When a reservoir is reported as "twelve feet low", the number is almost never a depth. It is the gap between the current surface elevation and the reservoir's full-pool elevation, and full pool is measured above mean sea level. That distinction is the single most common source of confusion in reading lake level reports.
A reference table of 44 US reservoirs across 12 states makes the point immediately. Full-pool elevations in it run from 172 feet at Toledo Bend Reservoir to 3700 feet at Lake Powell. Both figures are correct and neither tells you anything about how deep the water is. A reservoir sitting at 3700 feet is not deeper than one at 172 feet; it is higher up. Depth depends on the shape of the valley that was flooded, which the elevation figure does not encode at all.
This is why a percentage is usually more informative than a level, and why "down ten feet" means very different things on different lakes. On a steep-sided canyon reservoir, ten feet of elevation is a modest volume. On a broad, shallow impoundment, the same ten feet can expose a large area of lakebed and strand every boat ramp on the shoreline.
The second thing the table shows is that these numbers do not all come from one place. Of the 44 reservoirs, 10 publish through a USGS gauge, 12 through California's CDEC network, and 3 through the Bureau of Reclamation. The remaining 19 have no live public feed in the table at all. That is not an oversight in the data; it reflects how US reservoir reporting actually works. Operators differ too, with 17 of the 44 run by the Army Corps of Engineers and the rest split between Reclamation, interstate and international bodies, and local water control districts.
The practical consequence is that there is no single authoritative national feed to check. Update cadence, datum, and even the definition of full pool can differ between operators, so two reservoirs that look directly comparable in a summary table may be reporting on different terms. If you are comparing across states, it is worth knowing which agency publishes each figure before drawing any conclusion from the difference.
Current levels for these reservoirs, with the operator and gauge behind each number, are at lakelevelnow.com.