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Earth Sense: Water transfers can alter drainage basins

POSTED: November 25, 2012 1:00 a.m.

The water discussions in the Gainesville and Atlanta areas often bring up the term “drainage basin,” which is the same thing as a “watershed.”

A standard definition describes this as a system containing of a trunk or main river and its tributaries. Ideally, a drainage basin has the shape of a tree leaf, with the trunk river collecting water from the tributaries that flow into it at an angle.

The outer boundaries of the drainage basin are called divides. A divide can be a mountain or simply a ridge. Any elevation is a divide when it makes water run off in different directions on its sides. Water flows downhill on the earth’s surface, so the tributaries come together toward the trunk stream.

What’s often overlooked is that the groundwater system is part of the watershed. Water in the bedrock migrates slowly downhill until gravity forces it to emerge from beneath the bottom of the river, thus feeding the river. Withdrawal of water from the river results not only in a drop of the stream level, but also a decline in the groundwater table around it.

When water is taken from a reservoir, used by a municipality and then discharged in a different drainage basin than the one in which it started, an interbasin transfer is taking place.

Two local examples may serve. The planned Glades Reservoir above Gainesville will supply water to Hall County, and return it to the Chattahoochee River. The risk of interbasin transfer is small. But water taken from Glades is water that would have otherwise supplied Lake Lanier. Lanier is already in a delicate situation because it is fed by the smallest watershed of any in the U.S. serving a major city.

The Shoal Creek reservoir proposal is much worse. Damming up this tributary of the Etowah Basin would be transferring huge amounts of water to Atlanta, and thus the Chattahoochee.

 Interbasin transfers tend to dry up the areas where water is withdrawn and lower the groundwater table.

 In addition, Shoal Creek is home to at least two species of rare fish under federal protection, not found anywhere else in the world.  

With an estimated price tag of $650 million, this project has plenty of alternative solutions to the water needs of the area, such as conservation, improvements in usage efficiency and construction of cisterns to collect rainwater and reduce surface runoff.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears on Sundays.


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