Changes to Chehalis Basin gages

Found this information on a NOAA weather website. Folks fishing the Chehalis basin might want to take a look.

...Major Changes to Flood Stages Throughout Chehalis River Basin
Due to Changes in Stream Gage Reference Levels ...

Stream gage owners for rivers throughout the Chehalis River Basin
finished a project to convert all the base reference points (datum)
for their gages to a common one. This means the readings will be
different than what you are used to seeing and that the National
Weather Service (NWS) flood levels were renumbered to match the
changes. These changes affect rivers such as the Chehalis,
Skookumchuck, Newaukum, Satsop, and Wynoochee Rivers.

The stage height of the water level reported by a gage is measured
from a base reference point which can be an arbitrary number. For
example, if a gage had a base reference level of 5.0 feet and the
water level reads 12.0 feet, it means the height of the water level
is 7.0 feet above the base. Since the base level is usually given
for some level below the bottom of the streambed, the actual depth
of the water would be something less than 7 feet.

In the new system, called North American Vertical Datum 88 (NAVD88),
the base reference level is zero elevation and the measured water
level is the elevation above zero, similar to elevation above mean
sea level. It is much like the elevation of a highway over a
mountain pass or the elevation of a mountain peak. That means that
now for the same example gage above, instead of the gage reading
12.0 feet, it may read something like 176.0 ft.. A water level
reading of 176.0 feet would mean that the water level is 176.0 feet
in elevation above 0.

For the river gages that are part of the NWS river flood warning
system, the defined flood levels for minor, moderate and major
flood categories were converted to NAVD88 as well. Some conversions
changed levels by more than 300 feet. Below please find the link to
tables of flood warning points listing their old flood levels,
the newly converted flood levels, and the adjustment factors used to
arrive at the new levels. Levels were rounded to nearest one half
foot as needed.

https://goo.gl/L2cSZm

These changes have been made in the NWS Advanced Hydrologic
Prediction System (AHPS). The new levels for moderate and major stage are
also shown in AHPS. Please see the following website for AHPS information:

https://water.weather.gov/ahps2/index.php?wfo=sew

For more information or comments on the changes for the NWS flood
levels, please contact Brent Bower at brent.bower@noaa.gov or
206-526-6095 x228. All other questions should be directed to the
Chehalis River Basin Flood Authority.

Bushbear:

I'm sure the above is probably correct BUT last summer the "water height gauge close to Black Creek changed" and I wanted to know why.

I called the "USGS" and was told that there is change taking place....The person told me that the gauge would to "so many feet above sea level"


The following is from the Corp web page:


Gage height datum conversion to the NAVD88 datum for this streamgage was completed on May 9, 2018. All reported gage heights and those associated with discharge measurements and stage-discharge ratings have been converted and are consistent with elevations based on the NAVD88 datum. A conversion factor of 24.48 ft was used to convert the old gage heights to the currently reported instantaneous gage height, discharge measurement gage heights, and stage-discharge rating points.

All I know I understood everything when it was "how deep the water was at the Black Creek water gauge", now not so sure....

It is going to be a head scratcher for a while, I'm sure.

Might be some more answers in this link.

https://waterdata.usgs.gov/wa/nwis/current?type=ratings

Thanks for the heads up. Those who use gage height to decide when and how to fish (and certainly those who rely on those numbers to make decisions about boating safety) will do well to get familiar with the new data ASAP.

I have a feeling it will fall well short of rocket science. Probably not much to worry about. Think of it as a chance to learn something new about your favorite rivers!

I've always used CFS

I use CFS also. Will any of these changes affect the CFS readings?

"Will any of these changes affect the CFS readings?"

No. Rating curves stay the same; only the gauge height datum are changed.

I gauge by slabs at the ramp and the level of the tide below Fuller Bridge.