NOAK48 PAFC 171012

Public Information Statement
National Weather Service Anchorage AK
212 AM AKDT Fri Mar 17 2017

...Datum and flood threshold changes on several streams and 
rivers around Southcentral Alaska...

On or around April 17 2017 the National Weather Service (NWS) 
Forecast Office in Anchorage Alaska and the Alaska-Pacific River 
Forecast Center (APRFC) will be adding or modifying the existing 
flood thresholds at several area rivers. 

Of particular note, the flood thresholds at the river gage on the
Matanuska River near Palmer will be changing from a stage (water 
level height)-based threshold to a discharge (amount of water 
flowing through the river)-based threshold. For more information
please visit: www.weather.gov/afc/MatanuskaRiver 

A summary of all changes is found below:

Flood Category         Old Value             New Value 

Lowe River above Horsetail Falls near Valdez (Site: LHFA2) 
Action Stage            None                 11.0 FT 
Minor Flood             None                 14.0 FT 
Moderate Flood          None                 15.0 FT

Solomon Gulch Creek near Valdez (Site: SGLA2)
Action Stage            8.0 FT               16.0 FT 
Minor Flood             8.5 FT               16.5 FT 
Moderate Flood          9.0 FT               17.0 FT

Yentna River near Lake Creek (Site:YLKA2)
Action Stage           19.0 FT               Unchanged 
Minor Flood            20.5 FT               Unchanged 
Moderate Flood         21.5 FT               22.0 FT 
Major Flood            23.0 FT               24.0 FT

Iliamna River near Pedro Bay (Site: ILRA2)
Action Stage           71.0 FT               64.0 FT 
Minor Flood            72.5 FT               71.0 FT 
Moderate Flood         75.0 FT               Unchanged 

Matanuska River near Palmer (Site: MATA2) 
Action Stage            9.0 FT               15,000 CFS flow 
Minor Flood             9.5 FT               30,000 CFS flow 
Moderate Flood         10.8 FT               35,000 CFS flow 

These new and revised flood stages will be referenced in flood 
warnings, flood statements, and flood advisories. These changes 
are part of a recent re-evaluation of flooding impacts at these 
locations which included coordination with local emergency 

In addition, on or around April 17 2017 the published datum for 
several sites will be changing to match the datum used by the
United States Geological Survey (USGS - who owns and maintains 
these gages). The water levels reported by these gages will NOT 
change, and flood stages will NOT change unless noted above. This 
change simply brings NWS into full agreement with the official 
USGS information.

Site                     Old Datum            New Datum 
CHRA2 - Chester Creek     16.00 (NGVD29)         16.02 (NGVD29) 
KNKA2 - Knik River        30.20 (NGVD29)         33.57 (NAVD88) 
MATA2 - Matanuska River  170.90 (NGVD29)        185.24 (NAVD88) 
MONA2 - Montana Creek    245.00 (NGVD29)        250.00 (NAVD88) 
SXRA2 - Kenai River       35.34 (NGVD29)         40.40 (NAVD88)

Comments and questions about these changes should be directed to:

Andrew Dixon
Senior Service Hydrologist
NWS Anchorage
Andrew.Dixon at NOAA.GOV


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