A drainage basin is an area that drains all precipitation received as a runoff or base flow (groundwater sources) into a particular river or set of rivers. Canada’s major drainage regions are the Atlantic Ocean, Hudson Bay, Arctic Ocean, Pacific Ocean, and Gulf of Mexico. A lake can be defined as any inland body of water, usually fresh water, larger than a pool or pond. Canada is covered by as many as two million lakes. The largest set of lakes, the Great Lakes, straddle the Canada-US boundary and contain 18% of the world’s fresh water in lakes. Most Canadian rivers have developed since the last ice age. Almost 75% of the Canadian landmass contains water that drains northward into either the Arctic Ocean or into Hudson and James bays.