Every elevation on a USGS reservoir gauge is measured against a vertical datum — a theoretical reference surface that approximates mean sea level. The two datums you'll encounter on US lakes are NGVD29 (the National Geodetic Vertical Datum of 1929) and NAVD88 (the North American Vertical Datum of 1988).
NGVD29 was the official US standard from 1929 until the late 1980s. It anchored sea level at 26 tide-gauge stations along the US and Canadian coasts and propagated elevations inland from there. NAVD88 replaced it with a single primary anchor (Father Point/Pointe-au-Père on the St. Lawrence) and a much denser network of GPS-tied benchmarks. The two datums disagree because the earth's geoid isn't a perfect ellipsoid — at any given point in the continental US, the same physical lake surface has a different "elevation" depending on which datum you measure it against.
The disagreement varies by location, from about −1.5 ft in the Pacific Northwest to about +5 ft along the Gulf Coast. For most of the continental US it's in the −1 to +3 ft range. That's small compared to a reservoir's drawdown range, but it's not zero — at Lake Lanier, the NGVD29-vs-NAVD88 offset is around 0.5 ft, which is enough to confuse you if you're comparing Lakebrief's reading against a USACE bulletin published in a different datum.
Why does Lakebrief display each lake's datum prominently? Because operating agencies and USGS gauges don't always agree:
- The USGS gauge for Spruce Run reports parameter
62614, NGVD29 ft. Full pool from NJWSA is also published in NGVD29. Consistent. - The USGS gauge for Lake Mendota reports parameter
62615, NAVD88 ft. The Wisconsin DNR publishes target levels in NGVD29. The two are within a foot of each other but not identical — Lakebrief's water-level UI shows the live NAVD88 reading and the comments in the source file note the small datum offset. - USACE traditionally publishes Lake Tahoe in BOR Datum (US Bureau of Reclamation) — yet another datum used by some western reservoirs. Lakebrief's Tahoe entry uses the gauge offset from BOR datum and labels the displayed value accordingly.
For sailing decisions, the datum doesn't matter — what matters is "how far below full pool am I right now?" Both numbers come from the same datum, so the difference is correct regardless. Where the datum matters is when comparing against external sources: a fishing report quoting "the lake is at 850" can mean different things depending on the datum the author had in mind.
NOAA publishes a tool called VERTCON that converts between the two datums for any latitude/longitude. It's the right reference if you ever need to translate.