How reservoirs are managed
A reservoir's level is not left to the weather. Every dam has an operating plan that says what elevation to aim for on each day of the year, when to store water and when to release it, and who gets it. Knowing the plan explains most of what you see on the lake pages here.
The water control manual
For a Corps of Engineers project the plan is the water control manual, a document approved for that specific dam that sets the pool elevations, the rule curve and the release rules for floods and droughts. River authorities and cities that own their own dams have equivalent operating plans, and in Texas the state's watermaster program supervises releases on many river basins. These plans change rarely, through a public process, which is why the pool elevations on a lake page can stay the same for decades.
The rule curve
The heart of the plan is the rule curve, sometimes called the guide curve: a target elevation for each day of the year. On a typical flood-control lake it rises through spring to a summer pool, holds there through the recreation season, and drops in autumn to a lower winter pool so that the flood pool is as empty as possible before spring rains. Operators release water to follow the curve down and hold it to let the lake follow the curve up. When the lake sits above the curve after a storm, releases increase; below it in a dry year, releases are cut to the minimum the plan allows.
Multiple purposes, ranked
Most large reservoirs are authorised for several purposes, and the manual ranks them. Flood control almost always comes first: the flood pool is evacuated as soon as downstream channels can take the water, even if that means the lake drops below where boaters would like it. Water supply is next, delivered under contracts to cities and utilities that hold rights to a share of the conservation pool. Then come hydropower, navigation flows on river systems, minimum environmental releases, and recreation. Recreation is a real purpose and is why summer pools exist, but it ranks last when purposes conflict.
Floods
During a flood the operator's job is to store inflow that would otherwise overwhelm the river downstream. The lake rises into the flood pool, and releases are held at or below what the downstream channel can carry without damage, a figure fixed in the manual. Once the river recedes, releases are raised to the maximum safe rate to empty the flood pool for the next event. That is the reason a lake can stay several feet above full pool for weeks after the rain stops: the outlet works can only pass so much.
Droughts
In a drought the plan runs the other way. Releases fall to the minimum needed for downstream water rights and environmental flows, and the lake declines with evaporation and supply withdrawals. Many plans have drought triggers at set elevations that cut allocations in stages, which is why cities announce water restrictions tied to specific lake levels. The lake pages here show the bottom of the conservation pool so you can see how much working storage remains.
Systems of lakes
Where several dams sit on one river, they are run as a system. The White River lakes in Arkansas and Missouri, the Tennessee and Cumberland chains, the Columbia and Missouri main-stem dams are each operated together, with releases from an upstream lake timed against the storage available below it. A lake rising when it has not rained locally is often taking releases from upstream.
Where to see the plan for a lake
Each lake page links to the agency's own page for that project, and Corps district sites publish the current rule curve and release schedule. If you want to know why your lake is where it is, that curve is the answer nine times out of ten. For the seasonal pattern in particular, read the drawdown schedule guide.