At Nov 9, 5:42 a.m. EST the NOAA gauges read 74.69 m at Olcott (west, south shore) and 74.73 m at Cape Vincent (east end, St. Lawrence outlet), in metres above IGLD 1985. The two ends are within 3 cm of each other, so the lake is close to level. Averaged over 2026-11-08, the two gauges stood 16 cm above the November long-term average of 74.54 m.
Olcott, the westernmost NOAA gauge on the lake, and Cape Vincent at the head of the St. Lawrence. Lake Ontario is deep, so wind tilts it far less than Erie. Six-minute readings from NOAA, preliminary until NOAA verifies them; times in Ontario time.
The mean of each full day at each gauge, worked out on this site from NOAA’s six-minute readings, against the November long-term average (1918–2025).
Gauges on the US shore, run by NOAA. Oswego is the gauge NOAA GLERL lists as Lake Ontario’s master gauge.
| Gauge | Where | Latest (m, IGLD 1985) | 24 h | Reading time |
|---|---|---|---|---|
| Olcott (9052076) | west, south shore | 74.694 | −0.01 m | Nov 9, 5:42 a.m. EST |
| Cape Vincent (9052000) | east end, St. Lawrence outlet | 74.727 | +0.03 m | Nov 9, 5:42 a.m. EST |
| Oswego (9052030) | master gauge, south shore | 74.766 | +0.02 m | Nov 9, 5:36 a.m. EST |
The gauges on the Canadian shore are run by the Canadian Hydrographic Service of Fisheries and Oceans Canada; we link to their pages rather than reproduce their data: Port Weller, Burlington, Toronto, Cobourg, Kingston.
Lake-wide monthly means for 1918–2025, from the coordinated Canada–US series published by NOAA’s Great Lakes Environmental Research Laboratory (GLERL). The averages and records are worked out on this site from that series; the outflow has been regulated since 1960. Levels in metres above IGLD 1985.
| Month | Long-term average (m) | Record high (m) | Record low (m) |
|---|---|---|---|
| January | 74.58 | 75.16 (1946) | 73.81 (1935) |
| February | 74.62 | 75.27 (1952) | 73.78 (1936) |
| March | 74.69 | 75.37 (1952) | 73.94 (1935) |
| April | 74.89 | 75.65 (1973) | 74.03 (1935) |
| May | 75.03 | 75.80 (2017) | 74.11 (1935) |
| June | 75.06 | 75.91 (2019) | 74.19 (1935) |
| July | 75.01 | 75.80 (2019) | 74.14 (1934) |
| August | 74.90 | 75.58 (1947) | 74.00 (1934) |
| September | 74.75 | 75.41 (1947) | 73.91 (1934) |
| October | 74.62 | 75.22 (1945) | 73.82 (1934) |
| November (now) | 74.54 | 75.18 (1945) | 73.75 (1934) |
| December | 74.53 | 75.20 (1945) | 73.74 (1934) |
Highest monthly mean on record: 75.91 m, June 2019. Lowest: 73.74 m, December 1934, before regulation began. Chart datum is 74.20 m.
NOAA GLERL’s coordinated lake-wide series gives Lake Ontario a provisional mean of 75.12 m for September 2026: 37 cm above the September average for 1918–2025 (74.75 m) and 39 cm above September 2025 (74.73 m). The International Lake Ontario–St. Lawrence River Board’s weekly regulation summary for the week ending 23 September 2026 put the lake at 75.02 m against an average of 74.71 m for the time of year (the board’s average is for 1918–2022), with a weekly mean outflow of 8,390 m³/s.
The coordinated six-month forecast range, prepared by Environment and Climate Change Canada and the US Army Corps of Engineers and published on NOAA GLERL’s Great Lakes Dashboard, puts the October 2026 lake-wide mean between 74.79 and 74.98 m, against an October average of 74.62 m, and February 2027 between 74.45 and 75.06 m, against a February average of 74.62 m. The International Lake Ontario–St. Lawrence River Board also posts its own Lake Ontario forecast each Thursday.
Levels are in metres above the International Great Lakes Datum of 1985 (IGLD 1985), the common height reference used by Canada and the United States. The Canadian Hydrographic Service (CHS) explains that its zero sits at mean water level at Rimouski, Quebec, and that it has been in use since January 1992. Lake Ontario’s chart datum, also called Low Water Datum, is 74.20 m, and NOAA defines it as the surface from which charted depths are measured. Subtract 74.20 from a reading to get the water standing above charted depth. NOAA expects IGLD 1985 to be replaced by IGLD (2020), tentatively in 2027, and chart datums may be reviewed at the same time. The official lake-wide average is calculated from Port Weller, Toronto, Cobourg and Kingston on the Canadian shore and Rochester and Oswego on the U.S. shore.
Lake Ontario’s outflow has been regulated since 1960, mainly at the Moses-Saunders power dam between Cornwall and Massena, about 160 km down the St. Lawrence. The International Joint Commission (IJC) notes that the dam is owned and run by Ontario Power Generation and the New York Power Authority. The International Lake Ontario–St. Lawrence River Board sets the outflow under Plan 2014, which the IJC adopted on 8 December 2016 to replace Plan 1958-D. The plan allows more natural variation to help coastal wetlands and, according to the IJC, will often extend the fall boating season. The board may deviate from the plan when levels reach set trigger points. The IJC is clear about the limits: inflow from Lake Erie, rain, runoff and evaporation cannot be controlled, and most of the lake’s water arrives from Lake Erie. The CHS says Lake Ontario usually peaks in June and is lowest in December, with an average seasonal swing of about 0.6 m.
Lake Ontario set records twice in three years. On the coordinated lake-wide monthly series published by NOAA GLERL, May 2017 reached 75.80 m and June 2019 reached 75.91 m, the highest monthly mean since 1918. Ontario’s Special Advisor on Flooding gives the daily peaks: 75.88 m in late May 2017 and 75.92 m in early June 2019. The advisor’s review lists flooding on Toronto Island, in Clarington, Brighton and Prince Edward County, and along the Bay of Quinte and the Thousand Islands. Toronto Island Park closed in 2017, and preventive work kept the islands open in 2019. To relieve the shoreline, the board raised outflows to 10,400 m³/s, equal to the record outflows released in 2017. The review also notes that releasing enough water to lower Lake Ontario by one centimetre in a week raises flood levels below the dam at Montreal by 10 cm.
For 1918–2025 the same series gives Lake Ontario a highest monthly mean of 75.91 m (June 2019) and a lowest of 73.74 m (December 1934), a range of 2.17 m. Every monthly record low dates from 1934–36, before regulation began. Monthly record highs come from 1945–47, 1952, 1973, 2017 and 2019. The board advises shoreline users to be ready for at least the historical range, from about 75.73 m down to 73.56 m, and adds that, because future climate is uncertain, more extreme levels may be reached.
Lake Ontario is deep, so it tilts far less than Lake Erie. The CHS says storm surge on Lake Ontario rarely exceeds 0.5 m. The IJC notes that strong winds and waves can still change local levels on the lake and river by more than half a metre for a while. In a May 2026 notice the board warned that strong winds on top of even moderately high levels can flood vulnerable stretches of shoreline. Surges are largest at the ends of a long basin when the wind blows along it, which is why this page pairs Olcott at the west end with Cape Vincent at the east end.
Charted depths are measured from 74.20 m, so the lake’s height above datum is extra water over every shoal and launch. In August 2026 the lake averaged 75.28 m, about 1.08 m above datum and 38 cm above the August average. High water brings different problems: flooded docks and shoreline paths, and stronger wave run-up on breakwalls and piers. Conservation authorities regulate development along the shoreline and issue local flood messages. Ontario’s flood message system includes ‘water safety’ statements for conditions that could be dangerous for boaters and anglers even when no flooding is expected. The board’s weekday water-level updates and Thursday forecasts are a good check before a trip.
The Canadian Hydrographic Service runs the Canadian gauges. Its station pages for Port Weller, Burlington, Toronto, Cobourg and Kingston show recent readings. The CHS also publishes a Monthly Water Level Bulletin and a six-month forecast prepared with Environment and Climate Change Canada and the U.S. Army Corps of Engineers. The International Lake Ontario–St. Lawrence River Board posts water levels every weekday, a weekly regulation summary with outflows, and a forecast on Thursdays. We link to the CHS pages rather than reproduce their data.
The live values are the latest six-minute readings from NOAA’s Olcott (west) and Cape Vincent (east) gauges. On Lake Ontario they usually sit within a few centimetres of each other, so a widening gap on the 72-hour chart means wind set-up is under way. The 30-day chart of daily means shows the seasonal trend, which you can compare with the monthly average in the table. Recent NOAA readings are provisional and can be revised.
The live readings at the top come from NOAA’s Olcott and Cape Vincent gauges and update every six minutes, in metres above IGLD 1985. The latest lake-wide monthly figure is a provisional 75.12 m for September 2026, 37 cm above the September average for 1918–2025 (coordinated series published by NOAA GLERL). For the week ending 23 September 2026 the regulation board reported 75.02 m against an average of 74.71 m for the time of year (the board’s average is for 1918–2022).
Higher. The provisional September 2026 lake-wide mean was 37 cm above the September average and 39 cm above September 2025. The coordinated forecast range published in September 2026 runs from 74.79 to 74.98 m for October, above the October average of 74.62 m, and from 74.45 to 75.06 m for February 2027.
The International Lake Ontario–St. Lawrence River Board, acting for the International Joint Commission, sets outflows at the Moses-Saunders dam under Plan 2014. It controls only the outflow; inflow from Lake Erie, rain, runoff and evaporation are beyond its control, so it cannot fully control the lake’s level.
The highest lake-wide monthly mean since 1918 is 75.91 m in June 2019 (coordinated lake-wide series). Ontario’s Special Advisor on Flooding reported a daily peak of 75.92 m in early June 2019, above the 75.88 m reached in late May 2017.
Chart datum (Low Water Datum) for Lake Ontario is 74.20 m above IGLD 1985. Depths on nautical charts are measured from it, so a reading of 75.20 m means about 1 m more water than charted.