The Mattawa River rises in Trout Lake in north-central Ontario, 198.5 m above sea level, and drops 50 m over the 54 km distance to the Ottawa River. Its watershed encompasses 1170 km2 of the Canadian Shield. In 1988, a section of the Mattawa (43 km) was designated a Canadian Heritage River. In 2001, 2 other sections were designated, including the historic 11 km La Vase Portages, crossing the height of land between waters flowing east to the Ottawa River and west to Georgian Bay.

The Mattawa is part of a traditional Native route linking the St Lawrence River with the upper Great Lakes. The Algonquin led the first European travellers into the interior of the continent by way of the Mattawa. Étienne Brûlé was the first non-Native to see the Mattawa, in 1610. As it had been for the Native peoples, the Mattawa became the major westbound highway for the European traders and missionaries who followed Samuel de Champlain. Alexander Mackenzie called the Mattawa "La Petite Rivière," and considered it the most difficult section of the entire trans-continental fur-trade route. Nine of the original 11 La Vase portages are much as they were found by the voyageurs, and all still have their original French names. Mattawa is an Algonquian word meaning "junction of waterways" or "river with walls that echo its current."

Today, the Mattawa is still a busy canoe route, where recreational paddlers follow portages unchanged for over 300 years. Two provincial parks (Mattawa and Samuel de Champlain) protect most of its course. The town of Mattawa, Ont, lies at its mouth.