed, which may stretch many miles inland. Surface water runoff may come from rainwater, snowmelt, groundwater seeps or springs, and lawn or garden sprinkler systems. It may come from roofs through gutter pipes or from driveways, parking lots and roads.
Surface runoff over the face of a coastal slope gradually loosens and visibly removes exposed soil on the slope, resulting in the loss of up to half of the slope soils in some places. The volume of rainwater, snowmelt or artificially discharged water and the rate at which it arrives on the ground surface has a large influence on erosion.