Chehalis River

Due to its large drainage area, the Chehalis River tends to rise and fall slowly over a long period of time. The most predictable scenario for the Chehalis occurs when rains fall over all of southwestern Washington and all regional rivers and streams rise.

However, the Chehalis can also experience flooding when there is little or no rain in Thurston or Grays Harbor counties, but heavy rain in Lewis and Pacific counties. This causes flooding to occur later than normal. 

A third scenario occurs when heavy rain falls in Grays Harbor County, but not in Thurston or Lewis counties. Feeder streams can fill the Chehalis and cause water to "back up" into Thurston County.

Due to these varying scenarios, we urge floodplain residents to keep an eye on the river and to continue to monitor local media reports even if the rain has stopped and all other local rivers have receded.  Minor flooding (low-lying roads and pasturelands) occurs at a gage height of about 12.5 feet at the gage near Grand Mound. Moderate flooding (individual residences are threatened) occurs at about 15.5 feet, and major flooding (widespread threat to communities and major thoroughfares) at about 17.5 feet.

Chehalis River Gage near Grand Mound at Prather Road Bridge - River Mile 59.9

 

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

Historical Crests Feet CFS
(Cubic Feet per Second)

Anticipated Impacts

12/04/2007 20.23 79,500  
02/09/1996 19.98 75,500  
01/10/1990 19.34 65,800  
  19.0 61,100 The Chehalis River in Thurston County will cause severe near record flooding with deep and swift flood waters inundating the Independence Valley. Flooding will occur all along the river including headwaters, tributaries and other streams within and near the Chehalis River Basin.
11/25/1986 18.41 53,500  
12/29/1937 18.39 53,200  
  17.5 43,100 The Chehalis River in Thurston County will cause major flooding, inundating roads and farm lands in Independence Valley. Deep and swift flood waters will cover SR-12 and James, Independence and Moon Roads. Flooding will occur all along the river including headwaters, tributaries and other streams within and near the Chehalis River Basin.
  17 38,800 MAJOR FLOOD STAGE
  15.5 29,600 MODERATE FLOOD STAGE  The Chehalis River in Thurston County will flood several roads in Independence Valley with swiftly moving water including SR-12 and James, Independence, Moon and Anderson Roads. Flood waters will cut off access to and from the Chehalis Reservation and inundate nearby farm lands. Some residential structures may be threatened
  14 22,900 FLOOD STAGE  The Chehalis River in Thurston County will flood several roads in Independence Valley including James Road, Independence Road and Moon Road. Flood waters will also cover nearby farm lands.
  12.2 16,600 ACTION STAGE  The Chehalis River in Thurston County will locally spill out of its banks into nearby fields and over a few roads.
 

Chehalis River Gage at Centralia - River Mile 65

 


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

Historical Crests Feet CFS
(Cubic Feet per Second)

Anticipated Impacts

12/04/2007 74.78 71,050  
02/09/1996 74.31    
01/10/1990 73.50    
  73.40   The Chehalis River in Lewis County will cause severe near record flooding...with deep and swift flood waters inundating residential and business areas...roads including I-5...and farms. Flooding will occur all along the river including headwaters...tributaries...and other streams within and near the Chehalis River Basin. A stage of 73.4 feet corresponds to a Phase IV flood in the Lewis County flood system.
01/08/2009 72.40    
  72.0 51,100 MAJOR FLOOD STAGE-The Chehalis River in Lewis County will cause major flooding...inundating residential and commercial areas...farms...and many roads including I-5. Flood waters will be deep and swift in some areas. Flooding will occur all along the river including headwaters...tributaries...and other streams within and near the Chehalis River Basin. A stage of 72 feet corresponds to a Phase III flood in the Lewis County flood system.
11/25/1986 71.99    
  68.5 34,200 MODERATE FLOOD STAGE-The Chehalis River in Lewis County will flood some residential and commercial areas...with water encroaching upon the first floor of some homes and businesses. Swift flood waters will cover some roads. A stage of 68 feet corresponds to a Phase II flood in the Lewis County flood system.
  65.0 22,550 FLOOD STAGE-The Chehalis River in Lewis County will cause shallow flooding of farm lands and some roads. A stage of 65 feet corresponds to a Phase I flood in the Lewis County flood warning system
  63.0   The Chehalis River in Lewis County will spill over its banks into nearby pasture lands around Chehalis and Centralia
  61.0   ACTION STAGE

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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