River Stages Are All Relative. Einstein Was Right!

River stage is an important number. Communities relate to that number and take a variety of actions based on it. What happens when the frame of reference for that number changes? What are the effects that ripple through the community? Communities in the Chehalis River Basin in west central Washington are finding out. This is their story or maybe just the start.

If you live near a major river, especially one like the Chehalis River where the National Weather Service provides river forecasts at several locations, you may be familiar with the terms river stage and flood stage and what they mean to your community. And, it’s not just you. Other residents, businesses, ranchers, emergency managers, school officials, police and fire officials, radio stations, television stations, newspapers, and transportation departments are all aware of these terms. In fact, this awareness is likely to span your community and that knowledge has been engrained in river related activities for many decades.

Reporting river stages is more complex than it first sounds and it turns out Einstein was right. Everything is relative.

River stages are measured relative to a vertical datum. A vertical datum is a base measurement point from which all elevations are determined. The problem is gage datums can change. Vertical datums managed by the USGS, for example, depend on assumptions about the precise shape of the earth. These assumptions can change overtime as our ability to make detailed measurements of the earth’s shape improves.

Some river stages are reported relative to the National Geodetic Vertical Datum of 1929. (NGVD29) Others are relative to the North American Vertical Datum of 1988 (NAVD88). Still others are reported relative to a completely arbitrary datum. Unfortunately, differences in elevation at a point can be on the order of several feet, depending on the datum used.

In the Chehalis River Basin, NGVD29, NAVD88, and arbitrary datums were all in use. The mix of datums became a problem when comparing flood inundation maps, gage elevations, structure elevations, and FEMA maps. The Chehalis River Authority and the USGS resurveyed all of the river gages in the basin and converted all stage measurements to NAVD88.

This presentation discusses the extensive activities undertaken to coordinate the datum change with the stakeholders. The objective is to minimize the impact of unintended consequences resulting from changing everyone’s frame of reference in advance of the 2018-19 rainy season.