Lakes & reservoirs

Winter Drawdown: Why Reservoirs Are Lowered

By the Water Level Atlas team · Last reviewed Oct 21, 2026

Many reservoirs are lowered in fall and winter so there is empty space to catch winter and spring storm runoff, then refilled in spring for summer use. Both the Tennessee Valley Authority (TVA) and the US Army Corps of Engineers (USACE) operate many of their lakes this way. But the timing, depth and reasons vary from lake to lake, and some lakes do not draw down at all.

Why are reservoirs lowered in winter?

The main reason is flood storage. In a 2010 review, TVA's Office of the Inspector General spoke with two USACE district offices. The Corps confirmed that, like TVA, it lowers its reservoirs in winter to provide flood control ahead of spring rains, and raises them in summer for recreation and other goals. The report notes that each project has a guide curve that sets this seasonal pattern. See guide curves explained.

A lower lake can absorb more storm runoff and snowmelt before water has to be released downstream.

Are there other reasons for a drawdown?

Yes. Depending on the lake, these can include:

• Weed and vegetation control. At Leesville Lake in Ohio, the USACE water control manual says the lake is drawn down in winter "for flood control and vegetative control." Vegetation had been found to threaten the reservoir's capacity, so the lake is lowered to a level set each year by the Muskingum Watershed Conservancy District, usually near 958.0 ft, compared with a summer conservation pool of 963.0 ft (NGVD). Once the vegetation dies back, the lake refills to summer pool.
• Shoreline and dock work. Lower water gives access to shorelines, docks and structures. A USACE Mobile District operating manual lists "draw-downs because of shoreline maintenance" among the activities that can affect operations at Allatoona Lake in Georgia.

When does drawdown start and how far does the lake drop?

It depends entirely on the lake. Each operator sets its own schedule, usually in the lake's water control manual or operating policy. The TVA Inspector General's report makes the point that lakes have different shapes, so "the drawdown amounts, in terms of elevation, will be different for each reservoir." A five-foot drop on a broad, shallow lake affects the shoreline very differently from a five-foot drop on a deep, narrow one.

Weather also changes the plan. The Leesville manual notes that when two nearby lakes are drawing down, "any rain results in both lakes having to close back for flood control," which stretches out the drawdown.

When do lakes refill?

Refill normally happens in late winter and spring, as rain and snowmelt arrive. Operators often have a target level to reach by a set date. At Allatoona Lake, for example, the USACE manual says that in a dry year operators make decisions to ensure refilling the reservoir to 840 ft NGVD29 by May 1.

Refill is not guaranteed. In a dry spring the lake may stay below its summer target, and operators may cut releases to save water. Your lake's operator will usually post updates when this happens.

Do all lakes draw down in winter?

No. Some lakes hold a steady level all year. Others do the opposite of a drawdown.

Lac qui Parle in Minnesota is a USACE example. Its water control manual sets a winter conservation pool of 934.0 ft, one foot higher than the 933.0 ft summer pool, to reduce winter fish kills. The manual also records that an older, deeper winter drawdown was dropped after analysis showed it "was causing environmental damage while not significantly reducing downstream flooding."

Policies also change over time. TVA's 2004 reservoir operations policy raised summer and winter target levels at some tributary lakes to improve recreation. The Inspector General notes that higher winter levels reduce the flood storage available.

What should lake users check?

• The operator's published schedule or guide curve for your lake.
• Notices about delayed drawdown or refill.
• Whether elevations are on the same datum as other numbers you are comparing (see vertical datums).

For low water effects on boating, see low-water boating hazards, and follow your operator's and local authorities' notices.

Using this on Water Level Atlas

Our "normal range for the date" on lake pages comes from this lake's own past years, so a regular winter drawdown shows up as a seasonal dip in the normal band. If today's level sits inside that band in January, the lake is probably following its usual pattern. If it is well below, compare it with the same date in past years and check the operator's notices. You can also set an email alert for when the lake goes above or below a level you choose.