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Great Lakes Water Levels
This information was excerpted from Living with the Lakes, published by the U.S. Army Corps of Engineers and the Great Lakes Commission.
The Hydrologic Cycle
Water Level Fluctuations
Effects of Lake Level Fluctuations
Living Along the Shoreline

The Hydrologic Cycle

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Water, a renewable resource, is continually recycled and returned to the ecosystem through the hydrologic cycle. Moisture is carried into the Great Lakes basin most commonly by continental air masses, originating in the northern Pacific Ocean, that traverse the North American continent. Tropical systems originating in the Gulf of Mexico or Arctic systems originating in the north polar region also carry moisture into the basin. 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. Precipitation falling on the land infiltrates into the ground through percolation to replenish the groundwater.

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. Ultimately water flows out of each of the Great Lakes through their connecting channels and the St. Lawrence River to the Atlantic Ocean.

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. This vapor can remain as a gas, or it can condense and form water droplets, causing fog and clouds. Some of this moisture returns in the form of rain or snow, completing the hydrologic cycle. The best example of this is lake-effect snow squalls, which commonly occur on the leeward side of most lakes. Generally, much of the evaporated water is removed from the system by prevailing wind patterns.

Water Level Fluctuations

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SHORT-TERM FLUCTUATIONS

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.

Wind set-up, storm surge and seiche
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. This is called wind set-up or storm surge. 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.

Plant growth and ice development in the connecting channels
The natural growth of aquatic plants can affect the flow of water in the tributaries and connecting channels of the lakes. Plant growth decreases the flow of water by narrowing or partially obstructing the channel through which the water flows. Plant growth in part depends on the weather, and can vary from month to month and year to year. In the summer, aquatic plant growth in the Niagara River reduces its flow, on average, by about 2 percent.

An ice jam in an outlet river can drastically slow the flow of water out of one lake and into another. Water levels rise upstream of the jam and fall downstream. The effects are most noticeable on the water levels of the affected river, and of smaller lakes such as St. Clair and Erie.

On the St. Clair River, normal ice build-up can reduce the flow in the river by about 5 percent during the winter. A serious ice jam can reduce flows by as much as 65 percent for short periods of time. Ice jams can develop in a matter of hours, but it may take several days for the jam to be relieved and water levels and flows to return to normal.

SEASONAL FLUCTUATIONS

The lakes are generally at their lowest levels in the winter months. 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. With more water leaving the lakes than entering, the water levels decline to their seasonal lows.

As the snow 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. In the early fall, evaporation and outflows begin to exceed the amount of water entering the lakes.

The range of seasonal water level fluctuations on the Great Lakes averages about 12 to 18 inches from winter lows to summer highs. The timing of the annual peaks and lows varies geographically due to differences in climate across the basin. Seasonal rises begin earlier on the more southern lakes where it is warmer with peaks usually occurring in June or July. Lake Superior, the northernmost lake, is generally the last lake to peak, usually in August or September.

All water levels on the Great Lakes are measured relative to sea level and expressed relative to the International Great Lakes Datum (IGLD), last updated in 1985.

LONG-TERM FLUCTUATIONS

Long-term fluctuations occur over periods of consecutive 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 gage per lake until 1918 and observations 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 hydrograph presented on the graph on the following page. A hydrograph is a plot of water levels versus time.

Global warming and a phenomenon known as the 'greenhouse effect' could cause significant changes in long-term lake levels. Although debatable, most predictions indicate that global warming would cause prolonged declines in average lake levels into the future. These declines could create large-scale economic concern for virtually every user group in the Great Lakes basin. Dramatic declines also could compromise the ecological health of the Great Lakes, particularly in the highly productive nearshore areas.

Besides natural climatic variability and potential man-made climate change, other factors can affect long-term fluctuations, including changes in consumptive use, channel dredging or encroachment and crustal movement.

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.

To see what this means for water levels, an analogy can be made using a cup of water. As the cup is tipped, the surface of the water comes closer to the edge of the cup on one side and is farther from the edge on the other side. This is why water levels are measurably higher today at Duluth, Minnesota, and lower at Michipicoten, Ontario, on the opposite side of Lake Superior, than they were several decades ago. This tipping phenomenon is particularly significant for Lake Superior, and somewhat lesser for lakes Michigan, Erie and Ontario as their outlet channels are rising faster than the western shores of these lakes. As such, there is a gradual decrease in outflow capacities for each of the lakes over time.

Effects of Lake Level Fluctuations

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Erosion Processes

On the coast, natural forces causing erosion are embodied in waves, currents and wind. Most waves arrive at an angle to the shore. As successive wave fronts advance and retreat they set up a longshore current. As waves break, run up the shore, and return, they carry sedimentary material onshore and offshore. This sedimentary material is called littoral drift.

The energy in the moving water determines the size and amount of the material that will move and how far. The energy in a wave depends on the speed of the wind, its duration and the unobstructed water distance, or fetch, it blows over. Gentle waves move fine sand, whereas storm-generated waves move rocks and boulders. Materials picked up from shoreline areas are deposited wherever the water is slowed down and may be picked up again when the velocity of the water increases.

If erosion is not balanced by accretion, the depositing of sediment, the shore will be washed away. Erosion and accretion are two faces of the same process. These processes can occur at extremely slow rates or may occur dramatically in a short time.

Natural shores are nourished by material that has been eroded from other areas, becoming part of the littoral drift system. Attempts to reduce erosion by building shore protection structures, or armoring the shoreline in one area, will result in reduced littoral drift available, starving an adjacent area downdrift.

Fluctuating water levels can expose new surfaces to erosion. As seasons change, wind strength and direction also change, altering the path of waves and currents. Where ice forms, it redirects wave energies offshore protecting beaches, but can increase erosion of the lakebed. Ice may also exert tremendous forces that can weaken shore structures.

Gently sloping shores, whether beaches or wetlands, are natural defenses against erosion. The slopes of the land along the edge of the water form a first line of defense called a berm, which dissipates the energy of breaking waves. During high water periods, a berm can prevent water from moving inland. Dunes and their vegetation offer protection against storm-driven high water and also provide a reservoir of sand for replenishing the littoral drift and rebuilding beaches.

Although erosion is caused by natural shoreline processes, its rate and severity can be intensified by human activity. Dredging marinas and bulldozing dunes remove natural protection against wind and waves. Pedestrian and vehicle traffic destroy vegetation, degrade dunes, and weaken bluffs and banks. Docks, jetties and other structures interrupt the natural shoreline movement of water and redirect erosive forces, possibly in undesirable directions. Inappropriate building practices in high bluff areas can seriously reduce bluff stability. In particular, drainage patterns from new building construction can cause infiltration of runoff directly into a bluff and can weaken its normal cohesive forces. Wise management of shoreline construction and land uses can significantly reduce economic losses due to erosion.

Habitat Diversity

The region's glacial history and the tremendous influence of the lakes themselves create unique conditions that support a wealth of biological diversity, including more than 130 rare species and ecosystems. The Great Lakes are the only lakes of their size in a temperate climate. With the lakes' moderating effect on the climate, the ecosystem is able to provide habitat for a wide variety of species that otherwise might not survive. The Great Lakes - St. Lawrence River ecosystem features sand dunes, coastal marshes, rocky shorelines, lakeplain prairies, savannas, forests, fens, wetlands and other landscapes.

The place where land and water meet is by far the most diverse and productive part of the Great Lakes - St. Lawrence River ecosystem. This interface includes small wetlands nestled in scattered bays to extensive wetlands such as those along Saginaw Bay on Lake Huron, river-mouth wetlands such as the Kakagon Sloughs of northern Wisconsin and the enormous delta marshes of the St. Clair River. Nearly all species of Great Lakes fish rely on nearshore waters for everything from permanent residence, to migratory pathways, to feeding, nursery grounds and spawning areas.

Most common types of wetlands along the shoreline are marshes, where the vegetation can tolerate the large short- and long-term fluctuations in lake levels. In fact, these wetlands are shaped by dynamic lake processes, including waves, currents and changes in water levels. They occur in areas where the erosive forces of ice and wave action are low, allowing the growth of wetland plants. Many wetlands have species successions that are dependent upon water level cycles. Seasonal and long-term water level fluctuations also limit the invasion of woody plants at higher elevations and extensive beds of submersed aquatic plants at lower elevations. Individual wetland species and vegetative communities prefer, and have adapted to, certain water depth ranges, allowing wetlands to be more extensive and more productive than they would be if water levels were stable.

In addition to providing habitat, coastal wetlands play other vital roles. These include protecting nearshore terrestrial ecosystems from erosion by dissipating wave energy, and improving water quality in adjacent aquatic systems through sediment control and absorption of nutrients.

Commercial Shipping and Recreational Boating

Water levels have a profound impact upon the economic viability of commercial shipping and recreational boating on the Great Lakes. In the U.S., for example, the federal government maintains 71 deep-draft harbors and 745 miles of dredged channelways to support commercial navigation. Along the nearly 5,800 miles of U.S. Great Lakes and St. Lawrence River shorelines, the government also maintains 65 shallow-draft recreational harbors. The depths to which the harbors and approach channels are dredged have been subject to U.S. congressional authorizations, many of which date back to the 19th century.

The authorized depth for dredging varies with the type of traffic involved, ranging from a low of 9 feet deep in most recreational boating harbors to 30 foot deep in channels used for ocean-going freighters. Since some harbors serve both commercial and recreational purposes, it is common to see a deeper entrance channel near the harbor mouth for commercial vessels, with progressively shallower depths for recreational interests as one moves upstream.

Boaters should be familiar with and make regular practice of using navigation charts for the waters they expect to navigate. These navigation charts are published in the U.S. by the National Oceanic and Atmospheric Administration (NOAA) and by the Department of Fisheries and Oceans in Canada. All depths or soundings on the navigation charts are referenced to chart datum, also known as Low Water Datum. Chart datum is different for each lake and is expressed relative to IGLD 1985. Current and forecasted water levels are reported relative to chart datum. With an up-to-date chart and current water level information, navigators can find the depth of water available for transit. For example, if the water level is currently 3 feet above chart datum and the soundings on the chart are 8 feet below chart datum, then there is an actual depth of 11 feet at that location.

Boaters should always be aware that the Great Lakes, their connecting channels, and the St. Lawrence River are subject to fluctuating water levels on a short-term basis through storm events, through seasonal changes, and over longer periods due to climatic shifts. Boaters should always use caution and reduce vessel speeds when navigating unfamiliar waters.

Living Along the Shoreline

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Structural Options

A variety of structural options are available to shore property owners to protect and stabilize bluffs and beaches vulnerable to the impacts of lake level fluctuations and storm events. The best structural option depends upon the site characteristics. Professional design consultation is advisable. None of these options, however, are permanent solutions against the continued and relentless forces of nature. Many structures cause erosion downdrift, which can only be mitigated by replacing lost material. In most areas, without mitigation, the relatively thin layer of existing sand is stripped away, exposing underlying clay. The clay is rapidly and irreversibly eroded in a process called lakebed downcutting. This process lets larger waves attack closer to shore, increasing the failure rate of coastal structures and bluffs.

A revetment is a heavy facing, or armor, that protects the slope and adjacent upland from the erosive effects of wave scour. Revetments, which are best suited for gentle to moderate slopes, are comprised of three layers: armor, filter layers and toe protection. Typical armor materials, which include stone and gabions (wire baskets filled with stone) are designed to disperse wave energy that would otherwise impact the shoreline. The filter layer, comprised of graded stone, provides a stable foundation for the armor and permits groundwater drainage. Toe protection, which prevents settlement of the revetment and stabilizes the revetment's lakeward edge, is an extension of the armor material. Private revetments can temporarily protect some types of bluffs, but will likely cause erosion in downdrift areas by starving these shorelines of natural sand supply. Any beach present prior to construction will typically be lost.
Bulkheads (or seawalls) are retaining walls that prevent soil from eroding into a water body due to wave action. Construction can vary from thin structures that penetrate the ground like sheet piling to massive structures that rest on the surface such as poured concrete structures or stone-filled timber cribs. Bulkheads protect only the land immediately behind them by retaining soil at the toe of a bluff; they do not ensure the overall stability of the bluff and do not offer protection to adjacent areas. Bulkheads may worsen erosion downdrift in the same manner as revetments. In the long term, erosion of the lakebed will worsen immediately in front of the bulkhead.

Breakwaters are offshore structures typically placed parallel to the area of shoreline to be protected. Constructed of stone, steel, wood or concrete, breakwaters block and disperse wave energy, which can minimize shore damage. Breakwaters help build a beach in their protected shadow, but can worsen erosion downdrift by blocking transport of sediments along the shore.

Groins are structures that are placed perpendicular to shore and extend out into the water. Used either singly or in a series as part of a groin field, they trap and accumulate sand on the updrift side of the groin. Provided enough sand moves naturally along the shoreline, groins can be effective in building up beaches. Groins are typically constructed of the same materials used for revetments and breakwaters. Groins will aggravate erosion problems downdrift by blocking sediment transport along the shore.

Nonstructural Options

Nonstructural options for bluff stabilization and shoreline protection offer the shore property owner a variety of measures that have a strong land-use management emphasis.

Revegetation is a planting program to establish desired species for bluff and beach stabilization, which is among the least expensive of all protection measures. A variety of groundcover, including species of grasses, sedges and bulrushes, are effective at trapping sand particles and stabilizing beach and bluff areas. Upland species of grasses, shrubs and trees are effective in higher beach elevations. While useful for slope stabilization and erosion control, revegetation alone is not effective under conditions of heavy wave action in high bluff environments. Conversely, in areas of shallow relief, extensive coastal wetlands can effectively eliminate wave forces on adjacent beaches.

Bluff drainage is a measure that addresses seepage problems common to clay or composite bluffs. Seepage contributes to bluff instability when upper layers are saturated, slough off, and are ultimately carried away by wave action. Open joint tile drains, laid in a trench set back from the top of the bluff and back-filled with crushed stone, can help resolve shallow (less than 6 feet deep) groundwater drainage problems. Vertical wells with sump pumps can be used for deeper drainage problems.

Slope re-grading is a measure by which unstable bluffs can be re-graded to a more gradual or stable slope. Coupled with revegetation, this measure can be effective in reducing the rate of erosion and bluff recession, assuming the lakebed has not been irreversibly downcut.

Beach nourishment is the placing of quantities of sand, gravel, or stone on the shoreline by overland hauling or nearshore pumping from barges. The deposits serve as a buffer zone that slows erosion. Wave action carries the material offshore, where it can form sand bars that may cause waves to break farther from the beach. To extend its life span, beach nourishment often requires using larger and heavier deposits than would naturally occur, causing a change in beach characteristics. The useful life of a nourished beach depends upon the size and quantity of materials placed on the beach as well as the frequency and severity of storms that erode the deposits.

Relocation is the removal of structures vulnerable to damage from storm-induced flooding and erosion. This option recognizes that erosion and associated bluff recession is a natural process that, even with installation of structural protection, is difficult to stop entirely. Provided that the shoreline property is of sufficient size and depth to accommodate relocation of the structure(s), this option is often more cost-effective and reliable in the long-term than most structural options.

Community Measures

Additional nonstructural options entail the development and implementation of land-use and shoreline management measures that can prevent new damage from occurring. Many such techniques lend themselves to public policy actions, such as local ordinances, but also can be implemented by the individual property owner on a voluntary basis. These include:
  • erosion setbacks with minimum requirements for both movable and permanent structures;

  • flood setbacks and elevation requirements for new structures;

  • requirements/guidelines for shoreline alteration to ensure that updrift and downdrift impacts are considered and mitigated for;

  • real estate disclosure requirements to ensure that a prospective buyer is fully informed as to whether the property is within a mapped or known flood or erosion hazard area; and

  • adoption of hazard insurance programs that provide for mapping of hazard zones, establishing setbacks for new construction, and denying subsidized insurance for new construction or major renovations within the flood or erosion hazard area.
An additional nonstructural option available to both public and nongovernmental agencies and organizations is the implementation of conservation practices including the purchase of developed and undeveloped property in hazard areas for recreational use, habitat enhancement or other purposes.

Lake level fluctuations, storm events and related natural processes continuously reshape the coastal zone through flooding and erosion. These processes are an integral part of the ecosystem; it is neither economically feasible nor environmentally desirable to severely limit these processes.

Shoreline property owners should be cognizant of long-term lake level history so they will not be surprised by what happens in the future. While various private protective structures can be effective in temporarily protecting shorelines and associated buildings, none will be permanent. Ownership of shore property and structures has many benefits, but does require a thorough understanding and acceptance of the risks involved.

"Let the buyer beware" is sound advice to any prospective shore property owner. Every aspect of the property's history should be investigated thoroughly, particularly past flooding or erosion patterns and structural and nonstructural shoreline protection measures that either need to be maintained or possibly installed on the property. Selecting and implementing one or more management measures will be one of the most significant decisions shoreline communities and their citizens can make. Careful planning, including assistance from public agencies and reputable professionals, is advised.
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