Ask the Engineer: Where Does Our Water Come From?

You turn on the faucet and water comes out but where does it come from? We here in the Pacific Northwest have it good, nature does most of the work for us. Throughout the fall and winter when water use is down, precipitation falls in the mountains and turns into snowpack. When it starts to warm up in the spring, all that snow melts and sends us a steady flow of clear, clean water.

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Spada Lake is about 25 miles east of Everett

The water that comes out of your faucet has its beginnings in the Cascade Mountains. More specifically the Sultan Basin Watershed, considered one of the nation’s purest and most abundant water sources. A watershed is simply the area of land where all of the water that is under it or drains off of it goes into the same place. That place is Spada Lake or the Spada Reservoir.
photo 2 (1)

The Spada Reservoir was created in 1964 by the City of Everett in partnership with Snohomish County PUD. A dam was constructed on the Sultan River to hold back water, 50 billion gallons of water. Rain and snow melt from the surrounding Cascade Mountains flows into Spada Reservoir. The Sultan Basin Watershed covers an area of about 84 square miles (about 20 times the City of Mountlake Terrace) and the average annual rainfall is about 165 inches, or 5 times our local rainfall.

Water transmission line near Highway 2
Water transmission line near Highway 2

From Spada Reservoir, the water travels 8 miles through a pipeline to a hydroelectric powerhouse and then to the City of Everett Treatment facility at the Chaplain Reservoir. Here the water (about 50-million gallons a day) is filtered and disinfected and then travels in four 4-foot pipes towards Everett. Three of these pipes can be seen from the trestle on Highway 2 east of Everett. Two of these transmission lines carry treated drinking water, a third carried untreated water for industrial use at Kimberly-Clark paper mill up until the plant closed in 2012. A fourth line takes a southern route and can be seen from Homeacres Road west of Snohomish.

At this point much of the water goes to serve the City of Everett but a large portion is sold off to serve the majority of the remainder of Snohomish County. Over 50 water systems obtain their water from the City of Everett to serve over half a million residents.

The Alderwood Water District obtains water from the City of Everett and provides water to the City of MLT.

The City of Mountlake Terrace has over 81 miles of distribution water mains, three reservoirs with a capacity of 6.4 million gallons, and one pumping station which all work together to bring clean water right to your faucet.

Dustin DeKoekkoek, P.E. is a civil engineer with RH2 and designs public infrastructure projects all over the Pacific Northwest. Have a question about the topic covered here or for a future “Ask the Engineer” column? Email Dustin at or leave a comment below.  You can also connect with Dustin on LinkedIn here.
  1. Thanks for authoring this, I learned a lot. Can you define ‘treatment’ as used herein? I’m assuming removal of particulate material and chlorination. Ozonation or other steps as well?

    1. Thanks Bob and good question. There are 3 steps in the treatment.

      1. Coagulation and Flocculation
      A coagulant (alum) and polymer are added to allow small solid particles in the water to form into clumps so they can be easily filtered out.

      2. Filtration
      The water is then filtered through anthracite coal to remove finer particles.

      3. Disinfection
      There is an area at the water treatment plant where the chlorine is mixed with the water and is allowed to treat the water before it leaves the plant. I don’t think ozonation is used here. Ozonation does a great job disinfecting water once but it has no residual. Chlorine, on the other hand, will continue to disinfect in the transmission and distribution lines. During this step soda ash is added to control the pH and corrosiveness of the water as well as fluoride (that’s another article in and of itself.)

      1. I’d side-step the fluoride article unless you’re fully up to speed on Jenny McCarthy.

        Seriously, thanks for the added info.

  2. wanted to let you know how interesting and informative your story was. If its to be a series, I’m looking forward to more.

    Don’t know if you’ve been, but it’s beautiful up there. At 3,000 feet, I believe the snow should be off Olney Pass. Its paved until the switchback climb up to the pass, but the road is almost always in great shape, even if sometimes sloppy this time of year. You can take the left branch after the sign-in booth at the Pass to get down to within walking distance of the dam. The right hand branch leads down to a bridge across the South Fork and thence up and around to a large parking area with lake access as well as walking access along the old right-of-way (decommissioned) to both the Greider Lakes and Boulder Lake trails.

    In late Spring, Vesper, Del Campo and the entire back side of the Mineral City gap (Red Mountain) are covered in snow. It’s a sight.

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