How Lake Levels Are Measured

A lake or reservoir level is the height of the water surface above a fixed reference called a vertical datum. Most US reservoirs report elevation in feet above mean sea level, but the exact datum varies: some use NGVD 29, some NAVD 88, and some older projects use a local or historical datum. Two numbers measured against different datums can differ by a foot or more even when the water is at the same height, so this site always shows which datum a reading uses and never mixes series measured against different datums.

What the gauge records

Agencies such as the U.S. Geological Survey (USGS) and the U.S. Army Corps of Engineers operate gauges — typically a pressure sensor or a float in a stilling well near the dam — that record the water surface every 15 to 60 minutes. Some reservoirs, especially in the western US, are reported once per day.

Provisional and approved data

Recent readings are provisional: they come straight from the instrument and can be revised after the agency reviews them. Approved data has passed that review. We label provisional readings and update them if the agency revises them.

Why the latest reading may not match what you see

• Readings arrive with a delay; the time shown is when the agency measured the water, not when you loaded the page.
• Wind can pile water up at one end of a large lake (“setup”), and waves cause short-term noise.
• Equipment outages leave gaps. We show gaps as gaps — we never fill them in or treat missing values as zero.