Other Hydrological Issues
Because of the size of the system, it responds somewhat slowly to environmental
changes. Each lake has a large total volume relative to the amount of water
entering and leaving. For example, using simplifying engineering assumptions,
it has been calculated that a single drop of water deposited in Lake Superior on
average takes 190 years to leave through the St. Mary’s River.8 This relatively
prolonged hydrologic process means that the Great Lakes can require a
significant amount of time to process changes in chemical water quality.9 In
addition, the large surface area of the lakes, covering 94,000 square miles,
makes the lakes vulnerable to direct atmospheric pollutants that fall with rain
or snow and as dust on the lake surface.
The system is also somewhat slow in responding to hydraulic changes. It has
been estimated that up to 15 years is required for certain changes in the water
inputs to the upper lakes to be fully felt in the lower lakes.10 Nevertheless,
changes in the long-term average flows and levels of the Great Lakes are
somewhat predictable based on current knowledge and tools. Flows in the
channels that connect the lakes and the levels of the lakes themselves are of
critical importance to various users of the lakes, most notably the hydroelectric
power industry and navigation interests.