Lakes & reservoirs
What Is Full Pool? Lake Levels Explained
By the Water Level Atlas team · Last reviewed Oct 21, 2026
"Full pool" is the water level a reservoir is normally meant to hold. On most US Army Corps of Engineers (USACE) lakes it is the top of the conservation pool, which the Corps describes as the lake's normal or target operating level. Above it sits empty space kept for floodwater, so a lake can rise above full pool after heavy rain and still be working as designed.
What are the different pools in a reservoir?
A dam does not hold one single block of water. Operators split the space behind it into layers, often called pools or storage zones, and each layer has a job. Names differ between agencies and projects, but the usual layers, from top to bottom, are:
| Zone | What it is for |
|---|---|
| Surcharge | Extra, temporary space above the flood pool or spillway, used only in very large floods |
| Flood pool (flood control storage) | Kept empty in normal times so it can catch storm runoff and reduce flooding downstream |
| Conservation pool | The working water: water supply, hydropower, recreation, fish and wildlife, downstream flows |
| Inactive pool | Low water generally not used in normal operations |
| Dead pool | The very bottom, generally water that cannot be drawn out through the dam |
"Full pool" almost always means the boundary between the conservation pool and the flood pool.
Here is a real example from the Bureau of Reclamation's 2005 survey of Folsom Lake in California. The survey's capacity table lists surcharge storage between elevation 466.0 and 475.4 ft, joint-use storage (shared between flood control and conservation) between 425.8 and 466.0 ft, active storage between 327.0 and 425.8 ft, inactive storage between 205.5 and 327.0 ft, and a tiny amount of dead storage below 205.5 ft. Other projects use different names, but the idea of stacked zones is the same.
Is full pool the top of the dam?
No. Full pool is an operating target, not a safety limit. The USACE Fort Worth District lists these elevations for Canyon Lake in Texas: top of conservation pool 909 ft, top of flood pool and spillway crest 943 ft, and top of dam 974 ft. So at that lake there are 34 ft of flood storage above full pool before water reaches the spillway crest.
Why is my lake above full pool after heavy rain?
When more water flows into a lake than the dam is releasing, the lake rises. Once it passes the top of the conservation pool it is using its flood storage. The Corps says a lake can be above normal pool and still have plenty of flood capacity left.
Operators do not simply open the gates to drop the lake back to full pool. Releasing too much water too fast can make flooding worse downstream. So they store the floodwater for a while and let it out at a rate the river below can carry, following that lake's water control plan. The lake can stay above full pool for days or weeks after a big storm while the stored water is released. This is normal flood operation, not a sign that something has gone wrong.
Can full pool change with the season?
Yes, at some lakes. A few operators publish different target levels for summer and winter. Lac qui Parle in Minnesota is a USACE example where the winter level is higher. Its water control manual sets the summer conservation pool at 933.0 ft and the winter conservation pool at 934.0 ft (NGVD 1929), and the higher winter level was chosen to reduce fish kills. Many other reservoirs do the opposite and run lower in winter to make room for spring floods. See winter drawdown and guide curves.
So "full pool" at one lake may be a single fixed number, or it may depend on the date. Always check which value the operator is quoting.
What does "feet above or below full pool" mean?
It is the current lake level minus the full-pool elevation, both in feet.
- Example (round numbers): if full pool is 900.0 ft and the lake reads 897.5 ft, the lake is 2.5 ft below full pool. If it reads 903.0 ft, it is 3.0 ft above full pool and is holding floodwater.
The subtraction only works when both numbers are on the same vertical datum. Some lakes have older numbers on NGVD 29 and newer ones on NAVD 88, and the two can differ by a foot or more. See vertical datums.
What do "inactive pool" and "dead pool" mean?
These are the lowest layers. Folsom Lake's table shows how the levels stack, with inactive storage starting at 205.5 ft and only a few acre-feet of dead storage below that.
"Percent full" figures usually leave both the flood pool and the dead or inactive layers out. For example, the Texas Water Development Board says its percent-full figure is based on conservation storage and conservation capacity and does not count water in the flood pool. That is why a lake can show "100% full" while it is actually above full pool. See reservoir storage and percent full.
Using this on Water Level Atlas
Our lake pages show a "vs full pool" figure only when the operator publishes a full-pool level on the same datum as the live reading. Otherwise we show the current level, its change over 24 hours, 7 days and 30 days, and the normal range for the date. If a lake sits above full pool, check the NOAA flood status on the page and follow official warnings. Our methodology page explains where each number comes from.