The Mississipi river, along with its major tributary, the Missouri.
It is a river in the Canadian provinces of Ontario and Quebec.
An example of a watershed is the Mississippi River watershed, which drains water from 31 U.S. states and two Canadian provinces into the Gulf of Mexico.
Atlantic provinces
The Mississippi river.
Saskatchewan and Alberta
the Missouri river
The Missouri River
While America's most important river may depend on the region in which you live, traditionally the most important river has been the Mississippi. It is the largest river in North America, running 2,430 miles from Lake Itasca in Minnesota down to the Gulf of Mexico. It is the largest drainage system in North America and it's drainage basin drains parts of 31 American states and 2 Canadian provinces.
The Mississippi River and its tributaries drain all or part of 31 states and a small part of one Canadian province.
I'm not entirely clear what your question was. Most, if not all, of the territory covered by Canada's Prairie Provinces (Alberta, Saskatchewan and Manitoba), plus parts of North Dakota, South Dakota and Minnesota, drains into the Arctic Ocean. For the most part, the southern portions drain into Lake Winnipeg, then down the Nelson River into Hudson Bay, and the northern portions drain into the Arctic Ocean via the Mackenzie River.
Mississippi River