The Ottawa River is a Canadian river that for most of its length defines the border between the provinces of Ontario and Quebec. The river's source is Lake Capimitchigama in Quebec and it empties into the St Lawrence River at Montreal, Quebec.

The Ottawa River is 1,271 km (790 mi) long and drains an area of 146,300 sq km (56,487 sq mi) with an average flow of 1,950 cu m/s (68,864 cu ft/s) and a maximum flow of 5,351 cu m/s (188,969 cu ft/s) with a minimum flow 749 cu m/s (26,451 cu ft/s).