2018 Chehalis Basin Gage Height Conversion Project

 (updated 10-31-2018)

The Chehalis River Basin Flood Authority, United States Geological Survey, and Washington State Department of Ecology undertook a project in 2018 to improve the accuracy of flood information in the Chehalis River Basin. 

These organizations initiated a Chehalis Basin Gage Height Conversion Project to improve the accuracy of reported river height information.  More specifically, 27 river and stream gages in the Basin were resurveyed using the same gage height measurement standard (called "NAVD88").  Doing so better enables gaging interests to communicate and consistently share river height information, as well help with addressing issues associated with historically varied and inconsistent measurement methods used in the Basin.

While updating gage height measurements provide the Basin with many benefits (e.g., improved public safety, timelier flood preparation and response, potentially lower flood insurance premiums, greater inter-agency information sharing accuracy, etc.), it may also generate some confusion.  In anticipation of this, the Chehalis River Basin Flood Authority launched this Chehalis Basin Gage Height Conversion Project website as an information resource to help Basin interests better understand the new gage heights. 

Please explore the following links to learn more about the Chehalis Basin Gage Height Conversion Project:

A. Interactive Map of Affected Gages

B. Affected Gages (presented Upper Basin to Lower Basin)

C. Gage Height Conversion Table

D. Frequently Asked Questions

E. Additional Resources

F. More Information

Following table identifies stream gaging stations covered by the Chehalis Basin Gage Height Conversion Project.  For each gage, conversion factors are provided to convert old gage heights to equivalent new gage heights. 

Note: Above table of new gage height conversion factors is also available as a download here.

Q1.  What is NAVD88 and what are the key benefits of converting to NAVD88?

Answer:

A vertical datum is a method surveyors, engineers, and mapping professionals use to measure land elevations and water depths.  NAVD88 (North American Vertical Datum of 1988) is the official vertical datum of the United States, having superseded the older NGVD29 (National Geodetic Vertical Datum of 1929).  NAVD88 is a geodetic datum meaning it is based on physical land measurements (as opposed for example to more sophisticated and advanced satellite measuring systems).  Read more about NAVD88 here.  View an informative explanatory graphic from the Massachusetts Office of Coastal Zone Management here.

Converting to NAVD88 creates consistency across all elevation products in the Basin, including NWS flood forecasts, USGS stream gage data, topographic maps, flood inundation maps, FEMA flood insurance maps, hydraulic models, etc.

Q2.  Why is the Chehalis Basin Gage Height Conversion Project important to me?

Answer:

Having all elevation products throughout the Basin using the same method for determining elevation reduces the chance for error and misinterpretation of elevation information.  Accurate and consistent data are critically important to effective emergency response during flood events.

Q3.  How do I convert old gage height numbers I'm familiar with to new gage height numbers?

Answer:

Following websites provide additional, helpful information to better understand when Chehalis Basin rivers and major tributaries will flood.

• Lewis County Flood Phases -- Click here.

• Flood Warning System and Email Gage Alert System -- Click here.

• Thurston Community Alert -- Click here.

• NWS 10-Day Meteorological Forecast -- Click here.
  

• NWS Chehalis River Basin Rivers and Major Tributaries Forecast -- Click here.
  

• NWS Chehalis Basin Rain Radar -- Click here.
  

• WSDOT Road Conditions -- Click here.
  

• Atmospheric Rivers -- Click here.