System Controls Sediment Sources Glacial Influences Waves and Wind Coastal Platform Lake Levels Climate Topography and Bathymetry Seiches Landform Frequency Development


Welded barrier at Duluth, MN. (Photo by Dave Mickelson)

To develop and maintain themselves, lacustrine barrier systems need the same three requirements that oceanic barrier systems do (ample and continuous sediment input, the right wave action to form longshore drift, and a coastal platform large enough to support barrier landforms). However, some differences and similarities can be noted.

System Controls
Sediment Sources

As for all oceanic barrier systems, their lacustrine counterparts also rely on a constant and adequate supply of sediment. It must be enough to counteract all the losses due to wind and wave erosion. The main source of input is the longshore drift of sediment from rivers and bluff erosion, although the reworking of nearshore platforms, nourishment from backshore dunes, runoff and increased erosion from mainland construction and development, and aeolian transport should not be discounted.

Glacial Influences

Glaciers have been an extreme shaping force on the Great Lakes and the surrounding mainland, and they have some strong and unique influences on the development of their lacustrine barrier systems.

  1. Their extreme weight and pressure forced the continent to subside, and even today it is still uplifting in response. This process is called isostatic rebound, and it can vary by location.
  2. They left behind piles of debris at their edges as they retreated, called moraines, which would be instrumental in the formation of barrier landforms in some locations.
  3. They eroded and carried vast amounts of sediment deposited in bluffs, which would become a very important source of sediment for Great Lakes barrier systems.
  4. And, as mentioned above, dramatic shifts in lake levels occurred as the glaciers formed and later melted.

 

Waves and Wind

As for oceanic barrier systems, wind generates waves, which are the generating forces behind longshore drift. Waves are also incredible forces for shaping and destructing barrier landforms by eroding and transporting material.

Coastal Platform

Whether the platform is created by wave erosion due to high lake levels or the deposition of river sediment, a stable and shallow platform is a necessity for the development of lacustrine barrier landforms.

Lake Levels

Just as sea levels can cause the transgression or aggradation of oceanic barrier systems, lake levels exert similar influence over lacustrine barrier systems. Additionally, varying lake levels during glacial periods have been instrumental in forming wave-cut terraces, which create the necessary platforms for the development of barrier landforms.

Climate

Temperature and precipitation have similar influences in the Great Lakes as they do in marine environments, with one additional feature - the existence of winter ice cover and spring snowmelt due to the Great Lakes' location in the colder northern latitudes. Lake ice can cover lakeshores for 4 to 5 months, and it protects the shoreline from winter storms and their associated wave action. Spring snowmelt releases a great deal of water into the drainage basins of nearby rivers which can cause lake levels to rise, and it may trigger increased erosion due to runoff and mudslides.

Topography and Bathymetry

As in marine locations, the shape of the land near a coast can influence drainage, erosion, and sediment supply and type. The shape of the coastline can also encourage the formation of some barrier landforms by influencing the direction of longshore drift, refracting wave energy and encouraging development where the coastline suddenly changes direction.
The depth of the basin can influence a barrier landform's ability to form and grow, and will affect the wave environment and the strength of the waves allowed to reach the shore.

Seiches

While the Great Lakes do not experience any tidal influences, they do experience a phenomenon called seiches, which occur in closed water basins. Just like tides, seiches cause a rise and fall of local water levels, however, they do not occur regularly like the tides.

Back to the top


Landform Frequency

A welded barrier in Lake Michigan. (Photo by Steven Dutch)

Almost all barrier landforms that exist in oceanic barrier systems can be found in the Great Lakes, such as tombolos, welded barriers and barrier islands, however there are some noteworthy trends:

  1. Great Lakes barrier landforms tend to be smaller than their oceanic counterparts (Bona, 1990).
  2. Spits are common where glacial deposits can provide adequate sand-sized sediment. Sources can include eroding bluffs along nearby coasts and mainland deposits eroded by precipitation and carried into the lakes by streams and rivers.
  3. There are no large barrier island systems (like those found on the southern Atlantic coast of the United States) in the Great Lakes. The existing barrier islands tend to be islands that have been isolated from spits.


Back to the top


Development

While lacustrine barrier systems require the same three things as all oceanic barrier systems (ample and continuous sediment input, the right wave action to form longshore drift, and a coastal platform large enough to support barrier landforms), in the Great Lakes there appear to be many different reasons that barrier systems have come into existence. Two of these stories are examined in detail in the Case Studies section.

Back to the top

Continue to the next page
A COMPARISON OF OCEANIC BARRIER SYSTEMS AND GREAT LAKES BARRIER SYSTEMS
Copyright 2004, Jennifer Bruce