TheWA GrTEReat Lak LEVELSes S ne spring, Lake Erie's Huron by way of the St. Marys River. L water levels rose to the Lake Michigan also drains into Lake point that when a storm Huron through the Straits of Mackinac. E hit, the waves pounding The straits are so wide and deep that the the shoreline caused $5 water levels in lakes Michigan and Huron million in damage to cottages, homes and are the same. From Lake Huron, water V dikes. By the following spring, the water flows into Lake Erie via the St. Clair River, levels in Lake Erie had dropped 60 Lake St. Clair and the Detroit River. The E centimetres. Marina owners were worried water then flows into Lake Ontario the water would not be deep enough for through the Niagara River and the L some boats to dock. Welland Canal. Lake Ontario, in turn, empties into the St. Lawrence River. From Water levels in the Great Lakes rise and there, the water flows into the Gulf of St. fall all the time. They have been doing so Lawrence and the Atlantic Ocean. In total, R since the glaciers retreated about 10 000 water travels some 3 600 kilometres from years ago. Most changes in water levels the western end of Lake Superior to the are natural, but some are the result of E Gulf of St. Lawrence. n human activities. The purpose of this fact o sheet is to provide some background There is more to the Great Lakes than i T about the Great Lakes and St. Lawrence just the lakes. They are part of a com- g River and to explain why plex system of lakes, rivers and streams e A their water levels rise which drains large tracts of the r and fall. province of Ontario as well as o The Great Lakes and St. eight American i W Lawrence River form a chain of r states. In total, the Great lakes with each one draining Lakes basin, on both sides a into the next. Lake Superior, t Water travels some 3 600 kilometres of the border, measures the largest of the five Great from the western end of Lake Superior to the Gulf of St. Lawrence 774 000 square kilometres. n Lakes, drains into Lake O Environment Environnement Canada Canada Natural Factors Short-term changes Seasonal changes from the water, followed a number of years of below-normal snow and Sometimes, water levels rise or fall In general, water levels in the Great rain fall. In contrast, heavy rains because of the wind. These are Lakes are lowest in the late fall and and snow in the early 1970s and considered to be short-term early winter. This is because water mid-1980s raised the water levels changes since they rarely last on the surface of the lake at this in the Great Lakes to heights longer than a day. For example, time of year is warmer than the air which were above average. when strong winds blow over a lake above. As a result, water in one direction for several hours, evaporates rapidly. With more water A quick look at the graph of water they may push the water levels up leaving the lakes — in this case in levels on pages four and five shows at one end making the levels drop the form of water vapour — than that water levels in the Great Lakes at the other end. This is called a entering, water levels decline. rise and fall regularly. wind set up or surge. This phenom- In spring, when the snow melts, The earth moves ena has caused the water levels in water runs into the lakes. The lakes Lake Erie, the shallowest of the The earth rises in however, at this time of year, are Great Lakes, to rise by as much as the Great Lakes cooler than the air above. As a two metres at its eastern end within basin by up to 50 result, less water evaporates now the span of just a few hours. centimetres every 100 years — this than in the fall and early winter. is a hangover from the ice ages. When the wind stops blowing or With more water entering the lakes When this area was covered by changes direction, the water than leaving them, their water levels glaciers, their tremendous weight oscillates or flows back and forth usually rise, ultimately reaching compressed the earth underneath. like a pendulum until it stabilizes. their peak in the summer. When the glaciers melted, the land, This is called a seiche. The seasonal range between low now relieved of the weight, began An ice jam in a river which connects levels in the winter and high levels to rise. one of the lakes to another may in the summer runs between 30 It continues to do so today. The reduce the flow of water to a trickle. centimetres and 50 centimetres. land in the north and east rises This is what happened the day more rapidly than the land in the Niagara Falls ran dry. On March 29, Long-term changes south and west. Thunder Bay, for 1848, the residents of Niagara Falls example, rises more than 30 woke up to what was for them a Precipitation centimetres a century, while strange silence. Strong winds, Long-term changes Chicago does not move. The result currents and waves had jammed in water levels are is similar to tipping a bowl of water. hundreds of thousands of tonnes of usually the result of The water level on one side rises ice into the eastern end of Lake Erie heavier or lighter than normal and it falls on the other side. On between Fort Erie and Buffalo levels of precipitation. For Lake Superior, the water level at blocking the flow of water into the example, the low water levels in Duluth is rising 14 centimetres per Niagara River and reducing Niagara the Great Lakes during the 1960s, century, and is falling 22 centi- Falls to drips— for nearly 30 hours. which left many docks a good walk metres per century at Michipicoten. 2 Climate change to be getting warmer. Although Michigan and Huron may drop by opinions vary on the effect a 100 centimetres, while those in The earth's natural change in climate may have on the Lake St. Clair may fall by 90 climate system is Great Lakes, computer models centimetres, in Lake Erie by 80 never stable. It has suggest that supplies of water to centimetres and in lakes Ontario changed in the past the lakes may drop dramatically. and Superior by 40 centimetres and it appears to be The mean levels of water in lakes over the next 20 to 40 years. changing again. This time it seems Water Cycle TRANSPORT TRANSPORT (by wind) (by wind) TRANSPIRATION (evaporates CONDENSATION from plants) PRECIPITATION EVAPORATION (evaporates from lakes ) PRECIPITATION EVAPORATION (evaporates from land) PERCOLATION (into the ground and becomes groundwater) RUN-OFF (returns to lakes) (some groundwater seeps into lakes and rivers) The water levels of the Great Lakes are Not surprisingly, the affected by the amount of water rain or snow which falls directly flowing in and out of the lakes. The The processes which into the lakes increases the volume hydrologic or water cycle plays a make up the water cycle — of water in them, but so does some role in this process. evaporation, condensation, of the rain or snow which falls on precipitation and transpiration — The water cycle is the natural the ground. For example, when the are continuous but not consistent. recycling of water. It evaporates snow melts, some runs directly into They vary from season to season from oceans and lakes as well as the lakes and some percolates down and from year to year. from the land. Water vapour rises into the earth to become until it hits cooler air in the upper groundwater which may flow atmosphere, where it condenses to eventually into the Great Lakes. form clouds and returns to earth as Plants and trees take up some of the rain, snow, drizzle and other forms water in the ground and release it to of precipitation. the atmosphere through the process called transpiration. 3 above sealevel. in LakeSuperiorwasabout183.3metres level. Forinstance,in1995thelevelofwater surface andisexpressedinmetresabovesea O 4 the graphisheightoflake's the measurementonleftsideof ne importantpointaboutthegraph: Metres Metres Metres 173.8 174.8 175.8 176.8 182.8 183.8 176.6 177.6 182.6 183.6 173.6 174.6 175.6 176.0 177.0 183.0 173.0 174.0 175.0 184.0 173.4 174.4 175.4 176.4 177.4 183.4 176.2 177.2 183.2 173.2 174.2 175.2 73.8 74.8 75.8 73.6 74.6 75.6 74.0 75.0 74.4 75.4 74.2 75.2 Graph source:U.S. ArmyCorpsofEngineers 1865 1865 1870 1870 1875 La La La La 1875 La La La La La La ke ke ke ke ke ke ke ke ke ke 1880 O Er Er O 1880 s M s M St St Su Su nta nta ie ie . C . C pe pe ich 1885 ich 1885 rio rio lair lair rio rio iga iga r 1890 r n-H 1890 n-H uro 1895 uro 1895 n n 1900 1900 Monthly MeanWaterLevels 1905 1905 1910 1910 1915 1915 1920 1920 1925 1925 1930 1930 1935 1935 1940 1940 1945 1945 1950 1950 1955 1955 1960 1960 1965 1965 1970 1970 1975 1975 1980 1980 1985 1985 1990 1990 1995 1995 2000 2000 5 Human Factors Dredging dredged many times in the second from Lake Erie.