Lake Winnipegosis in the context of "Lake Winnipeg"

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.

The Red River Colony (or Selkirk Settlement), also known as Assiniboia, was a colonization project set up in 1811 by Thomas Douglas, 5th Earl of Selkirk, on 300,000 square kilometres (120,000 sq mi) of land in British North America. This land was granted to Douglas by the Hudson's Bay Company in the Selkirk Concession. It included portions of Rupert's Land, or the watershed of Hudson Bay, bounded on the north by the line of 52° N latitude roughly from the Assiniboine River east to Lake Winnipegosis. It then formed a line of 52° 30′ N latitude from Lake Winnipegosis to Lake Winnipeg, and by the Winnipeg River, Lake of the Woods and Rainy River.

West of the Selkirk Concession, it is roughly formed by the current boundary between Saskatchewan and Manitoba. These covered portions consisted of present-day southern Manitoba, northern Minnesota, and eastern North Dakota, in addition to small parts of eastern Saskatchewan, northwestern Ontario, and northeastern South Dakota. The lands south of the 49th parallel north ceased to be a part of the Red River Colony following the signing of the Treaty of 1818, in which the United Kingdom agreed to cede this territory to the United States.