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Public Information Statement
National Weather Service Seattle WA
400 PM PDT Fri May 16 2025

...NOTIFICATION OF MAJOR CHANGES TO FLOOD STAGES DUE TO CHANGES IN
STREAM GAUGE REFERENCE LEVELS FOR THE SOUTH FORK STILLAGUAMISH
RIVER...

The National Weather Service will be adjusting the flood warning
levels for the river forecast point, the South Fork Stillaguamish
River at Jordan Road Bridge gauge (WRAW1), effective June 4 , 2025.

The stream gauge owner Washington State Department of Ecology will
convert the base reference point (datum) for this gauge. his means the river stage reading will be
different than what you are used to seeing and that the National
Weather Service (NWS) flood levels need to be renumbered to match
the changes.

The stage height of the water level reported by a gauge is measured
from a base reference point which can be an arbitrary number. For
example, if a gauge has a base reference level of 5.0 feet and the
water level at the gauge reads 10.0 feet, it means the height of the
water level is 5.0 feet above the base. Since the base level is
usually given for some level below the bottom of the streambed, the
actual stream depth would typically be something less than 5 feet.
In the new system, called North American Vertical Datum 88 (NAVD88),
the base reference level is nearly mean sea level and the measured
water level is the elevation of the water surface above mean sea
level. That means that, for the same example gauge, instead of
reading 10.0 feet, may read 170.0 ft.. A water level reading of
170.0 feet of this example gauge means that the water level is 170.0
feet above sea level yet still the same 5 feet above the reference
level.

For the NWS river flood warning system, the defined flood levels for
minor, moderate and major flood categories have to be converted to
NAVD88 as well in order to match. The conversion factor for this
gauge is 196.54 feet. Below please find the listing of this flood
warning point`s old flood levels, and the newly converted flood
levels. Levels were rounded to nearest one half foot as needed

Here are the new and old flood stages:

S.F. Stillaguamish River at R Street

Flood Category            New Flow       Old Flow
Flood Stage/Minor Flood   211.0 Ft.      14.5 Ft.
Moderate Flood            212.5 Ft.      16.0 Ft.
Major Flood               213.5 Ft.      17.0 Ft.

If you would like more information about the National Weather
Service hydrologic services in the Seattle area, please contact
Brent Bower, Senior Service Hydrologist, at 206-526-6095.

$$