On a clear morning Mount Rainier fills the southeast sky from much of the lake, and for plenty of buyers that view is part of the draw. The lahar question tends to follow, and it’s a sensible one. The useful answer is about one parcel and the roads you drive every week, and a town name can’t give you either.
What a lahar is
A lahar is a volcanic mudflow. Rock, ash, soil and water move downhill together as a slurry thick enough to carry boulders and whole trees, and it behaves more like wet concrete than floodwater. At Mount Rainier, lahars sit at the top of the hazard list. The mountain holds a great deal of glacial ice, and parts of the volcano are built of rock that heat and water have weakened over a very long time. A flow can start with an eruption, or with part of a flank giving way, which can happen with little warning.
Lahars follow gravity and the river channels that drain the mountain. When they reach the lowland, they slow and spread. That’s why the hazard maps for Rainier look like long fingers reaching down the White, Carbon, Puyallup and Nisqually valleys, widening where each valley opens up.
The plateau has a mudflow in its past
Lake Tapps sits on the high, fairly flat country between the White River valley to the north and the Puyallup valley to the southwest. That ground has its own history. Thousands of years ago a massive collapse on Rainier sent the Osceola Mudflow down the White River and out across the lowland toward Puget Sound. Geologists have mapped its deposits across a wide area of the plateau country around Enumclaw and Buckley and beyond, and the USGS describes it in detail for anyone who wants the science.
So some of the ground in this part of Pierce County is, quite literally, an old lahar. The Osceola was a rare and enormous event, though. The flows scientists consider more likely within a lifetime are smaller, and they tend to stay in the valley bottoms, where the terrain funnels them. Hazard maps often show both kinds, so read the legend before you read the colors.
A plateau house and a valley house
For a home on the floor of the Puyallup or White River valley, in Sumner, Pacific or parts of Auburn, the lahar question is mostly about the house itself: whether it sits inside a mapped zone, how far it is to higher ground, and which road gets you there. Parts of the valley have outdoor lahar warning sirens and posted evacuation routes, and buyers there should learn both before closing.
For most homes around Lake Tapps, the house is the smaller part of the question, because the plateau stands above those valley floors. That is a general statement about the terrain. It doesn’t clear any single lot. The shoreline isn’t all at one height, the rare large scenarios on the maps reach farther than the common ones, and the edges of the plateau drop steeply toward the valleys. Get the answer for the parcel you’re considering.
How to look up one address
Mailing addresses around the lake say Lake Tapps, Bonney Lake, Sumner or Auburn, and every one of those names also covers homes down on the valley floor. Here’s how to check the parcel itself.
- Open the Washington Department of Natural Resources Geologic Information Portal, turn on the volcanic hazards layer, and search the address. Zoom in until you can see lot lines.
- Read the legend. Note which scenario each shaded area represents, since one map can show both the more frequent flows and the rare, very large ones.
- Check Pierce County’s hazard mapping and Pierce County Emergency Management’s lahar information for the same address, including the nearest evacuation routes.
- For the science behind the maps, read the USGS Cascades Volcano Observatory pages on Mount Rainier.
- Save a dated screenshot of what you found with your purchase file. If anything is unclear, call Pierce County Emergency Management and ask.
While the map is open, turn on the other layers. Landslide and steep slope areas along the plateau’s edges, and flood zones near the water, are much more everyday concerns for a lake house than the volcano. Our Buying on the Water page covers how to check slope, flood and the shoreline before you write an offer.
The White River connection
People who learn how the lake works sometimes ask a sharper version of the question. Lake Tapps is a reservoir filled with water diverted from the White River near Buckley, and the White River is one of the valleys a lahar could come down. What a flow would mean for the diversion, the reservoir and its dikes is an engineering and emergency planning question. I’m not the right person to answer it, and I won’t guess. Cascade Water Alliance, which owns and operates the reservoir, and Pierce County Emergency Management are the ones to ask.
Your week runs through the valley
This part is easy to miss. You can sleep on the plateau and still spend a large share of your week on the valley floor. SR 410 drops off the plateau into Sumner, where it meets SR 167. The roads toward Auburn come down into the valley too. The Sounder stations in Sumner and Auburn are down there, along with a lot of jobs, schools, stores and appointments.
The planning doesn’t take long. Learn the warning system and evacuation routes for the places you spend your days, alongside the place you sleep. Decide as a household where you’d meet if the roads between the plateau and the valley were closed for a while. If a child’s school or daycare is down the hill, ask what its plan is. I go through the daily drive off the plateau in more detail in whether Lake Tapps is a good place to live, including why you should test the route at the hour you’d really drive it.
Insurance and disclosure
This is where general answers run out. Ask your insurance agent, in writing, how a homeowners policy for this specific address treats volcanic eruption, earth movement and mudflow. Those can sit in different parts of a policy, and some policies exclude one or more of them. Ask whether flood coverage has any role for the property. Do it while your inspection contingency is still open, so there’s time to act on the answers.
Washington sellers fill out a disclosure statement about the property’s condition and known issues. How it applies to a regional hazard like a volcano is a question for a real estate attorney, and I’d rather send you to one than paraphrase the law. As a buyer, the maps are public, and checking them yourself is quicker than waiting to see what a form says. As a seller, if you know something specific about your property, talk it through with your agent, and an attorney where it makes sense, before you list.
Keeping it in proportion
Mount Rainier is part of living anywhere in this corner of Pierce County, much the way earthquakes are part of living anywhere near Puget Sound. The steady approach is the same for both. Look at the maps and make a household plan. Then buy the insurance that fits the house. An hour with the hazard layers before you write an offer settles most of the worry.
