The current from Lake Ontario to the Atlantic, a thousand miles. Zoom in anywhere, or jump to a place below. Tap the water for its speed.
Eleven hours from the forecast of October 11, 2026. Drag to hold a moment.
Nobody publishes one current map of the whole St. Lawrence, so this is several, laid end to end. The map loads the one under your view.
From Montreal to Île aux Coudres, the currents are Environment and Climate Change Canada's river forecast model, which works in 100 metre cells. The estuary and the Gulf are its ocean model, in 2 kilometre cells. Both are the forecast made on October 11, 2026. They are not a live feed. Each is averaged over 25 hours, which cancels the tide and leaves the water that is going somewhere.
Quebec City is the exception, because there the tide is the story. That reach plays the model hour by hour. The river is under a mile wide at the bridges, and for four or five hours of every tide it runs upstream at about 3 miles an hour. At Trois-Rivières the same tide only slows the river. It never turns it around.
Above Montreal there is no public model, so two reaches are computed here: Lake Ontario to the Moses-Saunders dam, and the dam to the Beauharnois powerhouse. The riverbed comes from Canadian Hydrographic Service soundings and NOAA charts, and the water is routed through it the way friction would spread it. Deep water carries more and runs faster. The slider sets how much water leaves the lake. Three NOAA water-level gauges raise and lower the charted depths as you watch.
The honest checks. In the Thousand Islands the model sends 48% of the river under the Canadian span of the bridge and 50% under the American one. But it needs an unusually rough riverbed to lose the height the gauges show between Cape Vincent and Ogdensburg, so its speeds above the dam are more likely too low than too high. No measured speeds were found to test it against. Below the dam, 85% of the river is sent down the Beauharnois canal. That share is Hydro-Québec's choice, and here it is a set number. The old riverbed past Les Cèdres is not drawn.
The whole-river view draws the river about 8 miles wide so it shows at that scale. There is no wind above Montreal and there are no eddies. Line speed on screen is exaggerated in slow water so you can see it move.