Why are lake levels dropping so quickly?
WHY ARE LAKE LEVELS DROPPING SO QUICKLY?
Article by Clyde Morris, LLA Vice President
Lately, we’ve been getting a lot of questions about why the lake level has dropped so much and so fast. Of course, we all know that when it doesn’t rain, the lake goes down. And as of this writing, river gauges tell us the Chattahoochee and Chestatee Rivers are supplying less than 350 cfs of inflow to Lake Lanier – a little more than half their average over the last year. But meanwhile, releases from Buford Dam are running 1,400 cfs. That difference between inflow and release volumes obviously explains the dramatic drop in Lanier’s level.
You may have heard folklore about the Corps “dropping the lake” back in the 60’s, 70’s, and 80’s during the fall to make room for all the rain that typically occurs during the winter. So, is the Corps “dropping” the lake now in preparation for the rainy season, and if so why so much and so fast? The answers may surprise you.
The Corps manages the Apalachicola-Chattahoochee-Flint River System (ACF) for multiple purposes, including not only recreation but water supply, water quality, hydroelectric power generation, flood control, downstream navigation, and fish and wildlife management. So, support for recreation is subject to lots of other considerations. The Corps has devised a set of rules that govern how it operates the dams in the system to support these diverse purposes. Here’s a primer.
The Corps’ rules are set to ensure minimum flows across the state line into Florida in order to protect four threatened or endangered species: the Gulf sturgeon and three species of mussel. Minimum discharges from Jim Woodruff Dam (the southernmost of the Corps’ three conservation storage reservoirs in the ACF) are determined according to three variables: composite storage in all three conservation reservoirs (divided into five level ranges or “Zones”), ACF Basin Inflow, and season of the year.
Releases are regulated according to sets of Basin Inflow threshold levels for each of the three seasons – Gulf sturgeon spawning season (March through May), non-spawning season (June through November), and winter (December through February). The specific thresholds and releases are contained in Table 7-2 of the ACF Water Control Manual (below). Column 3 of the table displays minimum releases from Jim Woodruff Dam and column 4 shows how much Basin Inflow can be retained for refilling the conservation reservoirs.
During the spawning period (March – May), the Corps regulates releases to minimize or avoid potential Gulf sturgeon “take” (death). It uses one set of thresholds for Zone 1 and another for Zones 2 and 3.
During the non-spawning season (June – November), the Corps uses a different set of thresholds and corresponding releases for Zones 1 – 3.
During the winter season (December to February), only one Basin Inflow threshold and corresponding minimum release (5,000 cfs) are used for Zones 1 – 3. No Basin Inflow storage restrictions are in effect as long as this minimum flow is met under the prescribed conditions. In theory, this provides an opportunity to refill the storage reservoirs during the rainy season.
In Zone 4, only the minimum release of 5,000 cfs is maintained, regardless of season. If storage drops into the Drought Zone, the minimum release is reduced to 4,500 cfs regardless of season.
What’s Happening Currently
To put these rules into context, we start with ACF composite conservation storage, which dropped into Zone 2 in early September. ACF Basin Inflow was 14,000 cfs on September 1 but dropped below 10,000 cfs on September 6 and ultimately to 2,000 cfs by September 13; the current rate is 5,000 cfs. According to Table 7-2, releases from Woodruff after September 6 should have been equal to Basin Inflow, with a minimum of 5,000 cfs. But releases from Woodruff didn’t drop below 10,000 cfs until September 14 and are still running over 8,000 cfs. Why, you ask? Because of one little-known but very important detail.
The Corps will not allow the Apalachicola River to drop more quickly than it deems safe for protection of the threatened and endangered species. The rate at which it ramps down the discharge – the “ramp rate” – is calculated as the difference between the daily average river stages of consecutive calendar days. The Corps sets the maximum ramp rate at three to six inches of drop per day.
OK, so we get why the Corps has continued to send so much water across the state line to Florida. But while total ACF Basin Inflow was dropping precipitously between September 1 and 13, Lanier inflows averaged a mere 771 cfs while releases from Buford Dam averaged more than 2,500 cfs – much more than was coming in and even more than total ACF Basin Inflow. So why was the Corps letting so much water out of Lanier?
Because Lake Lanier holds about 65% of the conservation storage in the ACF, our lake has to supply whatever additional water is needed to support down-ramping until releases no longer exceed Basin Inflow (or the minimum of 5,000 cfs). And in the process, the Corps seeks to keep the levels of all three conservation reservoirs in the same storage Zone. So, the Corps releases as much water as is required to keep West Point and Walter F. George from dropping into a lower zone than Lanier.
If not for the ramp rates, the Corps could have reduced the amount of water it released from Buford Dam much more quickly, thereby slowing down the rate at which the lake was dropping. Unfortunately for us, Lanier is the resource of last resort for the ACF system, and that’s why the Corps was emptying so much more water over the last few weeks than was flowing in.
And there you have it: it’s not as simple as just dropping the lake before the rainy season or cutting releases to match the amount of water coming into Lanier.
Those who followed LLA’s efforts in the Water Wars may recall that we fought against the ramp rates the Corps proposed for its update of the Water Control Manual (finalized in 2017). Our thinking was that the threatened and endangered species thrived for millennia with large natural variations in the day-to-day flows of the Apalachicola River and therefore do not need artificial ramp rates to protect them. Unfortunately, the U.S. Fish and Wildlife Service insisted on the ramp rates, so what we see happening now is that Lake Lanier is supplying a disproportionately large portion of the water those rates require. And that’s why lake levels are dropping so much more quickly than they otherwise would.