How to read a USGS lake and reservoir gauge

The U.S. Geological Survey runs thousands of water monitoring stations across the country. Most sit on rivers, but several hundred measure lakes and reservoirs. Each station records the water surface every 15 minutes or so and sends the data in by satellite or cell modem, usually within an hour.

The site number

Every station has a number, such as 02334400 for Lake Sidney Lanier near Buford, Georgia. Short numbers (8 digits) follow a river-basin order. Longer 15-digit numbers encode the latitude and longitude of newer sites. On Lakeglance the gauge number is listed on each lake’s water level page, with a link to the official USGS page.

Parameter codes you will see

USGS labels each kind of measurement with a five-digit code:

• 00062: reservoir water surface elevation above a datum, in feet
• 62614: lake or reservoir elevation above NGVD29
• 62615: lake or reservoir elevation above NAVD88
• 00065: gage height, measured against a local reference point at the station
• 00010: water temperature, in degrees Celsius
• 00035 and 00036: wind speed and direction, at the few lake stations with a weather mast

Elevation codes can be compared with dam documents and ramp elevations. Gage height cannot; it only tells you whether the water is rising or falling at that station. See gage height.

Provisional data

Recent readings are marked provisional. USGS hydrographers review them later, and a small share get corrected, for example when a sensor drifted or ice formed around the intake. For day-to-day boating the provisional numbers are reliable, but treat a sudden one-hour spike or a flat line with some suspicion.

Daily values

USGS also publishes a daily mean for each day once it is complete. Lakeglance uses daily means for the 30-day chart and for the monthly normals, which keeps short wind-driven spikes from distorting the trend.