Nisqually River

Flooding on the Nisqually River is related largely to the amount of water released from LaGrande Dam (in Pierce County near the southeast corner of Thurston County). This, in turn, is related to how much water enters Alder Lake and is released from Alder Dam. Feeder streams such as Ohop, Yelm, and Taxwax creeks also influence flooding, as does high tide in the Nisqually Delta.

The National Weather Service issues a flood warning for the Nisqually River when their forecast model indicates the river will reach a stage of 12 feet or higher at the McKenna gage.  Minor flooding (low-lying roads and pasturelands) occurs when the flow rate at La Grande exceeds about 8,000 cfs (cubic feet per second) or a gage height of about 8.75 feet at the McKenna gage.  Moderate flooding (individual residences are threatened) in Thurston County occurs when the flow rate exceeds about 15,000 cfs or a reading of 10 feet or over at the McKenna gage. A high tide could flood the banks at Hayko Lane, 6th Avenue, Riverbend Campground and Riverside Manor apartments.  Major flooding (widespread threat to communities and major thoroughfares) occurs when the flow rate tops 16,500 cfs or a gage height of 10.5 feet at McKenna.

Recently work was done in the Nisqually Delta to restore the natural estuary habitat.  It is unknown exactly how this reclamation will affect the anticipated flooding impact levels shown below.

 

Nisqually River Gage at McKenna, River Mile 21.8

 

Nisqually River at McKenna

Source: National Weather Service, Advanced Hydrologic Prediction Service (click graph to open site)
Graph above reloads every three minutes

 

Nisqually River Gage at LaGrande, River Mile 40.4

Historical Crests Feet Cfs
(Cubic Ft per Second)

Anticipated Impacts

02/08/1996 17.3 50,000  
  16.0 37,000 The Nisqually River will cause severe near record flooding from LaGrande downstream through McKenna to the mouth. Deep and swift flood waters will inundate roads, farms and residential areas, including the residential care facility in McKenna. Erosion will likely cause severe damage. Flooding will occur all along the river including headwaters, tributaries and other streams within and near the Nisqually River Basin.
  14 26,500 MAJOR FLOOD STAGE  The Nisqually River will cause major flooding from LaGrande downstream through McKenna to the mouth. Deep and swift waters will flood roads, farms and residential areas, including the residential care facility in McKenna. Erosion may cause severe damage. Flooding will occur all along the river including headwaters, tributaries and other streams within and near the Nisqually River Basin.
01/29/1965 13 23,300 MODERATE FLOOD STAGE  The Nisqually River will flood from LaGrande downstream through McKenna to the mouth. Swift waters will flood roads, farms and some residential areas including the residential care facility in McKenna. Erosion will likely damage properties along river banks.
11/30/1995 12.48 21,700  
12/26/1980 12.39 21,400  
12/12/1955 12.38 21,400  
  12 20,200 The Nisqually River will flood from LaGrande downstream through McKenna to the mouth. Flood waters will flow over some roads and through farms and residential areas. Erosion may damage some properties. High tidal levels on Puget Sound will cause flooding along the lower reaches, threatening homes along Riverside Drive, Conine Street and Conine Avenue.
  10 14,700 FLOOD STAGE  The Nisqually River will flood at the lower end near the mouth. High tide levels on Puget Sound may increase the amount of flooding. The Nisqually River will also spill over its banks between LaGrande and McKenna.
  8 9,970 ACTION STAGE

 

 

Source: National Weather Service, Northwest River Forecast Center (click graph to open site)
Graph above reloads every three minutes

 

Nisqually - Alder Lake Reservoir

Note: Flood prediction is not an exact science. Although gage readings and historical data are excellent forecasting tools, please remember that rivers continually change. There are also local factors that can contribute to flooding such as stream and creek discharge into a river, snowmelt and damming caused by fallen trees and other debris. Therefore, during flood situations floodplain residents should not rely solely on gage readings and historical flood levels, but should keep an eye on the river and stay tuned to local media reports.

During flood situations, Thurston County Emergency Management posts additional information and media releases on its emergency information page.

 

Thurston County Emergency Management
9521 Tilley Rd. SW
Olympia, WA 98512

Phone: 360-867-2800
Fax: 360-867-2811
 
 
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