The Great Lakes are a series of interconnected freshwater lakes that comprise the largest freshwater system on Earth and represent about 21 percent of the world surface fresh water supply. Primarily located along the U.S.-Canadian border, the Great Lakes include Lake Superior, Lake Michigan, Lake Huron, Lake Erie and Lake Ontario. The Great Lakes connect to the Atlantic Ocean through the St. Lawrence River to the northeast (from Lake Ontario); to New York Harbor via the Erie Canal/Hudson River to the southeast (from Lake Erie); and to the Gulf of Mexico by way of the inland river system from Chicago to the south (from Lake Michigan). 

The Great Lakes region is the aquatic footprint of Eastern North America and the legacy of thousands of years of geologic history. The Wisconsin Ice Sheet invaded from the north about 10,000 years ago, then retreated, then invaded again, scouring the landscape and leaving large freshwater lakes.

The edges of the Great Lakes are the moraine of the glaciers that came and went leaving clays, silts, sands, gravels and boulders at each cycle. This became the potting soil for the deciduous forests that supplied the lumber for pioneer homes and barns, and later, construction materials for America's 19th-century Great Lakes cities. 

In the early 1800s, the race was on to occupy the western frontier of the Great Lakes for the valued resources and the transportation of goods from the interior to worldwide markets. Communities sprang up and evolved into cities and major metropolitan areas, including Erie, Cleveland, Toledo, Toronto, Detroit and Chicago. These became the major ports for shipments of farm commodities and finished goods. New York initiated more western expansion by building a canal from the Hudson River to Lake Erie, and the Canadians duplicated the effort by building the Lachine, Richelieu and Welland Canals to access the lakes and all their resources.

Season

Mid-June to mid-September provides the best weather and sea conditions on the Great Lakes, although hardy local folks extend the season a month longer either side of that. In more remote locations, marina facilities may not be open before June or after September, although the public docks on both sides of the border ensure you can find a place to tie up. The cruising season is shorter on Lake Superior than on the other lakes, largely because of spring and fall storms and fog. For early and late season cruising, a cabin heater, in addition to warm clothing, is a must, and it can be a comforting accessory even in summer. Although the days are shorter at the two ends of the season, the air is often clear and bright and the harbors uncrowded.

Weather Patterns

The general Great Lakes weather pattern is summertime high-pressure systems of fairly long duration, punctuated by lows coming through from the southwest. Air temperatures are cooler on the upper lakes than on the lower lakes, yet on bright, sunny days, the temperature occasionally climbs into the 90s. Daytime temperatures average in the 70s, with nights in the low 50s. Bear in mind that these are land temperatures; it is always colder on the water. It is possible to have chilly days and downright cold nights anywhere on the Great Lakes. 

Rainfall varies from season to season, but July and August tend to be the driest months. Thunderstorms during the summer leap up quickly, so keep a careful weather watch. Given the right conditions, any one of the lakes can build up a steep, uncomfortable chop with lightning and even waterspouts in what seems like no time. Storms on Lake Superior can feature wave heights of over 20 feet; waves well over 30 feet have been recorded there.

Fog

Between late spring and late fall the shore can be shrouded in fog when the land surrounding the lakes heats up much warmer than the water. This warm season fog occurs when moisture in the warm air condenses as it flows over the cold lake. Like ocean fog, lake fog can descend suddenly and last for days. Although most likely to roll in late at night or in the early morning and burn off by mid-day, fog can occur at any time and any place. 

Water Temperatures

Water temperature rises over the course of the summer in all the lakes, from south to north, reaching the highest temperature in September. By early July, swimming in the northernmost shallows is comfortable at about 68 degrees. Boaters cruising in the early and late parts of the season need to be aware of the dangers of hypothermia. Even in mid-summer, the water only reaches 70 degrees, except in the most sheltered of bays. Be sure to have a "man overboard" plan but it's best to use your skills to avoid such weather.

Learn more about the cruising characteristics of the Great Lakes in the Great Lakes Volume 1 Edition of Waterway Guide.