A significant problem facing Carleton was that the river did not lend itself to amphibious operations. Major Henry Livingston, who commanded the American forces at La Prairie, noted that the river “is very unsafe to navigate. The rocks often projecting just out of the water above a mile from either shore & some but a few Inches under the surface & very dangerous for Battoes or Canoes to strike on.” 
Perhaps for this reason, Carleton decided to make his attack near Longueuil, in a place where the water was so shallow that men could wade to shore from an island [Île Ste Hélène] well out in the river. This permitted the attack to be made in two parts: “a number were to wade and the rest to come with their boats.”
The nearest American defenders were 2 miles away in the village of Longueuil. This force, under the command of Lieutenant-Colonel Seth Warner, included all or nearly all of the Green Mountain Boys, and 5 or 6 companies of the 2nd New York regiment. There, the officers took lodging in some houses while the men encamped in or about an ancient stone fort. 
Carleton organized the Canadian militia into “brigades,” and asked the prominent citizens of Montreal to select one from their ranks to lead the Canadians and Indians. They chose Claude-Nicolas-Guillaume de Lorimier, a 31-year-old volunteer who had previously distinguished himself in operations around Fort Saint-Jean. 
A view of Montreal from Mont Royal. The city and its suburbs are visible in the middle ground. At center left is Île Ste Hélène. The battle of Longueuil was fought between this island and the far shore of the river.The fort at Longueuil. This drawing shows the fort's appearance in the early 19th-Century, before it was demolished.
Maps of the area between Île Ste Hélène and the southern shore of the St. Lawrence, dating from 1866 (top), 1915 (middle), and 1952 (bottom). The river is quite shallow at this point, and in various places the sandbars create small islands. A comparison of the three maps suggests that these small islands would change over time.
1. Simon Sanguinet, a resident of Montreal, claimed in his memoir that there were 130 regulars, 80 Native Americans, and 800 Canadian militia. Other sources suggest that Carleton's army was smaller, especially in terms of Canadian militia. For example, the day after the battle Lieutenant John Fassett (a copy of his journal is available here) learned from a Canadian prisoner that Carleton had commanded “660 men, that 100 were Regulars, and the rest Canadians and a few Indians.”
2. A copy of Livingston's journal is available here.
3. Sanguinet and de Lorimier stated that the attack was made from Île Ste Hélène. The quoted passage is from Fassett's journal and it refers to a prisoner's description of Carleton's plan of attack.
4. cf. journals by Fassett and Livingston.
5. This description is based on de Lorimier's memoir. In September he co-led the party that halted the first American advance on Fort Saint-Jean. When Moses Hazen was captured by the British on September 18, de Lorimier was given the assignment of taking him back to Montreal. At this time, almost everyone making the journey between Montreal and Fort Saint-Jean was captured by the Americans; de Lorimier was successful despite having a prisoner in tow. Afterwards, de Lorimier claimed that John Brown offered him the position of major with the Americans if he would switch sides. He did not.