Should Cobb residents be concerned about a reservoir proposed way upstream on the Chattahoochee River in Hall County near Gainesville? Possibly so.

Plans for the Glades Reservoir in Hall County and also the Bear Creek Reservoir in south Fulton mean the Chattahoochee is in imminent danger, according to the Washington, D.C.-based American Rivers organization. Cobb County gets a bit more than half (around 45 million gallons per day) of its water supply from the Chattahoochee, which runs for 22 miles along its border with Fulton County. (The rest is drawn from Lake Allatoona, fed mostly by the Etowah River.)

American Rivers claim the two dams are “boondoggles” that pose such a radical threat to the environment. And they ranked the Hooch as No. 3 on its annual list of the 10 most endangered rivers in the country.

American Rivers also contends that the dams are overly expensive, reduce recreational opportunities (though no doubt creating alternative ones) and would harm habitat for fish and wildlife.

“It drowns the river upstream,” American Rivers’ director Jenny Hoffner said of dams. “It’s no longer a river. You are changing the ecology completely … It completely changes it from being a flowing river to being a lake, which has a completely different assemblage of wildlife.”

The group also contends that the lakes would translate to more evaporation, which might in fact be the case after shady riverbanks are replaced with broad expanses of unshaded lake exposed to the summer Georgia sun. Slow-moving lake water also evaporates faster than river water, Hoffner points out. Her group argues that five million gallons of water per day might be lost via evaporation at the two proposed dams. But no such analysis has actually been done as yet.

While all that might just be an attempt at headline-grabbing by an attention-hungry watchdog group that seems to have never met a dam it liked, there remains the possibility that the Hall County dam could negatively impact Cobb’s water supply. But in the absence of any comprehensive modeling of the Chattahoochee watershed for the proposed dams, it is impossible to determine the Hall dam’s actual impact on Cobb, according to Cobb-Marietta Water Authority director Glenn Page. As long as the releases from Lake Lanier and Morgan Falls dam are sufficient to keep Cobb’s intake valve (just downstream from Morgan Falls) “wet,” Cobb will be fine. If, for some reason, the river level drops below the level of the valve, it would be a different story.

“We share American Rivers’ appreciation of the resource, but also need to be mindful that our population centers, which drive our economy and quality of life, have to put their straws into something,” Page told the MDJ. “If managing the resource to meet Georgia’s needs in the most efficient and cost-effective manner, while observing all environmental regulations and minimizing impacts on other users, includes constructing a few more reservoirs, then we should do it.”

Construction of more dams for drought-relief is a valid and, most likely inevitable step for many Georgia localities. Yet even so, it is in Cobb’s interest to keep a wary eye upstream.