LAKE OF THE OZARKS, Mo. — Every winter, Ameren—the utilities company that owns and operates Bagnell Dam, which impounds Lake of the Ozarks—drops the Lake’s water level by about six feet.
It’s called the “winter drawdown,” and its purpose is to prepare the Lake for the inevitable heavy spring rains. Having six feet of wiggle-room gives Ameren a buffer against storms that can bring rain heavy and fast. With 1,150 miles of shoreline and multiple rivers and creeks feeding into Lake of the Ozarks, that’s an enormous drainage area, and heavy rains can raise the water level quickly.
The Big Rain
But sometimes those heavy rains come in January, too; that’s why Lake of the Ozarks is at its summertime water level (between 659-660 feet of elevation, measured at Bagnell Dam), and the dam’s 12 turbines are running at nearly full generation: passing nearly 35,000 cubic feet per second (cfs) as of Thursday morning. Going into last weekend, Bagnell Dam’s Director of Hydro Operations Warren Witt pointed out, the Lake’s water level had been dropping, as Ameren followed the guide curve that gives operators a target to aim at throughout the year. (View the guide curve here.) By mid-January, the guide curve has the Lake level at 657 and falling. “Last Friday morning we were at 658,” Witt said. “The big rain came, and it went up two feet!”
That “big rain” was what drenched the Lake area on Friday and Saturday, before turning to ice and snow. In all, Witt says the area received an average of 4.5 inches of rain, with some areas receiving as much as 6 inches. And all of it came in less than a 12-hour period. Dam operators opened the floodgates, and Truman Dam—which discharges into Lake of the Ozarks—was shut off completely. As a result, Truman Lake rose by 4.5 feet. And Lake of the Ozarks rose to nearly full-pool, cresting at 659.77 feet of elevation before beginning to fall early Sunday morning.
More Flooding Ahead
This week, Truman Lake has been discharging extra water in an attempt to get back to normal levels, and as a result, Lake of the Ozarks has remained in the mid-659 range. But the Lake is under a flood watch again this weekend, as another storm system threatens to bring snow, ice, and rain through the Midwest. Witt says the one to two inches of rain expected are not a major concern, although the additional rain will continue to delay the winter drawdown a bit longer. “How many inches in how many hours” is one of the major factors that determines whether Lake of the Ozarks will be much affected by a particular rainfall event, Witt explained. The other factor is ground conditions, he said: a four-inch rain in the summer would not have resulted in a nearly two-foot rise in the Lake level like last weekend’s storm did. That’s because the ground is drier so it soaks up more rain before major runoff takes place, and the presence of leaves on the trees slows the rainfall as it heads toward the Lake. Winter rain is a different thing altogether, especially if the ground is recently saturated, or worse: frozen.
So Witt’s primary concern about this weekend’s rain is the order in which the precipitation is expected to change forms: it’ll be snow first, then sleet/freezing rain, and then rain. Witt says a significant amount of freezing rain could have a greater impact on the Lake level, as the subsequent rainfall moves quickly across frozen surfaces, toward the Lake. However, he said, “if we start seeing very much rain, Truman will back off on their discharge so we don’t go up very much.
Lake residents and second-homeowners remember grimly the massive flooding in late-December of 2015 that plunged many Lakefront homes into several feet of water. That flood came on the heels of the July 2015 flood that also damaged or destroyed many Lake homes; those two floods rank among the 10 worst the Lake of the Ozarks has ever seen. The way the Lake was developed provides a very slim margin of error for flooding, as evidenced by many homes that were filled with water twice in 2015. So Bagnell Dam operators keep a vigilant eye on the weather forecast, as well as what’s coming into the Lake of the Ozarks from upstream. Truman Lake, by contrast, was built for flood control, and Truman Dam can help lessen flooding at Lake of the Ozarks.
When Will The Lake Level Drop?
“We always shoot to be down at 654 by mid-February,” Witt said. So while these January rains have undone some of the winter drawdown that took place in late-December, that just means Ameren will drop the water level more quickly as mid-February approaches. “How fast we drop the lake is very dependent on continued rains and snowmelt… it’s not unusual at all for it to be mid- to late-January ’til we start to get some good dropping of the Lake,” he pointed out. But as to exactly when the Lake level will begin dropping… it’s tough to say.
Ameren publishes hourly reports of the water level measured at Bagnell Dam and the Osage River, as well as Bagnell Dam discharge amounts. View the hourly reports here.
Lakefront homeowners shouldn’t expect to start seeing more of their seawall for a little while yet: Witt said after this weekend’s rain, Truman Lake will have to drop by several feet, so Ameren will be passing that water quickly through the much larger Bagnell Dam. For comparison: Truman Dam’s generators can pass a maximum capacity of about 15,000 cfs without the use of floodgates; Bagnell Dam’s generators can handle 40,000 cfs.
Plenty of factors will affect how quickly Truman Lake can get down closer to its normal levels, Witt said, but until that happens (normal elevation for Truman Lake is about 706 feet, measured at Truman Dam), Lake of the Ozarks’ water level won’t be moving much.
Regardless, he emphasized, Lake of the Ozarks’ level should be down to 654 by mid-February, and that’s where it will remain until mid-April, when Ameren will allow spring rains to begin refilling the Lake to its target of 659 by Memorial Day weekend, when vacationers return again, for one of the Lake’s biggest weekends of the year.
2019 Flooding Coverage
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2015 Flooding Coverage
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