The second largest use of Great Lakes water is to supply cooling water to the
thermoelectric power industry. The Great Lakes Commission estimates that 40
billion gallons per day are used to cool reactors and condensers used in the
generation of electric power. This represents roughly 4% of the total use
reported by the Commission for calendar year 1992.
Potential ecological effects from this use of water are largely attributable to
the near field alteration of thermal regimes due to increased temperature and
the possible release of trace contaminants used as cooling water additives. In
terms of hydrological alteration, the cumulative effects of losses in wet cooling
towers may merit attention. In compiling data for the 1992 use report, the ten
jurisdictions each estimated the losses of cooling water to predict consumptive
uses. The estimates ranged from “negligible” to 14% of water used.
The remaining 2% of water used in the basin is divided among all other uses
– industrial supply, public supply, domestic use where public supplies are not
available, irrigation, livestock, and navigation. The majority of the water is used
for cooling industrial operations.
Even though the total volume of water use in these remaining categories is
small, ecological impacts can be significant. For example, in the upper watershed,
municipalities or industries often/can withdraw water from a stream, use
it, and then return treated wastewater at a location significantly downstream.
The intervening stretch of river or stream can be dewatered to some degree,
affecting the viability of local and perhaps regional fisheries. Similarly, if
withdrawals are made from ground water, and the water is returned directly to
surface streams, the ecological integrity of the watershed can be degraded
because of reduced base flows in the streams. Such streams can be dry in the
summer, and subject to erosion in wet weather and sedimentation during lower
flow periods.
The cumulative impact of extensive ecological degradation of the basin’s
headwater streams will be a more fragile open lake system that requires
intervention to maintain a healthy fishery and adequate water quality.