Sea Level Variations of the United States 1854–2006

The United States National Water Level Network (NWLON) was established in the 19th Century to ensure the Nation’s nautical charts, shoreline maps, and elevations relative to homes, levees, and other coastal infrastructure were accurately referenced to sea level. In support of this mission, NOAA’s Center for Operational Oceanographic Products and Services and its predecessors have determined sea level for the United States since the mid 19th Century. While climate change was not a concern during the mid-1800s, the accurate determination of sea level was critical for navigation and marine boundary determination. To meet these important requirements, technology, procedures, and processes were developed to the highest scientific and engineering standards.

At the turn of the 20th Century it was realized that there was a need to account for a rise in sea level and the first National Tidal Datum Epoch was established. Today this Epoch is updated every 20 to 25 years. The Supreme Court recognized these standards and procedures in the landmark 1936 case of Borax, Ltd v. City of Los Angeles when legally defining sea level. Due to those initial efforts and the continued dedication of those charged with the responsibility for monitoring sea level for the United States, we can accurately determine relative (local) mean sea level along the Nation’s coastline today. These observations also play an important role in monitoring change in global sea level.

As we monitor change in sea level into the 21st Century, the statement made by Alexander Dallas Bache, the Second Superintendent of the Coast Survey, is as relevant today as when it was stated more than 150 years ago, “It seems a very simple task to make correct tidal observations; but, in all my experience, I have found no observations which require such constant care and attention” (1854).

Michael Szabados, Director, Center for Operational Oceanographic Products and Services