The Arctic Watershed
North of this watershed all flowing water eventually reaches Hudson Bay, while south of it all watercourses form part of the Great Lakes drainage system. The height of land follows an erratic course of some 2250 km across Ontario, ranging from 32 to 280 km north of Lakes Huron and Superior. This watershed was declared the inland boundary of the tract surrendered to the Crown by Ojibwa Indians in the Robinson Superior Treaties of 1850. It was also widely considered to be the southern limit of Rupert's Land, the vast, ill-defined Hudson's Bay Company territory transferred to Canada in 1870, and it figured prominently in the Ontario-Manitoba boundary dispute of 1883-4.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.