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History and ecology of the Great Lakes

Erosion control structures on Great Lakes

The Great Lakes constitute the largest body of fresh water in the world. These incredibly large lakes were formed by glaciation and are still surrounded by the silt, sand, gravel and clay that the retreating glaciers left behind. There are some areas of rock outcroppings, but even many of the rocky shoreline areas are more recently formed sandstone and limestone, which can be susceptible to erosion. Lake Michigan and Lake Superior are also fairly unique when you compare the size of the watersheds to the size of the waterbodies. The Lake Superior Watershed is only 1.5 times larger than the lake surface area; Lake Michigan is half as large as its watershed. While this means that a specific impact on land can have less cumulative impact on the water, it also means that the lake levels are influenced by much more than rainfall in the watershed.

Water levels

The Great Lakes basin is subject to harsh, rapid changes in weather and temperature. Each year, Great Lakes waters change from cold and ice-covered to warm enough for swimming in as little as four months. Changes in the lakes’ water levels may occur hourly and daily, season to season and over many years. Lake levels are determined primarily by precipitation, evaporation, river and groundwater flows. The water levels of the Great Lakes fluctuate at different time scales to different forces.

Very short-term water level changes are caused by wind and storms. These short-term (hours to days) effects can be dramatic and can cause the lake levels from one side of the lake to the other to vary by several feet for a short time. Each of the Great Lakes has an annual rise and fall cycle driven by the timing of precipitation, snowmelt and evaporation. In general, the lakes are at their lowest levels in the winter and highest levels in summer or fall. The annual change in water levels is from 11 to 20 inches.

In addition to the storm-caused and annual changes, data shows that the long-term lake levels change on a 10-year to 30-year cycle. On Lake Michigan, the difference between the recorded low levels and the recorded high levels is greater than 6 feet. A six-foot difference in water level can significantly change how the shoreline looks and acts. Lake Michigan experienced a prolonged period of low water levels between the high waters of the 1980s and record lows of 2013. Marinas and harbors were dredged and shoreline property had exposed beaches. Commercial freight traffic was impacted and some property owners no longer had access to open water. Three of the five Great Lakes recovered their water levels at a record pace, with Lake Superior gaining 2.3 feet from January 2013 through November of 2015, and Lakes Michigan and Huron increasing by 3.2 feet in that same period. That’s the fastest ever for Superior and the second-fastest for Huron and Michigan. Property owners that had a beautiful sandy beach now have to worry about erosion.

Erosion on the Great Lakes shorelines

Erosion is the process where wave energy moves material from the shore out to greater water depths. Erosion on the Great Lakes occurs even during low water levels. Not only is the glacial till material susceptible to wave erosion, the lakes experience storm surges. As the wind blows across the surface of a Great Lake, energy is transferred from the wind to the water surface. This energy generates currents and builds waves. The lakes respond to strong winds more quickly with waves and storm surges than with currents. Storm winds cause rapid changes in water levels. As the wind blows across many miles of open water, it drags some water towards the downwind side of the lakes. This causes a temporary rise in water level along the downwind shore and a lowering of water on the upwind shore. Storm surges typically raise the water level one to two feet on the open coast and two to five feet in bays. On top of the rise in water level, these storms include high winds that cause high waves. Fortunately, much of the Lake Michigan and Lake Superior coastline has shallow water. As large storm waves approach shallow water, they will break and reform into smaller waves. Rising lake levels and/or lakebed erosion (reflected wave energy causing lakebed downcutting) 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.

Changes in a beach shoreline are often called erosion, but most are caused by a process called “sand starvation.” Waves usually approach a beach at a slight angle, creating a “push” against the beach in the alongshore direction, which causes sediment to move laterally. As long as the same amount of sand comes into the beach at one end and goes out the other, it doesn’t matter how big or small the waves are. It only matters when the ratio of sand coming in to sand going out changes. This sideways movement of sand is called littoral drift. Along the Lake Michigan shoreline, littoral drift generally occurs in a southerly direction. The balance of sediment budget in the nearshore environment can determine the stability of coastal bluffs.

When waves strike the shoreline straight on, there is sand in the system that is carried onshore with a wave and offshore with the undertow (return of the wave). This sand can be trapped by intrusions along the shoreline but will continue to move with high waves or to settle during calm waters. If the nearshore area is deep enough or the undertow strong enough, the sand can be moved into deeper water where it settles on the lakebed. If the sand settles into deep enough water, wave action no longer touches the bed and the sand is lost to the littoral drift system.

In the Great Lakes, other forms of shoreline processes occur as well. One shoreline change called an ice shove or push is due to the formation and movement of ice during winter. Though ice may grow as a solid sheet across the lake, the stress and friction of the wind blowing across the ice surface force it to move. It will push against the shoreline and frequently ride up, bulldozing shoreline material with it. In extreme cases, it may freeze around an object such as a rock or pile and lift it from its foundation.

Wind will carry sand off the beach and pile it inland, building tall dunes that can then march farther inland. However, in general, most changes seen on Great Lakes shores are triggered by wave action.

Nearshore environment

The nearshore environment, located where the land and water meet, plays an important role in ecosystem functions in the Great Lakes. The nearshore environment is affected by waves, currents and water level fluctuations as well as coastal protection structures such as breakwaters, jetties, groins and harbors. The type of sediment forming the beach and the underwater slope of the lakebed can affect habitats and biological communities. For instance, a steeper lakebed has deeper water, which can mean higher waves and more energetic circulation patterns. This energy is concentrated in the nearshore area and can transport bottom sediments and redistribute macroinvertebrates that live in the water. The composition and porosity of the sediment are associated with different species of fish and benthic macroinvertebrates. Structures that would occupy existing beaches or the shallow nearshore areas along the great lakes have impacts on these unique and limited habitats. In the simplest terms, structures use space that would otherwise be available to the organisms that would usually be there.

Beach nourishment

The most effective shore parallel structure is the simple beach. Its broad shallow sloping surface causes waves to break gradually until all energy is lost. Studies have shown that flat beaches can reduce the offshore wave height by 40%. Structures and natural steeper slopes can create wave run-ups that are twice the height of the offshore waves. The sand of the beach is highly mobile so that the beach width and profile are not consistent and frequently not predictable. Waves are continually moving sand in the system. During calm waters, many beaches reach an equilibrium beach profile by net onshore sand movement. But storm waves create an undertow which moves the sand back out to deeper water.

For beaches to be self-sustaining, there must be a source of sand from the updrift area that is not obstructed by a natural or man-made intrusion. Sand is not destroyed in the system, but it may be moved into an area where it is “trapped” and no longer moved by waves.

Beach nourishment is a man-made enhancement of the sand in the littoral drift system. Rather than let calm water build up the beach, sand is placed on the beach. To be effective over the long term, beach nourishment is an ongoing process.

Vegetation as erosion control

Vegetation can help stabilize beach areas from wind and wave erosion. There are a few species of plants that are recommended for use in the Great Lakes region. First plant these species: Marram (dune) grass, wheat grass, wild rye, and dune willows. Once these plants are established and flourishing, plant sand cherry and choke cherry. After the sand and choke cherry are flourishing, you can add cottonwood and basswood. Avoid walking through these areas and do not let any motorized vehicles operate in these areas. Many sand beaches on the Great Lakes are colonized by deep rooting plants during low water levels. Unless you are planning to do invasive species removal for phragmites, keeping these plants in place is critical to protecting the dunes from wind and waves.

Bluffs and bluff stability

A bluff is stable as long as the soil’s resistance to failure remains greater than the forces that can cause failure. Loads placed on the bluff that can contribute to failures include the weight of the soil itself, buildings and other bluff-top structures, and the water in the bluff. When the loads become greater than the strength of the soil in the bluff, sections of the bluff break loose and slide towards the lake. In Wisconsin, another factor that contributes to bluff instability is frost action. On the natural bluff face, freezing and thawing can break up the material. Concrete pieces are also susceptible to frost cracking. Many of the bluffs also contain clay materials. Soliflucton, which occurs in spring when frozen clays begin to melt and the thawing soil flows down the face of the bluff, can move natural bluff material and fill material.

Bluff stability also depends, to some measure, on the slope of the bluff face. The degree to which any particular angle is stable depends on the soil types and the amount of water in the soils that make up the bluff. Different soil types, climate changes, erosive forces, and man-made changes on the bluff and in the watershed can all cause short and long term changes in how water content affects the bluff. There can even be differences in the makeup of the clay that can affect the stability. In the red clay shores of Lake Superior, a stable slope may require a 1:3 slope or flatter. The improper placement of fill can accelerate/cause bluff failure. The action of dumping fill itself can increase the chance of bluff failure. Driving a dump truck, crane, or dozer to the bluff edge, or depositing the fill material near the bluff top, can add to bluff load and result in bluff failure. Groundwater in the bluff increases the chance that slumping will occur, as the water both adds to the soil load and lubricates the soil, weakening its resistance to failure. “Some material used for fill is simply inappropriate for bluff stabilization and only adds trash to the lake.” (UW Sea Grant Pennants). Fill made up of small rubble, asphalt pavement, reinforcing wire and rod, lumber, etc. will soon litter the lake water and lake bed.

Any material added to the bluff face must include a plan for managing groundwater in the bluff itself. Groundwater seeps are not always visible in the bluff face, especially in the summer and fall season. Impermeable bluff fill can cause groundwater to pool in the bluff, eventually causing a blowout. Several methods have been tested for groundwater management in the bluffs, but the constantly changing surface and drastically seasonal rainfall events can cause failures. Any material designed to concentrate water in the bluff itself can be susceptible to shifts and failures, which create a build-up of water pressure that will eventually breach the surface of the bluff face. Yard waste and household garbage kill underlying vegetation and add weight to the upper portion of a slope, which can cause a potential slide, thereby damaging established vegetation down below.

Bluffs and toe erosion

The glacial till soils of the tall bluffs are very susceptible to erosion from waves. This erosion at the base of the bluff is called bluff toe erosion and can cause the bluffs to oversteepen and slump. Continued slumping exposes even more erosive materials to the waves and often causes major bluff failures. Many property owners on the Great Lakes look to protect the toe of the bluff from erosion by constructing revetments, using large rocks, granite blocks or other forms of material (often several tons each) laid over filter fabric and stone. The most effective revetment will be designed to have the first layer of rocks trenched into the lakebed, stone high enough to prevent overtopping during storm events, and at least two layers of armor (outer layer) stone. Caution! Revetments can cause damage to neighboring properties and construction access can be problematic. Professional assistance is encouraged. DNR and US Army Corps of Engineers permits are needed for placement of a revetment.