If a drop of water could talk, what a story it would tell.

On average, a drop of water which finds its way into Lake Superior from runoff or rainfall takes more than two centuries to travel through the Great Lakes system and along the St. Lawrence River to the ocean, says Environment Canada. To be precise, water which entered Superior in 1794 - the year the reign of terror came to an end in France following the French revolution - didn't make it to the Atlantic until 1998.

The travelling time is based on retention times, or how long, on average, it takes for each of the lakes to replace its water with new water.

To get a grip on this theory, think of each of the lakes as a 10-gallon bathtub with the drain slightly open and the tap running slowly. If one gallon of water flows in and another out every minute, after 10 minutes you will have emptied 10 gallons and added 10 new gallons. One drop might come in through the tap and go out the drain in only a few seconds, while another drop might stay in the tub for an hour or more, but the average length of stay is 10 minutes.

In Lake Superior, the tap is rain and runoff and the drain is the St. Marys River, which flows into Lake Huron. After 173 years, much of the water in Superior has flowed out of the lake and been replaced with new water. In Lake Huron replacement averages 21 years; Lake Erie, 2.7 years and Lake Ontario, 7.5 years. Add the numbers together and you will see that drops of water which fell into Lake Superior in 1794 worked their way to the Atlantic slightly more than 204 years later.

The time span is much less for lakes closer to the Atlantic. Water you were swimming in at the beaches of Grand Bend on Lake Huron in 1967, when Canadians were celebrating Canada's 100th birthday, is also arriving at the Atlantic in 1998, while water which poured into Lake Erie in 1988, when the Canada-U.S. Free Trade Agreement was signed, also made it to the ocean in 1998. If your son or daughter is 7 1/2 years old, water now streaming into the Atlantic was in Lake Ontario about the same time he or she was born.

It should be noted that all of the water in each of the Great Lakes is never completely replaced. For instance, 37 per cent of the water that was in Lake Superior 173 years ago is still there, says Environment Canada.