The water levels of the Great Lakes are determined by the amount of water flowing in and out of the lakes. Very simply, water inflow (precipitation, runoff, and water from streams and groundwater) minus water outflow (evaporation and water flowing out of the system) equals a change in lake water levels. While climate and weather patterns are the primary driver of Great Lakes water levels, there are also man-made forces that influence water levels. Several human activities have affected levels and flows of the Great Lakes over the years including intentionally regulating water levels, dredging of navigation channels, diversions (transfer of water from one watershed to another), and water withdrawals. However, human effects on lake levels have been relatively small, compared to the changes caused by the natural factors.
Low lake levels affect many interests including shipping, power generation, tourism, fishing, the ecology of the Great Lake ecosystem, shoreline property owners, and recreational boating. Periodic low water conditions can be beneficial for lake ecosystems. It consolidates sediments, allows new plants to colonize the lake bed and it provides habitat for rare plants and shorebirds. When water levels return, this expansion of plants becomes habitat for fish and wildlife, removes nutrients from water, and can increase water clarity.
At the same time, low water levels can adversely affect other interests such as commercial navigation, recreational boating, and marinas. During low level periods, lake carriers transporting iron ore, coal, grain, and other commodities are forced to carry fewer goods. Also, as water levels recede, marinas have fewer slips to sell to boaters and often dredge boat slips, channels, and harbor to accommodate boater needs costing millions.
The primary contributor to algae accumulation along the Great Lakes has been the introduction of zebra and quaqqa mussels. The mussels have increased water clarity in the lakes, which allows sunlight to penetrate deeper and support more algae growth.
There is some evidence that botulism outbreaks correspond to low water level events. Historically, larger bird die-offs as a result of Type E botulism have occurred during periods of low or rapidly declining water levels, and water level fluctuations. The mechanism behind this possible link still needs to be researched but is likely to be related to warmer water and sediment temperatures during low water events.
Water levels in the Great Lakes continuously rise and fall and have fluctuated throughout the history of the Great Lakes. In general, the Great Lakes system experienced extremely low levels in the late 1920s, mid-1930s, the mid-1960s, and again over the last decade. Water levels climbed during the late 1960s and 1970s and were above average for some time reaching record highs in the late 1980s. After three decades of water levels above the long-term average, levels declined and experienced an unprecedented period when water levels for Lake Michigan-Huron and Superior fell below their long-term average for 15 years. Lake Michigan-Huron even set an all-time record low in January of 2013 of 576.02 feet IGLD 1985, surpassing the previous record low of 576.05 feet from March of 1964.
All water levels are determined based on a network of water level gauges. These gauges are maintained by the National Ocean Service in the United States and the Canadian Hydrograph Service in Canada. They are spread around each lake in order to provide a lake-wide water surface elevation. The U.S. Army Corps of Engineers (USACE) is responsible for collecting and disseminating information on Great Lakes water levels in the United States.
On any given day, the location of the water’s edge can change dramatically. Day-to-day fluctuations are caused by wind and wave action along the shore. Short-term fluctuations are known as wind set-ups, storm surges, or seiches. A wind set-up or storm surge is when strong, persistent winds blow across the surface of a lake in one direction and force an increase in water levels in the direction of the wind making the water levels drop at the other end. When the wind stops blowing or changes direction, water levels oscillate back and forth several times like a pendulum before returning to equilibrium. This is called a seiche. These short-term fluctuations typically last anywhere from a couple of hours to a couple of days, but have resulted in water level differences in excess of 9.9 feet.
Despite their size, the Great Lakes are not large or deep enough to be affected by the tidal forces of moon and sun. While a very small tide has been recorded on the Great Lakes, less than two inches; water levels in the Great Lakes change not because of the tides, but because of meteorological effects imposed on the longer-term changes in the amount of water in the lakes. The minor tidal changes on the Great Lakes are inconsequential compared to the much greater fluctuations of the lakes caused by changes in wind and atmospheric pressure. As a result, the Great Lakes are considered to be a non-tidal water system
The diversion of Great Lakes water to locations both inside and outside of the Great Lakes Basin has been source of concern and questions for many years. The Chicago diversion is the largest and best known out-of-basin diversion of the Great Lakes. In 1967, the U.S Supreme Court capped the diversion at 3,200 cubic feet per second per day or 2.1 billion gallons per day — its current level.
At present, more water is diverted into the Great Lakes Basin through two northern diversions than is diverted out of the Basin at Chicago. The Long Lac and Ogoki diversions redirect water into Lake Superior from the Albany River system in northern Ontario. Combined, the flow of water in the Long Lac and Ogoki diversions averages 5,400 cubic feet per second. This means that the Long Lac and Ogoki diversions divert over 2,000 cubic feet per second greater than the amount of water diverted out of Chicago. This makes the Chicago and other smaller diversions of water from the Great Lakes currently negligible.
Water is taken from the Great Lakes Basin for a variety of reasons – for drinking, bathing, laundry, agriculture, industry, and so much more. The majority of water withdrawn is returned to the Basin through runoff and discharge. However, some of the water withdrawn is made unavailable. The term consumptive use refers to any quantity of water that is withdrawn from the Great Lakes system and not returned. Examples of consumptive use include water that evaporates from irrigated fields, lawns, and golf courses and water incorporated into dairy products, canned foods, drinks, and chemicals. Approximately 5% of the water that is withdrawn from the Great Lakes watershed is consumed and is lost from the Basin. At this time, the 5% of water lost does not appear to be placing significant pressure on the Great Lakes.
While the primary reasoning for low lake levels is natural weather conditions, it is also widely accepted that global warming either will or is already dramatically altering what would otherwise be a natural occurrence. While there are still many unknowns about how climate change will unfold in the Great Lakes region, scientists do anticipate that air and water temperatures, evaporation rates, ice cover, seasonal precipitation, and water levels will change. Global warming is predicted to result in shortened winters, decreased ice cover, and increased evaporation from the lake; the result being lower lake levels.
The major influences on the hydrology of the lakes and their connecting channels are weather and climate. Precipitation, surface water runoff, and groundwater flows provide water to the Great Lakes system and evaporation and outflows decrease water quantities in the Lakes. We cannot control most of those forces. We possess the ability to distribute water supplies provided by nature, but we cannot control those supplies. However, even managing and distributing water supplies is difficult given the extensive surface area of the lakes which means changes in water levels from controls require a significant amount of time to actually take effect. Therefore, we can alter and alleviate lake level extremes, but we do not have significant effect on long-term lake level trends.
Forecasts of Great Lakes water levels are typically based on computer simulation models. Mathematical relationships have been generated between the measurements of water levels and the rate of flow within the connecting rivers of the Great Lakes system in attempt to predict future water levels. The models are able to generate forecasts of water levels for each lake. However, these forecast models fundamentally depend on accurate seasonal variations of weather patterns. Because there is large variability in weather forecasting, water levels can vary widely from what is predicted.
It is technically feasible to install engineering structures within the Great Lakes Basin to raise water levels. In particular, a few structures have been initially evaluated as a means to compensate for the permanent lowering of water that resulted from dredging in the St. Clair River including a set of submerged sills along the bottom of the river that could act as speed bumps or an adjustable, inflatable “flap gate” across the St. Clair River to control flows.
It is likely that any large engineering structure would take decades to obtain the necessary approvals and permits and install the devices. Additionally, raising the water levels would produce winners and losers among regions, sectors of the economy, and local ecosystems. For example, commercial navigation would benefit as ships could carry heavier loads, but reduced flows to the lower lakes and their connecting rivers would mean less hydropower at places such as the Niagara and St. Lawrence rivers. Higher water on Lake Huron would replenish the wetlands of Georgian Bay, but the structures would leave less water for Lake Erie and other downstream waterways for a least a decade, potentially harming their wetlands and fish habitat until the system reaches stability. In addition, structures could degrade spawning grounds of endangered fish such as lake sturgeon, for which the St. Clair River is a crucial nursery.