The Cedar River ends its run from the Cascades in downtown Renton, emptying into the southeast corner of Lake Washington about 11 miles from Seattle. The town grew up at that river mouth, and for its first decades it flooded from both the Cedar and the old Black River. Those floods shaped the city's engineering history, and the Cedar's behavior in December 2025 showed they are not entirely a thing of the past.
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How the Cedar River Was Moved, and Why It Still Floods
A major flood on the lower Cedar in 1911 led Renton to dig a diversion canal, finished in 1912. A few years later the Lake Washington Ship Canal lowered the lake by several feet, the lake stopped draining south through the Black River, and the Cedar was turned to flow into Lake Washington instead. The Black River nearly disappeared, and with it the threat of annual floods.
Reduced is not eliminated. On December 11, 2025, the Cedar River broke its all-time flood records during the atmospheric river storms that hit western Washington. Homes and businesses near the channel are the most exposed when that happens. River water carries mud and sewage, so flood cleanup after it enters a building involves removing and disposing of soaked materials, then disinfecting before drying begins.
Wartime Growth and the Houses It Left Behind
Boeing built its Renton factory during World War II, and the population jumped from about 4,488 in 1940 to more than 16,000 by 1950. A lot of housing went up quickly in that era and in the decades after. Homes of that age commonly have crawl spaces, older galvanized or early copper supply lines and water heaters that have been replaced several times in tight closets or garages. Typical problems include:
- Slow drips under a kitchen or bathroom floor that rot the subfloor before anyone notices.
- Water heater failures that soak a garage or hallway.
- Crawl spaces that collect water every winter because of poor grading or blocked downspouts.
A provider doing water damage repair should open up only what is wet, confirmed with a moisture meter, rather than gutting whole rooms by default.
Lakefront Towers, The Landing and New Construction
The north end of the city has changed a great deal. The Landing, a 68-acre residential and commercial development, the Southport hotel and office complex on the lakefront and planned residential towers at Quendall Terminals have added a lot of multifamily housing. In newer buildings, the common leak sources are appliance hoses, sprinkler heads and window or deck membranes rather than old pipes. When water reaches units below, the building manager should close the source valve, and each owner should notify their insurer and the association. Keep moisture readings for every affected unit, which makes sorting out responsibility easier later.
Rain, Slopes and Mold on the Plateaus
Renton has a pattern of warm, dry summers and cloudy, wet, cool winters, and the hills rising toward the East Renton Plateau and Cougar Mountain send runoff downhill during long storms. Basements and crawl spaces on sloping lots get the runoff. In those cool months, wet drywall and insulation can grow mold within days if the space is not dried mechanically. If you smell mustiness or see staining after a leak has been fixed, ask for mold remediation with containment, and make sure the moisture source has been corrected first.
Areas In and Around Renton
Calls come in from across Renton, including:
- Downtown Renton
- North Renton
- The Landing
- Southport
- Port Quendall
The same line also serves nearby communities such as:
Renton Water Damage Questions
How do I know when the Cedar River is rising?
King County's Flood Alert System sends free email, text or phone notifications for the Cedar River, and its Flood Warning app shows current river conditions and forecasts. The county's Flood Warning Center is also available by phone during flood events. Flooding in the area is most common from October through April, so sign up before the wet season starts.
Does Renton sit in the Green River valley too?
Renton is counted among the Green River valley communities downstream of the Howard Hanson Dam. When seepage was found at the Howard Hanson Dam after the January 2009 storm, officials handed out sandbags across the valley, including in Renton, Kent, Auburn and Tukwila, as a precaution. If your property is on the valley floor, check FEMA's flood map for your address rather than assuming the dam removes all risk.
Is flooding from the river covered by my homeowners policy?
Standard homeowners policies generally exclude flooding from rivers and overland water. That coverage usually comes from a separate flood policy, often through the National Flood Insurance Program, and new policies typically have a waiting period. Sudden internal leaks, such as a burst supply line, are a different kind of claim and are often covered.