http://geosurvey.ohiodnr.gov/lake-erie-geology/water-levels
Lake Erie Water Levels
by D. Mark Jones
Lake Erie’s water level constantly undergoes natural fluctuation. Daily changes, seasonal cycles, and long-term changes in the level of the lake may occur. While some believe human activities and technology have a large impact on Lake Erie levels, the opposite is true. Indeed, Erie’s level at any moment is the result of complex interactions between climate, wind, precipitation, bathymetry (the shape of the lake bottom) and the levels of the upper lakes (Superior, Michigan, and Huron). A strong comprehension of these elaborate interactions can take years to develop, however, the key concepts are very simple to understand.
The Great Lakes System
The Great Lakes are comprised of five of the world’s largest lakes, as well as Lake St. Clair, connected by rivers and falling from about 600 feet of elevation at Lake Superior down to sea level where the St. Lawrence River meets the Atlantic Ocean. The Great Lakes system stretches about 2,000 miles from the west end of Lake Superior to the Gulf of St. Lawrence. Twenty percent of the world’s fresh water and 95 percent of the fresh water in the United States is contained in the Great Lakes and their interconnecting rivers. The Great Lakes basin (the land surface that drains into the Great Lakes) is about twice the surface area of the Great Lakes themselves.
Why do lake levels change?
A lake of any size is a dynamic system, subject to constant change. Sometimes slightly more water comes in than goes out, or vice versa, and the lake level changes in response. The Great Lakes are no different, only much more complex. Some of the factors affecting lake levels are discussed below.
Precipitation
Changes in precipitation (rain and snow) are a main cause of lake level fluctuations. Precipitation anywhere in the Great Lakes basin, whether over the water or somewhere inland, will end up in the Great Lakes. Sufficient precipitation will raise their levels. To complicate things, there is usually a time lag so that increased rainfall or snowfall may not manifest itself as a higher lake level until as much as a year later. The following hydrograph (a chart of water levels) shows the close relationship between precipitation and lake levels.
Evaporation
With almost 100,000 square miles of surface area, the Great Lakes lose a significant amount of water to evaporation. If Lake Erie's inputs and outlets were blocked off, evaporation alone would cause its level to drop 36 inches in one year. Evaporation is greatest not when the weather is warmest, but when the temperature difference between the water and the air is greatest. Such conditions occur in the fall, when the air has cooled but the water still retains some of the heat gained during the summer. Evaporation can continue throughout the winter as well. Lake Erie, the shallowest and southernmost lake, is also the warmest and does not always freeze over. If ice cover is not significant, the open water continues to lose vapor to the dry winter air, dropping water levels.
Precipitation and evaporation together tend to create seasonal cycles in lake levels. Water levels tend to be higher in the spring and summer—a response to winter snowmelt and spring runoff—and lowest in the winter due to summer and fall evaporation. The hydrographs below demonstrate this phenomenon.
Changes in precipitation and evaporation can have long-lasting effects. In the late 1990s, a pattern of mild winters, with less precipitation and reduced ice cover, resulted in a dramatic lowering of Great Lakes levels that lasted into the early 2000s.
Precipitation and evaporation are the most obvious and most significant factors in changing lake levels. Other factors include wind, crustal rebound, and, to a much lesser degree, the human activities of dredging, flow diversion, flood control, and power generation.
Wind
Strong winds blowing for an extended period (several hours or more) not only create waves but can also produce wind set-up, the tilting of a lake's surface. A few times a year along Lake Erie, storm winds coincide with the lake’s southwest-to-northeast orientation. Southwesterly winds blowing along the lake’s length can pile water up at one end of the lake (Buffalo), leaving the other end (Toledo) with significantly lower water. The reverse can also happen, when nor'easter storms striking the east coast of the United States send winds down Lake Erie from the northeast. When the storm winds subside, water at one end of the lake rolls back towards the other end, like a wave created when a tub of water is tilted—a phenomenon known as a "seiche." In about twelve hours this rolling wave has traveled to the opposite end of the lake, raising its level from abnormally low to abnormally high. The cycle of sloshing back and forth continues until the lake’s surface returns to equilibrium.
Crustal Rebound
Many people know that glaciers contributed to the creation of the Great Lakes. The ice in Ohio reached as far south as the present-day Ohio River. When the massive deposits of ice retreated, Earth's crust beneath began to rebound, similar to a couch cushion rebounding when you stand up after sitting on it. That rebound continues to this day, on the order of only inches per century. But the rebound is unequal. For example, geologists have recognized the remnants of ancient beaches in northern Ohio. If one were to follow the same ancient beach trace from northeastern Ohio to Buffalo, New York, the elevation of that ancient beach is well over 100 feet higher in New York because of greater crustal rebound there. Such tilting of one end of a lake basin higher than another causes water to pile up at the lower end of the basin. Over a century, this factor alone can cause water levels to rise the better part of a foot.
What about human-caused effects? Dredging, diversion, and flood control are human activities that can have a very minor effect on the levels of the Great Lakes. Some lakes are affected more than others.
Dredging
Dredging is the removal of sediment accumulations to deepen a channel and make it suitable for navigation. Scientists estimate that dredging of the St. Clair River (between Lake Huron and Lake St. Clair), by lowering the outlet from Lake Huron, has lowered the elevation of Lakes Michigan and Huron by 10 inches over the last century. Dredging is unlikely to affect Lake Erie the same way because the shipping channel that leads out of Lake Erie, the Welland Canal, was built to a designed depth. Unlike a channel of natural origin such as the St. Clair River, it is unnecessary to deepen the canal.
Diversions
Diversions are structures that add water to a basin from elsewhere or that bypass a natural outlet from a basin. There are several diversions in the Great Lakes where water is artificially removed from or added to the basin. However, only two diversions exist in the Lake Erie basin—the Welland Canal and the New York State Barge Canal (a successor to the historical Erie Canal). By bypassing the Niagara River, the natural outlet of Lake Erie, these diversions remove water from Lake Erie slightly more quickly than if they did not exist. Hence, the lake is slightly lower than it would be without these outlets. The Welland Canal has lowered Erie's level by about five inches. The mainly-recreational New York State Barge Canal’s effect is less because it takes only about 1,000 cubic feet of water per second from the Niagara River, approximately equal to the flow of a small river such as the Cuyahoga. These diversion structures are outlets and do nothing to block the lake’s outflow or raise its level.
Flood Control
Only two of the Great Lakes, Superior and Ontario, could be described as having structures on them that specifically control the outflow of water. The St. Mary's River and the St. Lawrence River, outlets of Lakes Superior and Ontario respectively, have structures to regulate lake outflow for flood control and navigation purposes; but they are not designed for regulation of lake levels, which, as we have shown, are dependent upon many other, mostly natural factors. The impact of regulation on Lake Superior is to raise that lake’s level about four tenths of a foot, with no effect on the levels of lower lakes. The controls on Lake Ontario and the St. Lawrence River obviously can have no effect on Lake Erie because they are at least 325-feet lower than Lake Erie.
Power Generation
Currently, three power generation facilities at Niagara take water from the Niagara River above Niagara Falls and discharge it below the Falls. A popular belief is that these activities have raised Lake Erie's level through damming. However, the power plants at Niagara are unlike hydroelectric plants in the western United States that use tall dams to impound large amounts of water. The Niagara power facilities use the natural elevation drop of the Niagara River to generate power. Therefore, there are no tall dams on the river. The overall effect of the power plants actually is to somewhat lower the level of the Niagara River. To ensure that the Falls remain visually impressive with less water going over them, weirs (low dams) deepen the water slightly in the vicinity of the Falls and an "International Control Structure" helps to spread the flow of water across the full width of the Horseshoe (Canadian) Falls.
It is possible to measure how much each of these human-made factors removes from or contributes to the overall system and arrive at a net effect on the level of Lake Erie. Taking into account the controls on Lake Superior and the various diversions, the overall effect of artificial controls on Lake Erie's level is -0.3 feet. In other words, Lake Erie is about four inches lower than it would be without controls.
Over the Long Term
We have shown how lake levels can change over hours because of winds and how they vary seasonally because of precipitation. What about even longer terms? Geological evidence shows that over the millennia since Lake Erie was formed, its level has varied widely. About 5,000 years ago, the lake stood about 46 feet lower, which would have put the Ohio shoreline 2 to 3 miles farther out than it is today. The other Great Lakes experienced similar "lowstands," evidence for which includes submerged tree trunks—an indicator that forests once stood where water is today. The reasons for these lowstands are not clearly known, but climate was probably responsible.
References
Eckel, P.M., 2002, Botanical evaluation of the Goat Island complex, Niagara Falls, New York: St. Louis, Res Botanica, Missouri Botanical Garden Web site, last accessed June 23, 2009.
Great Lakes Commission, 1986, Water Level Changes-Factors Influencing the Great Lakes: Boyne City, Mich., Harbor House Publishers, 13 p.
National Oceanic and Atmospheric Administration (NOAA), 2002, Water Levels of the Great Lakes: Ann Arbor, Mich., NOAA Great Lakes Environmental Research Laboratory, 1 sheet.
Quinn, F.H., 1999, Anthropogenic Changes to Great Lakes Water Levels, in Great Lakes Update: U.S. Army Corps of Engineers, Detroit District, v. 136, p. 1–4.
Strand, Gail, 2008, Inventing Niagara—Beauty, Power, and Lies: New York, Simon and Schuster, 352 p.
U.S. Army Corps of Engineers, 1987, Fact Sheet-Water Levels of Lake Erie: U.S. Army Corps of Engineers, Buffalo District, 6 p.
Last update September 23, 2009
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