Chehalis, the Lewis County seat, fills a valley where the Newaukum River joins the Chehalis River, and both have a habit of leaving their banks in winter. The 1996 flood was rated a 100-year event here, then the December 2007 storm and the 2009 flood each shut down Interstate 5 and the railroad through town. The valley floor and the city's hillside historic districts face very different water problems, and it helps to know which one you are dealing with.
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The Newaukum, the Chehalis and Dillenbaugh Creek
Three waterways matter most here. The main river winds through the valley, the Newaukum joins it there, and Dillenbaugh Creek drains a watershed of more than 17 square miles near the Port of Chehalis. When a long Pacific storm parks over Lewis County, all three can rise at once, and the lowest ground on the valley floor is where water tends to collect first.
Flood protection is still a work in progress. The Chehalis Basin Strategy has funded pump station and levee improvements at the Chehalis-Centralia Airport, and a flood retention dam near Pe Ell has been proposed to limit damage from the largest floods. In December 2025, a long-lasting atmospheric river flooded roads in Napavine just south of town and closed parts of State Routes 6 and 508, a reminder that big events are not a thing of the past.
Hillside Historic Homes and Older Construction
Chehalis has three National Register historic districts: the downtown commercial core, the Pennsylvania Avenue-West Side district and the Hillside district. Older houses up the slope are generally less exposed to river water, but they bring their own issues. Plaster walls, original wood floors, aging roofs and galvanized or patched plumbing are common, and basements dug into a slope often take on groundwater during long wet spells.
For leaks in these homes, the goal is to dry original materials in place where possible. A careful water damage restoration crew will use moisture meters to map how far water has spread inside plaster and under flooring, remove as little historic fabric as the drying allows and document conditions before and after.
Wettest in November, Damp Until Spring
Chehalis averages roughly 45 inches of precipitation a year, and November is usually the wettest month at a little over 7 inches. The rain is persistent more than intense, which means problems build slowly: gutters clogged with fir needles overflow, crawl spaces stay wet for weeks and roof flashing that held up fine in October starts leaking by December. Winter cold snaps add frozen and burst pipes to the list, especially in unheated garages and crawl spaces.
What to Do in the First 48 Hours in Chehalis
- If river water is rising, follow Lewis County Alert notices and leave before roads close; I-5 itself has gone under in past floods.
- Once it is safe to return, shut off electricity to wet areas and photograph everything, including the water line on walls.
- Report damage to Lewis County when a damage hotline is opened, as the county did after the December 2025 flood.
- Pull out wet carpet, pad and soaked drywall and get air moving; floodwater-soaked materials are usually not salvageable.
- Keep receipts for pumps, fans, tarps and debris hauling for your insurance and any assistance claims.
For homes hit by river water or sewage backup, professional flood cleanup includes cleaning and disinfecting the structure before it is dried and rebuilt.
Stopping Mold in Slow-Drying Valley Homes
Cool, humid winters slow natural drying, so a wet wall or crawl space can stay damp long enough for mold to start within a day or two. Run dehumidifiers instead of relying on open windows, keep the heat on and check hidden spots such as wall cavities, the underside of subfloors and behind cabinets. A spot or two on tile or painted trim can be wiped down. Larger areas, growth inside walls or anything in a flooded home should be handled through proper mold remediation with the moisture source corrected first.
Chehalis Neighborhoods and Surrounding Towns
Calls come in from across Chehalis, including:
- Downtown Chehalis
- Pennsylvania Avenue-West Side
- Hillside Historic District
The same line also serves nearby communities such as:
- Centralia
- Napavine
- Adna
- Winlock
- Toledo
Common Questions
Which parts of Chehalis flood most often?
Low ground on the valley floor near the two rivers and Dillenbaugh Creek carries the greatest river flood risk, while homes up the hillside generally face less. Check your exact address on FEMA's Flood Map Service Center and the Lewis County flood warning phases map for the Centralia and Chehalis area before assuming you are safe.
How do I get flood warnings for Chehalis?
Sign up for Lewis County Alert, the county's emergency notification system. The county emergency management site also links to river gauge readings and National Weather Service river forecasts, so you can watch both rivers during a storm and decide early whether to move belongings or leave.
Where can I get sandbags in Lewis County?
Lewis County Emergency Management keeps a sandbag FAQ and a sandbagging technique video on its flood information pages, and those pages are updated during flood events. Sandbags help redirect shallow water away from doors and vents but will not keep out deep river flooding, so pair them with moving valuables up and having a plan to leave.
Can a historic house here be dried without tearing out the plaster?
Often, yes, if drying starts quickly and the water was clean, such as from a supply line. Plaster can be dried in place with dehumidifiers, air movement and sometimes small access holes, while moisture readings confirm progress. Plaster soaked by floodwater or loosened from its lath usually has to be removed. Before demolition starts, have the provider walk you through what stays, what goes and the reasoning.