Gulf of Mexico
It might be hard to imagine, but a raindrop that falls within a small area of southern Alberta and Saskatchewan drains all the way south to the Gulf of Mexico. Some 21,600 square kilometres of Canada's land mass lies within the Missouri River watershed, which flows into the Mississippi River watershed, which constitutes the bulk of the Gulf of Mexico watershed. The Gulf of Mexico watershed dominates the United States, with 31 of the 50 states sitting within its area.
The Milk River in southern Alberta originates in Montana, winds its way across the border into Canada, then flows back to Montana and empties into the Missouri River.
Saskatchewan's Poplar River, another tributary of the Missouri River, also takes a detour into Canada before flowing into Montana. Likewise, the Missouri River tributary Big Muddy Creek flows through southern Saskatchewan before heading stateside.