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 139.53 feet at the gage near Grand Mound. Moderate flooding (individual residences are threatened) occurs at about 142.53 feet, and major flooding (widespread threat to communities and major thoroughfares) at about 144.03 feet.