The smallest Great Lake by area still out-volumes Erie
Lake Ontario, easternmost of the Great Lakes, covers about 19,000 square kilometres split between Ontario and New York—the chain’s smallest surface. Niagara River water enters; the St. Lawrence Seaway begins here on the way to the Atlantic.
Roughly 10,000 square kilometres of its surface lie in Canada and 9,000 in the United States, yet its greater depth lets it hold more water than Erie upstream. The drainage basin spans 64,030 square kilometres. Besides the Niagara, the Don, Humber, Trent, Cataraqui, Credit, Genesee and Oswego rivers drain into it. Champlain labelled it Lac Saint Louis in 1632; by 1641 the Iroquoian Oniatarí:io—“beautiful” or “shining” lake—was already in use beside later Frontenac tags.
Wisconsin ice deepened a Silurian Ontarian River valley; when the St. Lawrence opened below sea level, the basin briefly became an Atlantic bay tied to the Champlain Sea before isostatic rebound lifted the outlet. Roughly 2,000 metres of ice had pressed the land down, and the St. Lawrence area is still springing back by about 30 centimetres a century, tilting the bed south so that the southern shore floods and river mouths turn into bays. Baymouth bars build lagoons from Prince Edward County to Toronto Bay, once chosen for Upper Canada’s capital perch. The Welland Canal restores ship passage around Niagara’s drop.
An eleven-minute seiche normally shifts the surface by just 1.9 centimetres, though wind and pressure can magnify it. The lake is too deep to freeze over entirely; ice usually covers between a tenth and nine-tenths of it, though in 1877 and 1878 it came close to total cover. North-west winter winds load up with moisture over the open water and dump lake-effect snow on the southern and south-eastern snowbelt.
In spring, lake breezes hold back fruit blossom until frost danger has passed, and in autumn they postpone the first frosts on the south shore. Around the shores, deciduous woods of maple, oak, beech, ash and basswood belong to what Canada classes as the Mixedwood Plains Ecozone.
Source: Lake Ontario
