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. Canadas major drainage regions are the Atlantic Ocean, Hudson Bay, Arctic Ocean, Pacific Ocean and Gulf of Mexico. Rivers are organized into networks, each with its own recharge area upstream, and drainage channel and mouth downstream. Networks are ordered from ocean to main river to secondary rivers to streams which correspond to ocean basins, river basins, sub-basins, sub-sub-basins, and so forth. The boundary of a watershed is called a drainage divide.