The current across the whole lake, hour by hour, from NOAA's forecast model. Tap the water for its speed.
This is a forecast model, not a measurement. NOAA runs the Lake Ontario Operational Forecast System four times a day. It divides the lake into 64,453 triangles, from about 200 m across near shore to 2.5 km in the middle, stacks 20 layers of water in each one, and pushes them with the wind, the rivers and the water level.
The page reads two of those layers straight from NOAA's files: the top twentieth of the water and the bottom twentieth. The last 12 hours come from runs driven by observed weather. The next 48 are the forecast. A new run arrives about every six hours. To load on a phone, the page keeps about 16,000 points of the model's mesh: all of the open lake, and a thinned copy of the fine triangles along the shore.
Wind does most of it. The Niagara River pours in at the west end with the water of the four lakes above it, and the St. Lawrence takes it out at the east. It would need about six years to replace the water in the lake.
A few small bays and harbours have no colour. The model leaves them out. Line speed on screen is exaggerated in slow water so you can see it move. Up the Niagara, the Niagara River map shows where this water comes from.