Why two sites show a different lake elevation

Two pages can quote the same reservoir on the same morning and disagree by a foot without either of them being wrong. There are three reasons, and they are easy to separate once you know what to look for.

1. A different vertical datum

An elevation is a height above a reference surface, and the United States has more than one. The current national vertical datum is NAVD 88; older records and older engineering drawings often use NGVD 29. The gap between them is not one number: it varies with location, so the two datums do not convert by adding a constant. The National Geodetic Survey documents both at ngs.noaa.gov/datums/vertical.

A site that mixes datums across lakes produces a table where the numbers look comparable and are not. This site prints the agency figure for each lake without shifting it, and where a page derives a depth it subtracts two elevations that are on the same datum.

2. A different gage

A reservoir is not a flat sheet. Wind, inflow and release points move the surface, and the gage that produces the number sits at one place. For Lake Powell the reading comes from USGS site 09379900 at Glen Canyon Dam, parameter 62614, the reservoir water-surface elevation series. A site quoting a different site number is quoting a different point on the water.

Storage-based figures add a second step. Percent full is storage divided by capacity, and storage comes from a rating table that converts elevation to volume. Two sites can agree on the elevation to the hundredth of a foot and still disagree on percent full.

3. A different hour

USGS instantaneous gages update every 15 to 60 minutes. Daily reservoir storage summaries are published once a day. A page built at 09:00 and a page built at 15:00 can legitimately differ, and a page that caches its build for a week can be six days behind while looking live.

Every reading on this site carries its own timestamp, and the lake pages print it next to the gauge. Check the timestamp before you check the number.

Applying it to a lake here

Lake Powell reads 3,536.20 ft today. Full pool is 3,700 ft and dead pool is 3,370 ft, so the surface is 166.2 ft above the dam's hard floor. The Lake Powell depth page shows the same elevation with the depth it implies at the dam, and the dead pool guide explains the two operating floors the number is measured against.

Updated October 28, 2026.