Lake Erie is the shallowest of the Great Lakes, and it has the shortest "detention time" of any of the Great Lakes. Detention? Does this mean that it is the best behaved lake, you ask? No: it means that a drop of water moves through Lake Erie in 2.6 years, compared with 6 years in Lake Ontario, 22 years in Lake Huron, 99 years in Lake Michigan, and 191 years in Lake Superior. (You can read about this in Wikipedia.)
Every 6 years Lake Erie replaces all of its water. If the ratio of detention time to replacement time is the same for all of the Great Lakes, then how many years does it take Lake Ontario to replace all of its water? Lake Superior? Does this mean that Lake Erie is the cleanest of the lakes?
Solution
13.8 years for Lake Ontario; 440.8 years for Lake Superior. Lake Superior is the cleanest lake. Lake Erie has pollution problems, largely from runoff of fertilizer from farms near the lake.