It is the main island of the Hochelaga Archipelago at the confluence of the Saint Lawrence and Ottawa rivers.

These waterways, connecting Lac des Deux-Montagnes with Lake Saint-Louis, constitute one of the places where the Ottawa flows into the St. Lawrence.

After coursing about 55 km (34 mi), the Prairie, at the northeastern tip of the island, joins the St. Lawrence.

Around a southern point near the center of the island, the St. Lawrence narrows into the Lachine Rapids, then widens into the Bassin de La Prairie past that southern point, and narrows again at the Montreal neighborhood of Cite Du Havre before flowing towards Quebec City and the Atlantic Ocean.

[5] This lake, and several other watercourses, have been drained or covered over as the city developed, while areas on the island's shores have been reclaimed and extended.

At the centre of the island stand the three peaks of Mount Royal, a volcanic intrusion (not an extinct volcano, contrary to popular belief).

This short mountain (elevation 233 metres (764 feet) is a member of, and gives its name to, the Monteregian Hills.

However, in recent years, the government has acknowledged this problem and is working on long-term solutions to alleviate the congestion.