The island is approximately 50 km long and 16 km wide at its widest point, and has a shoreline of 266 km. It is the second largest island in the Saint Lawrence River, after Anticosti Island in the Gulf of Saint Lawrence. The Island of Montreal is the largest island in the Hochelaga Archipelago, which is formed by the confluence of the Ottawa and St. Lawrence rivers. The St. Lawrence widens into Lake Saint-Louis southwest of the island, narrows into the Lachine Rapids, then widens again into the Bassin de La Prairie before narrowing again and flowing toward Quebec City and the Atlantic Ocean. Saint Helen's Island and Notre Dame Island are in the Saint Lawrence southeast of downtown Montreal, and Nuns' Island is located southwest of downtown. The river forms the Lachine Rapids to the west of the island between Lake Saint-Louis and the Bassin de La Prairie, and the :fr:Rapides Sainte-Marie|Rapides Sainte-Marie between the island and Saint Helen's Island. The Ottawa widens and becomes Lac des Deux-Montagnes north-west of the island. Natural watercourses on either side of Île Perrot, as well as the Sainte-Anne-de-Bellevue Canal on the western tip of the Island of Montréal, connect Lac des Deux-Montagnes and Lake Saint-Louis. Another outlet of Lac des Deux-Montagnes, the Rivière des Prairies, flows along the north shore of the island, separating it from Île Bizard and Île Jésus, and flows into the St. Lawrence at the northeastern tip of the Island of Montreal, while the Rivière des Mille-Îles separates Île Jésus from the northern mainland. . Man has altered the topography of the island significantly, as evidenced by historical maps that name a lake St. Pierre in the island. 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. The southwest of the island is separated by the Lachine Canal between Lachine and Montreal's Old Port; this portion of the island is partially divided further by the Canal de l'Aqueduc, running roughly parallel to the Lachine Canal, beginning in the borough of LaSalle and continuing between the boroughs of Le Sud-Ouest and Verdun. Most of the bedrock of the island consists of Chazy limestone, with some Utica shale. At the centre of the island stand the three peaks of Mount Royal, a volcanic intrusion. This short mountain is a member of, and gives its name to, the Monteregian Hills.
The first French name for the island was l'ille de Vilmenon, noted by Samuel de Champlain in a 1616 map, and derived from the sieur de Vilmenon, a patron of the founders of Quebec at the court of Louis XIII. However, by 1632 Champlain referred to the Isle de Mont-real in another map. The island derived its name from Mount Royal, and gradually spread its name to the town, which had originally been called Ville-Marie. In the Kanien’kéha, the island is called Tiohtià:ke Tsi or Ka-wé-no-te. In Anishninaabemowin, the land is called Mooniyaang.
