Dead pool and minimum power pool
A big reservoir has more than one bottom. The streambed is the physical bottom. Above it sit two operating floors: minimum power pool, where the turbines stop, and dead pool, where water can no longer reach the outlet works at all. On a dam page the three numbers answer three different questions, and only one of them is about boats.
The two floors, defined
Minimum power pool is the surface elevation below which a hydroelectric plant can no longer generate. The penstocks that feed the turbines sit above the lower part of the storage band, so once the surface drops under that line the generators come off. The dam still holds water. It just cannot turn it into electricity.
Dead pool is the lower line, the elevation below which water can no longer pass the dam. The outlet works sit below the penstocks, and nothing else takes water downstream. Dead pool is therefore the hard floor of the usable reservoir: below it the dam stops being a dam and becomes a wall.
The published figures
These elevations come from the operating agency, not from an estimate. For Glen Canyon Dam the figures are the Reclamation operating landmarks; for Hoover Dam they are the same set of Reclamation landmarks on the lower Colorado.
Where the water sits today
The gap between the current surface and each line is the number that matters operationally. It is recomputed from the live reading every time this page builds, so it cannot drift.
Why the two lines are not the same line
The order is fixed by the plumbing, not by policy. Full pool is the top. Minimum power pool is where generation stops. Dead pool is where release stops. Between the two lower lines the dam can still pass water downstream through the outlet works while producing no power, which is why a reservoir can be "dead" for hydropower and alive for the river.
The gap is not the same size on both dams. On Lake Powell the power floor sits 120 ft above dead pool. On Lake Mead it sits 55 ft above it, because the two dams were built with different outlet and penstock elevations.
See the live gap on each lake
The Lake Powell page recomputes the buffer above both lines on every build, and prints the streambed elevation so you can see how much water column is left at the dam: Lake Powell water depth at Glen Canyon Dam. The lowest daily pool since the reservoir first filled was 3,516.6 ft, which is 147 ft above dead pool. For the opposite end of the same river system, the Lake Mead water level chart shows the same band against Hoover Dam.
Elevations on this site are the agency's own datum, printed as the source states them. If two sites quote different numbers for the same lake on the same day, read why two sites show a different lake elevation before you assume one of them is wrong.
Updated October 28, 2026. Figures re-read from the site's own band constants at build time.