Lake Agassiz

Lake Agassiz (/ˈæɡəsi/ AG-ə-see) was a large proglacial lake that existed in central North America during the late Pleistocene, fed by meltwater from the retreating Laurentide Ice Sheet at the end of the last glacial period. At its peak, the lake's area was larger than all of the modern Great Lakes combined. It eventually drained into what is now Hudson Bay, leaving behind Lake Winnipeg, Lake Winnipegosis, Lake Manitoba, and Lake of the Woods.
First postulated in 1823 by William H. Keating, it was named by Warren Upham in 1879 after Louis Agassiz, the then recently deceased (1873) founder of glaciology, when Upham recognized that the lake was formed by glacial action.
During the last glacial maximum, northern North America was covered by an ice sheet, which alternately advanced and retreated with variations in the climate. This continental ice sheet formed during the period now known as the Wisconsin glaciation, and covered much of central North America between 30,000 and 10,000 years ago. As the ice sheet disintegrated, its meltwaters created an immense proglacial lake.
Around 13,000 years ago, this lake came to cover much of what are now southeastern Manitoba, northwestern Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan. At its greatest extent, it may have covered as much as 440,000 km (170,000 sq mi), larger than any currently existing lake in the world (including the Caspian Sea) and approximately the area of the Black Sea.
At times the lake drained south through the Traverse Gap into Glacial River Warren (parent to the Minnesota River, a tributary of the Mississippi River), east through Lake Kelvin (modern Lake Nipigon) to what is now Lake Superior, and northwest through the Clearwater Spillway to the Mackenzie River System and the Arctic Ocean about 13,000 years ago.
The ice returned to the south for a time, but as it again retreated north of the present Canada–United States border around 10,000 years ago, Lake Agassiz refilled. The last major shift in drainage occurred around 8,200 years ago. The melting of remaining Hudson Bay ice caused Lake Agassiz to drain nearly completely. This final drainage of Lake Agassiz has been associated with an estimated 0.8 to 2.8 m (2.6 to 9.2 ft) rise in global sea levels.
Lake Agassiz's major drainage reorganization events were of such magnitudes that they significantly impacted climate, sea level, and possibly early human civilization. The lake's enormous freshwater release into the Arctic Ocean has been postulated to have disrupted oceanic circulation and caused temporary cooling. The draining of 13,000 years ago may be the cause of the Younger Dryas stadial. Although disputed, the draining at 9,900–10,000 years ago may be the cause of the 8,200 yr climate event. A study by Turney and Brown links the 8,500-years-ago drainage to the expansion of agriculture from east to west across Europe; they suggest that this drainage may also account for various flood myths of ancient cultures, including the Biblical flood narrative.
The lowest point between the drainage of Hudson Bay and the Gulf of Mexico is in the Traverse Gap between the U.S. states of Minnesota and South Dakota. It lies between Lake Traverse and Big Stone Lake. This continental divide is about 300 metres (980 ft) above sea level. When Lake Agassiz existed, the gap was the outlet to River Warren. The outflow from the melting glaciers filled Lake Agassiz and then drained through the gap to the Gulf of Mexico. This mass of moving water eroded a valley 2–5 kilometres (1.2–3.1 mi) wide and from 100 to 125 feet (30 to 38 m) deep. Today, this valley contains the Minnesota River, joined by the Upper Mississippi River at Fort Snelling, Minnesota.
North of the gap, the Red River of the North flows from Lake Traverse north through the former lakebed of Lake Agassiz to Lake Winnipeg.
During the Lockhart Phase, water accumulated in the Red River valley of North Dakota and Minnesota. As the water reached to the top of the divide to the south, the water drained into the ancestral Minnesota and Mississippi River systems. This occurred while the Laurentian Ice Sheet was at or south of the current Canada–US border. As the ice sheet melted northward, an early Lake Agassiz covered southern Manitoba, the Minnesota and Ontario boundary country, and along the Red River south of Fargo, North Dakota. The Lockhart Phase is associated with the Herman lake stage (335 metres (1,099 ft)), the highest shoreline of Lake Agassiz. The Big Stone Moraine formed the southern boundary of the lake. During the Lockhart Phase the lake is estimated to have been 231 metres (758 ft) deep, with greater depths near the glacier.
As the ice sheet melted northward, Lake Agassiz found a lower outlet through the Kaministikwia route along the modern Minnesota–Ontario border. This moved water to Lake Duluth, a proglacial lake in the Lake Superior basin. From there the water drained south via an ancestral St. Croix and Mississippi River systems. The lake drained below the Herman lake beaches until isostatic rebound and glacial advances closed the Kaministikwia route. This stabilized the lake at the Norcross lake stage (325 metres (1,066 ft)). The average depth of Lake Agassiz during the late Moorhead Phase was 258 metres (846 ft). Drainage from Lake Agassiz continued to flow southward out of the ancient Minnesota and Mississippi River systems into the Gulf of Mexico.
During the Emerson Phase, lake levels and drainage patterns continually fluctuated. The lake switched from a southward outlet to a northwestern outlet, and may have been static without a significant outlet during this phase. Isostatic rebound changed the elevation of the land, and this, combined with changes in the volume of meltwater from the ice margin and the closure of the Kaministikwia outlet in the east increased the size of the northern end of the lake. One hypothesis postulates that the lake was a 'terminal lake' with water inflows and evapotranspiration being equal. Dating of the glacial moraines shows that the Clearwater and Athabasca River system and Lake Nipigon and Minong basin were still ice-covered. A period of precipitation and meltwater input balance with the rate of evapotranspiration may have existed for a short period of time. During this phase, the Clearwater and Athabasca River system outlet opened. Isostatic rebound opened the southern outlet for a time, creating the Norcross (325 metres (1,066 ft)), Tintah (310 metres (1,020 ft)), and Upper Campbell (299 metres (981 ft)) beaches. The south outlet was permanently closed at the end of Emerson Phase.
The opening of the Kaministiquia outlet to the east initiated the onset of the Nipigon Phase. The lower lake level ended the southern outlet through the ancestral Minnesota and Mississippi River systems. The ice sheets advanced and blocked the northwestern outlet through the Clearwater and Athabasca systems. There were several other low level outlets into the Lake Minong basin, including the Kaministiquia and the Lake Nipigon outlet. These allowed large amounts of water to flow from Lake Agassiz into Lake Minong. A series of ice advances and retreats between 10,500 and 9,500 YBP blocked the Lake Nipigon outlet and the other low level outlets, creating intermittent catastrophic outbursts of water into the Lake Minong basin.
These large inflows of water raised Lake Minong lake levels and flowed into Lake Algonquin in the Lake Michigan and Huron basins. These outbursts refilled the Lake Michigan and Huron basins, which are extreme low water levels of Lake Chippewa (Lake Michigan basin) and Lake Stanley (Lake Huron basin). This was due to isostatic rebound of the northern shorelines combined with the opening of the North Bay outlet of the Lake Huron basin. These repetitive outbursts from Lake Agassiz flooded the Lake Minong basin, then flowed over into the Lake Stanley basin, and then flowed through the North Bay drainage route into the Champlain Sea (present day St. Lawrence lowland). The shifting ice sheet created fluctuating drainage channels into the Lake Nipigon and Superior basins. A dozen beaches were created during short periods of stability. Towards the end of the Nipigon Phase, Lake Agassiz reached its largest geographical size as it joined with Lake Ojibway in the east.
The Ojibway Phase is named for the glacial lake along the ice front in northern Ontario. Lake Ojibway merged with Lake Agassiz at this time. Isostatic rebound of glaciated lands that were south of the ice sheet created a long linear lake from the Saskatchewan–Manitoba border to Quebec. This long lake drained through the eastern outlet at Kinojevis River [fr], into the Ottawa River valley. Lake Agassiz-Ojibway drainage raised sea levels. The results can be seen in Nova Scotia, New Brunswick, and eastern Maine. Marine records from the North Atlantic have identified two separate episodes, linked to northern hemisphere cooling in 8,490 YBP and 8,340–8,180 YBP. These may be linked with the Ojibway Phase of Lake Agassiz and may indicate large amounts of drainage from the Ottawa River valley and the Tyrrell Sea (ancestral Hudson Bay).
The Laurentide Ice Sheet continued to recede. Continued warming shrank the ice front towards present day Hudson Bay. Here, the Lake Agassiz northward outlet drained into the Tyrrell Sea. This breach dropped the water level below the eastern Kinojevis outlet. The drainage was followed by the disintegration of the adjacent ice front at about 8,480 YBP. This brought on the end of Lake Agassiz. The ice sheet continued its northward retreat to Baffin Island, leaving the North American mainland around 5,000 YBP.
Numerous lakes have formed in this glacial lake basin. The best known are the Great Lakes of Manitoba; Lake Winnipeg, Lake Manitoba, and Lake Winnipegosis. A cluster of smaller lakes surround these, including: Cedar Lake, through which the Saskatchewan River flows; Lake Dauphin, south of Lake Winnipegosis and tributary to it; and Lake St. Martin, on the Fairford or Little Saskatchewan River, the outlet of lakes Manitoba and Winnipegosis. In northern Minnesota, there are Roseau, Thief, Mud, and Maple lakes, besides three large lakes of that state, Rainy Lake, the Lake of the Woods, and Red Lake.
Glacial Lake Souris formed along the Manitoba and North Dakota border, forming a crescent around the west side of the Turtle Mountains. Lake Souris had three successive outlets: the Sheyenne River, the Pembina River, and finally the Assiniboine River. Initially, Lake Souris' southern bay drained into the Sheyenne River, a tributary of the Red River, which in turn flowed into Lake Agassiz. However, after the ice sheet had retreated enough to uncover Turtle Mountain, the northern bay of Lake Souris found an outlet at the "elbow" of the modern Souris River; the elbow is about 18 miles (29 km) southwest of the present mouth of the Souris River. From this elbow, the lake's waters flowed southeast and entered the Pembina River, now a tributary of the Red River, and the Pembina, in turn, entered Lake Agassiz at its Assiniboine embayment. When the ice sheet retreated north of the Assiniboine River, Lake Souris drained via that river into Lake Agassiz. (Pelican Lake in Langs Valley of Manitoba occupies what was once the northern shore of Lake Souris.)
The lower part of the Saskatchewan River basin near the river's mouth at Cedar Lake was clear of the ice-sheet before Lake Agassiz began to drain to northeast. Lake Saskatchewan existed on about 135 miles (217 km) of the North Saskatchewan River between Saskatoon and Prince Albert, Saskatchewan. A few miles east of Lake Saskatchewan's outlet, near the modern junction of the north and south branches, it entered Lake Agassiz. This Saskatchewan embayment extended for 400 miles (640 km) along the modern Saskatchewan River route.
Raised beaches, many kilometres from any current water, mark the former boundaries of the lake. While the Red River gradually descends from south to north, these old strandlines ascend as one goes north, due to isostatic rebound since glaciation.
The highest shore of Lake Agassiz is called the Herman Beach. It is named for Herman, Minnesota, in Grant County. The Herman Beach is the highest shoreline and can be traced from the historic outlet at Lake Traverse on the border of Minnesota and South Dakota. The beach fluctuates between 973 and 976 feet (296.5 and 297.5 m) above sea level. The altitude of Lake Traverse at 971 feet (296 m) above sea level at the Traverse Gap at Brown's Valley is at 980 feet (300 m). This was the south outlet of Lake Agassiz.
The Herman Beach displays numerous deltas from the major rivers that entered Lake Agassiz. In Minnesota and North Dakota, these include the Buffalo River Delta, Sand Hill River Delta, Sheyenne River Delta, Elk Valley Delta, and the Pembina River Delta. In Manitoba, there is the Assiniboine River Delta.
Fourteen shorelines of Lake Agassiz have been identified, which lie below the McCauleyville beaches. These formed when the River Warren could no longer receive the outflow of the lake. This occurred when a lower outlet was found and the lake shrank with the release of the lake's waters. The three highest shorelines are named the Blanchard beaches, and the next five in descending order are the Hillsboro, the two Emerado, and the two Ojata beaches, from towns on or near their course in North Dakota.
The fertile soils of the Red River Valley, now drained by the Red River of the North, were formed from lacustrine deposits of silt from Lake Agassiz.
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