Lake Ontario

Long ago, a huge sheet of ice covered this land.

As the ice melted, a lake much bigger than today's filled the low ground.

As the last ice age ended, a huge ice sheet called the Laurentide Ice Sheet melted back from this part of North America. As it retreated, its meltwater filled a large lake called Lake Iroquois, which covered the Lake Ontario basin and the low land around it. A 2005 report from Woods Hole Oceanographic Institution says it was three times the size of today's Lake Ontario.

At first the lake drained through a channel near Rome, New York, and along the Mohawk River valley. When the ice moved back far enough, the water found a lower way out, and the 2005 report says the lake level fell by 120 meters.

Porreca, Briner and Kozlowski (Geomorphology, 2018) describe Lake Iroquois as a large proglacial lake that occupied the present Lake Ontario basin and adjacent low-lying areas, dammed against the retreating Laurentide Ice Sheet. Its outflow first ran through a spillway near Rome, New York, and along the Mohawk River valley; when the ice retreated north of the Adirondack Uplands, the lake found a lower outlet at Covey Hill, causing a significant flood.

The timing is debated. Donnelly and colleagues dated the drainage to 13,350 years ago (WHOI, 2005), with the lake level dropping 120 m; Porreca and colleagues say the rerouting is hypothesized at about 13,200 years ago but has not been directly dated, and their beryllium-10 ages on Moss Island average 14,800 ± 1,300 years.

Water from other big lakes flows into Lake Ontario.

Then it flows out in a river all the way to the ocean.

Lake Ontario is the final link in the Great Lakes chain. Water from Lake Erie arrives through the Niagara River, and Lake Ontario drains out through the St. Lawrence River, which carries its water to the Atlantic Ocean.

The lake holds about 1,640 cubic kilometers of water. On average, water stays in the lake for about six years before it flows out. The lakewide plan says the lake is big enough to change the weather nearby and shape its shores with powerful waves.

In NOAA GLERL's table, Lake Erie's outlet is the Niagara River (with the Welland Canal), and Lake Ontario's is the St. Lawrence River; the lakewide plan calls Lake Ontario the final link in the Great Lakes chain and the gateway to the Atlantic Ocean. Its land drainage area, 64,030 km² including the Niagara River, is about 3.4 times its 18,960 km² water area.

Its retention time is about 6 years, compared with 2.6 years for Lake Erie and 191 years for Lake Superior. The plan adds that the lake is large enough to influence local weather and to shape its shorelines with powerful waves.

Lake trout live in the lake. People are working to help them come back.

Ducks like these dive down to eat shellfish.

Lake Ontario has dunes, beaches, islands and coastal wetlands. The lakewide plan says its meadows and wetlands are nurseries and migration stopovers for many species. Since 1983, scientists have surveyed lake trout to check efforts to bring them back; in 2024 they surveyed the whole lake for the first time since 2008.

Zebra and quagga mussels, which came from elsewhere, now filter the water and have changed how food moves through the lake. Audubon says lesser scaups on the Great Lakes may feed heavily on zebra mussels.

The lakewide plan describes a mosaic of dunes, coastal wetlands, barrier beaches, cobble and bedrock shores, islands, tributaries and deep offshore waters. In the 2022 State of the Great Lakes assessment it cites, lake trout were Fair and Improving, walleye Good and Unchanging, and lake sturgeon Poor and Improving. A USGS report (O'Malley et al., 2025) notes that lake trout rehabilitation has been assessed since 1983; the 2024 gillnet survey, the first lake-wide one since 2008, found relatively few naturally produced fish in US waters and a higher share in Canadian waters.

The plan says invasive zebra and quagga mussels have altered nutrient cycling and water clarity, moving nutrients toward the nearshore and the lake bottom. Audubon's guide notes that lesser scaups on the Great Lakes may feed heavily on the introduced zebra mussel.

Swim only where a grown-up says it is safe.

Wear a life jacket on a boat. Cold water can be dangerous.

NOAA warns that rip currents, narrow channels of fast-moving water, happen along the shores of the Great Lakes as well as on ocean coasts. If one pulls you, NOAA's advice is to swim parallel to the shore, not straight back against it.

Cold water is a danger too. The US National Weather Service says the shock of falling into cold water makes people gasp, and that wearing a life jacket greatly increases the chance of survival.

NOAA's National Ocean Service notes that rip currents occur along the shores of the Great Lakes, can move at up to eight feet per second, and are best escaped by swimming parallel to the shore. The National Weather Service adds that cold shock can be just as dangerous in water of 10 to 15 °C as in water near freezing, causing involuntary gasping and rapid breathing in the first minutes, and that wearing a life jacket significantly increases the chance of survival.

PlayOwl's own advice: watch diving ducks and other water birds from the shore without chasing them, and check beach conditions before swimming.