Why Lakes Drop in Winter
Many reservoirs are lowered on purpose in fall. Here is why, and how to tell a planned drawdown from a drought.
Short answer: Many reservoirs run by the Corps of Engineers and TVA are lowered on purpose every fall. The empty space holds winter and spring floods. In spring the lakes are filled again toward their summer level.
Room for floods
A reservoir that is full in winter has no space to catch a big storm. Operators of flood-control lakes follow a guide curve: a target level for each day of the year. The curve drops in fall, stays low through winter and rises in spring, when the risk of floods eases and the water is needed for summer.
The size of the drop depends on the lake. Some fall a few feet; tributary reservoirs in the mountains can drop tens of feet between summer and winter.
Texas is different
Most Texas water-supply reservoirs do not follow a winter drawdown. Their level is set by rain and use, so a low lake in Texas usually means drought. The pass-through lakes of the Highland Lakes, like Lake LBJ, are kept near a constant level, but they can be lowered for a few weeks in winter for dock and shoreline work when LCRA announces it.
Drawdown or drought?
Look at the same date in past years on the lake page. If the lake is about as low as it is every winter, it is the planned drawdown. If it is far lower than the same day in most years, it is a dry year.
Where this data comes from
General operating practice of the US Army Corps of Engineers and TVA. Check each operator’s own guide for the exact targets of a lake.
Lake Levels Daily is an independent site, not affiliated with TWDB, LCRA, USACE, USGS, NOAA, TVA or any lake operator. Check official sources before making safety decisions.