How to read a lake elevation

Elevation, not depth

Dam operators report the height of the lake's surface in feet above a fixed reference surface called a vertical datum. For most U.S. reservoirs that datum is roughly mean sea level, so the number is a lake's elevation above the sea: Lake Mead's full pool is about 1,229 ft, Lake Powell's about 3,700 ft, and many Southeastern lakes sit a few hundred feet up.

The number tells you nothing about how deep the water is. What matters is the change: if the reading falls from 1,071.0 to 1,068.0, the water surface is three feet lower everywhere on the lake, at the dam, at your dock and at the ramp.

Why you cannot compare two lakes by elevation

A lake at 600 ft is not lower than a lake at 1,000 ft in any useful sense. Each reservoir has its own full pool, its own shape and its own normal range. To compare lakes, compare each one with itself: distance from its own full pool, percent of its own storage, or how today ranks against its own past years on the same date. That is how the ranking orders lakes.

Even feet below full pool can mislead between lakes. A shallow lake with a broad floodplain loses a lot of surface area per foot, while a deep canyon reservoir can drop ten feet and still look nearly full from the water. Percent of conservation storage, shown where the agency publishes it, is the fairest single number.

The datum on each lake page

Every lake page names the datum its readings use. Across the 340 lakes on this site:

NGVD 1929

The National Geodetic Vertical Datum of 1929, the older national reference. Most dams were designed and surveyed on it, so most Corps lakes still report on it, often written “ft NGVD” or simply “ft msl”.

NAVD 1988

The North American Vertical Datum of 1988, the newer reference used for most surveys and maps made since the 1990s. The same point has a different number on each datum. The difference depends on where you are and runs from near zero to a few feet across the country. NOAA's free coordinate conversion tool converts between them for any location.

Local project datum

Some projects report against their own reference. The numbers can still look like sea-level elevations, but they are only reliable when compared with other numbers from the same agency for the same lake: the full pool, your dock elevation from the project office, or the lake's own past readings on this site.

Why this matters for your dock or property

If you compare a lake reading with an elevation from a survey, a plat, a flood map or a GPS app, make sure both are on the same datum. Mixing NGVD 1929 and NAVD 1988 can put you off by a foot or more, which is the difference between a floating dock and a grounded one. The simplest way around it is to measure your dock against the lake itself, explained in how to find your dock or ramp elevation.

Provisional readings

Readings arrive from the gauge as provisional data. Agencies occasionally correct them later, for example after recalibrating a sensor. We read the latest values on every update, so corrections show up here automatically. A sudden spike or drop of many feet that reverses within hours is almost always a sensor glitch; we set those aside as described in how we measure.

More lake level guides

• What “full pool” means: Full pool, flood pool and dead pool: the layers of a reservoir, and why a lake above full pool is not a full lake.
• Why lakes are lowered in winter: Why many Corps lakes drop several feet every fall on purpose, and which ones are drawn down the most.
• Will my dock or ramp be dry?: A tape-measure method to find the elevation of your dock or ramp, and what to watch for when the water is low.
• Who decides how full a lake is: How the Corps of Engineers and Reclamation decide how much water to hold or release, and why a lake can fall after rain.