Der Mississippi River ist ein Nebenfluss des Ottawa River im Osten Ontarios in Kanada. Er ist 169 km in length from its source in Upper Mazinaw Lake, has a drainage area of 3750 km², and has a mean discharge of 40 m³/s (source: Atlas of Canada). It rises east of the Kawartha lakes and flows northeast through Crotch, Dalhousie, and Mississippi Lakes, past Carleton Place, through the municipality of Mississippi Mills and the town of Almonte, then north to join the Ottawa River just east of Arnprior.
Tributaries include the:
The river originally powered textile mills. Today, it provides hydroelectric power.
The Purdon Conservation Area, a wetland in this river's watershed, contains Canada's largest native colony of showy lady slipper orchids, some 16,000 plants.
The origin of the river's name is something of a mystery; although its current spelling may be derived from that of its much larger American cousin, it is most certainly a corruption of a different native name, as the translation 'great water' would not apply to a relatively minor tributary of the Ottawa, definitely the largest river in the area. Instead, the name may originate from "Mazinawzeebi", Algonquian for 'painted image river', referring to the pictographs found on Mazinaw Lake, though this is by no means proven.