Learn More: River Water Levels

What does this mean?

Water levels in rivers typically follow rainfall patterns, rising and falling during periods of wet weather and drought. By examining seasonal and year-to-year data, one can get a picture of how recent flood or drought events compare to historical patterns.

  • Latest Value is the most recent data available for the water body.
  • The Historic Norm for the Month is the average (mean) of all values for this water body for the current month over the entire period of record of the data.
  • Historic Range is generated by reporting the minimum and maximum values for this water body over the entire period of record.
  • FEMA Values may include NHWE Flood and/or FEMA 100-year Flood values, whichever are available:
    • NHWE stands for "Normal High Water Elevation" and refers to the customary high-level water level of a water body. It is usually defined by the public works department of local government and is defined as the landward edge of a water body during normal hydrological conditions. It is usually determined by examining vegetation patterns and high-water marks on trees.
    • The FEMA 100-year Flood elevation is defined by the Federal Emergency Management Administration as the water level which has a one percent chance of being reached or exceed in any single year.

How are the data collected? (Methods)

River water levels are measured at stationary gages which may be read manually by visually inspecting a ruler-like gage, or periodically transmit their data to a receiving station or via a communications network. Both manual and automatic types must be calibrated at installation to a known reference point called a vertical datum. The two datums usually used are NAVD88 and NGVD29. For more about vertical datums, visit the National Geodetic Survey's website.

Fun Fact: Why does the USGS use the spelling ‘gage’ instead of ‘gauge’?


To convert an elevation value from one vertical datum to another, use the National Geodetic Survey's NGS Coordinate Conversion and Transformation (NCAT) tool. You will also need to know the latitude and longitude of the gage's location.

Caveats and Limitations