One Quick Question: Why are there so many 100-year flood events?

Information in this story is accurate up to 3 p.m. Wednesday, December 10. Flood conditions develop rapidly and may have changed. Please check Skagit County's website for updated information about evacuation levels and flood forecasts.

Parts of Washington are experiencing 100-year flood events. Skagit County is on Evacuation Level 2: SET, meaning short notice evacuation is likely in areas within the 100-year flood plain. This includes parts of Mount Vernon, Burlington, Sedro-Woolley, La Conner, Edison and as far east as Marblemount.

As of 3 p.m. December 10, the Skagit River is expected to crest to 48.01 feet near Concrete at 10 a.m. December 11 and to 42.29 feet near Mount Vernon at 4 a.m. on Dec. 12. Historical record crests for the Skagit River were recorded at 42.21 feet near Concrete and 35.19 feet near Mount Vernon in 1990.

In addition, the Samish River is expected to crest at 14.7 feet near Burlington at 10 p.m. December 10, a little more than half a foot more than its historical crest of 14.18 feet.

The term “100-year flood plain” may sound like you can expect a flood at this magnitude every 100 years. In reality, it’s a bit more complicated. The term refers to the recurrence interval of a flood event, or how likely it is the flood event will occur each year. Flood events in the 100-year flood plain have a 1-in-100 (or 1%) chance of occurring each year. 

On the Skagit River, a crest over 35.6 feet near Mount Vernon is considered within the 100-year flood range, while a crest must exceed 37 feet in Concrete to fall within the range. 

Since 1990, the Skagit River has experienced five 100-year flood events near Mount Vernon and seven events near Concrete. This means the Skagit River has experienced 100-year events at each location on average every seven years and every five years, respectively.

The most recent event for both locations occurred in November 2021. 

Prior to 1990, the Skagit River experienced 100-year flood events three times in 84 years near Mount Vernon and seven times in 175 years near Concrete for an average of once every 28 years and 25 years, respectively.

So, why do we see 100-year flood events so often?

We asked Geology Associate Professor Allison Pfeiffer.

Allison Pfeiffer: "Great question! The short answer, as you might have guessed, is climate change, but the details are pretty interesting, and unique to each region. 

First off, let’s define 100-year flood: there is a 1-in-100 (1%) chance that streamflow will get that high in any given year. Think of it like rolling the weather dice each year to figure out what the biggest flow will be. This is how hydrologists, emergency managers, and insurance companies have talked about flood frequency since the 1960s. 

The problem is that climate change has weighted the dice! Over the past decades, climate change has increased the intensity of these peak streamflow events, such that the whole paradigm of flood frequency analysis is breaking down. Now, the events we would have defined as 100-year events have a more than 1% chance of occurring in any given year. 

In Western Washington, this increase in peak flow intensity is driven by a combination of warmer temperatures shifting our precipitation from snow to rain, as well as an intensification of the “atmospheric river” storms. As the atmosphere gets warmer, it can hold more water, meaning it has more moisture to dump on us! In the fall, our big storms bring rain to the lowlands but snow up high in the alpine. Snow water gets trapped on the landscape, then gradually melts out, often months later in the spring. Rain, however, flows quickly into rivers within hours or days, causing floods. As temperatures rise, the snow line shifts higher up, meaning that more and more of the Cascades are getting rained on rather than blanketed in snow (R.I.P., my winter break ski plans). This is especially true in the fall, which is why our biggest floods tend to hit in November and December, rather midwinter. Professor Bob Mitchell and his students have modeled changes in Nooksack River peak flows over the coming century. Their predictions suggest that our peak flows might carry about 50% more water by the end of this century. Yikes. 

Think of flood recurrence like rolling the weather dice each year to figure out what the biggest flow will be. There’s a 1% chance that you’ll “roll” a 100-year (or larger) flow. This is how hydrologists, emergency managers, and insurance companies have talked about flood frequency since the 1960s.

In the Nooksack River, the flooding problems are especially unruly because the river channel changes so much. Folks might remember the November 2021 floods that spilled out over the banks near Everson and flowed into Canada. One man died, there was more than $1 billion in damage, and hundreds of thousands of livestock drowned. 2021 wasn’t unique. There have been about five such overflows (though mostly much smaller) in just the past decade. They’re happening more frequently now because flows are increasing and because the river channel has narrowed and filled in with gravel over recent decades. So, there’s more water and a smaller conduit. This means more flooding. Those overflow events are an incredibly thorny problem for engineers, geologists and politicians (the Canadians are livid when the Americans don’t keep their wild river “under control”). Professor Doug Clark and I are collaborating with a group of scientists and engineers at UW and Whatcom County flood managers to better understand the history of river change in the Nooksack, in hopes of being able to better predict, manage and communicate risk in the future. 

The other river I’m anxiously watching this week is the Skagit. As of Wednesday, mid-day, the current river forecast predicts that it will crest on Thursday night at 4 feet above the record stage in Mt Vernon, set in 1990. High flows in 1951, 1995, 2003, and 2021 came within a foot or so of that 1990 record and caused plenty of damage. We’re about to blow past that record into uncharted territory.

If you’re out of harm’s way this week, it’s a great time for a walk to a local creek to see how these big storms change our landscape! Look for twigs and grass (‘flotsam’) stranded on trees or bushes that mark the high-water line, sand freshly deposited by overbank flows, and newly formed or rearranged gravel bars. Rivers are wondrous, evolving features that integrate the geologic, atmospheric and human stories happening upstream. They’re awake and busy right now. 

Of course, stay safe if you’re out while the flows are still raging. Never try to drive across a flooded road with any current. Cars are surprisingly buoyant and can get swept away!"

Addendum:

12/11 at 10 a.m. by Pfeiffer:

"Late on Wednesday afternoon (Dec. 10), the river forecasts changed substantially, increasing the predicted flows in the Nooksack to nearly 2021 levels. This was a dramatic turn of events from the much less scary forecast earlier in the day. As of Thursday morning, the Nooksack seems to have crested at Everson with overflows about 1’ lower than the 2021 peak. Likely not record breaking but nonetheless devastating for the families who are still rebuilding their lives after the 2021 event. That water is flowing across fields and through the town of Sumas, filling the former Sumas Lake (now home to thousands of people and hundreds of thousands of livestock) across the border in Canada."

Mikayla King (‘17) covers the College of Science and Engineering and Woodring College of Education for University Communications. Reach out to her with story ideas at kingm24@wwu.edu.