New scientific information tells us that all hydrological alterations of
aquatic systems have ecological consequences. While these consequences
depend on the exact nature of the specific alteration, they have more to do with
how the water is used, where in the system it is used, and when water is used,
than how many gallons of water are used. A relatively small volume of water
permanently removed from a sensitive habitat may have grave ecological
consequences. Similarly, the rapid addition of water to a stream reach during
spawning season can eliminate a source of young fish for an important fishery.
Recent scientific investigations have identified that the biotic composition,
structure, and function of aquatic wetland and riparian systems depend largely
on the hydrologic regime.17 The hydrologic regime includes not only the absolute
quantity of water at a given time, but also the frequency with which certain flows
occur over a given time interval, the rate at which flow conditions change, the
duration of various flow conditions, and the range of flows on a given system.18
The biologically important time intervals for these variables can be as short as an
hour and as long as several years. All of these conditions describe the ability of a
given aquatic system to move materials and to support species and natural
communities that have evolved in response. Moreover, these conditions control
critical biological events including the ability of exotic species to establish
themselves; dispersal of native species; cues for spawning, hatching and migration
of native species; and changes to food webs and encroachment of vegetation.
In addition to the waters of the Great Lakes basin themselves, the exhaustible
natural resources that are threatened by future water development projects
are those that depend on the basin’s waters in one way or another. They include
the natural communities of the open lakes – phytoplankton, zooplankton,
planktivorous fish, piscivorous fish and avian predators; natural communities
on the coasts – wetlands, dunes, beaches, and shorelines; the plants, animals
and natural communities of the rivers and streams tributary to the Great Lakes –
species of fish, insects, plants, herps and mammals; riparian wetlands;
embayment lakes; riparian plant communities; and upland pond and wetland
communities fed by ground water. These natural resources are sustained not
only by the waters of the Great Lakes basin but also by the movement of energy,
materials, and biota in those waters.