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Water Supply & Treatment

Our shared waters

The Cedar River Municipal Watershed is 90,563 acres of land owned by the City of Seattle. The watershed is carefully managed to supply clean drinking water to 1.3 million people in the greater Seattle area as well as downstream water flows for salmon, lakes, and locks.

99.8% of the land within the watershed’s hydrographic basin, from the Cascade Crest along the north and south ridges and downstream to Landsburg diversion dam, are owned by Seattle. This land is protected for water supply, flows, and wildlife habitat.

Quality water

The City of Seattle is required by law to maintain a clean drinking water supply.

To ensure clean, high-quality water, the City restricts public access. Watershed management is guided by a Habitat Conservation Plan. The Cedar River is an unfiltered surface water supply that produces some of the best water in the world.

Cedar River water is:

  • Screened to remove debris
  • Chlorinated to remove microbial contaminants, such as bacteria and viruses
  • Fluoridated for dental health protection
  • Ozonated for odor and taste improvements and Giardia control
  • Disinfected with ultraviolet light to disable microbial contaminants such as chlorine-resistant Cryptosporidium
  • Supplemented with lime for pH adjusted corrosion control to minimize lead leaching in older plumbing systems.

Cedar River water meets or exceeds all federal standards for drinking water. Daily, more than 50 samples are tested before and after treatment at Seattle Public Utilities Water Quality Lab for a variety of waterborne disease indicators, minerals, chemicals, and contaminants.

From mountain forests to faucet

Chester Morse Lake

Melting snow and rain are gathered and stored in two reservoirs -- Chester Morse Lake and the Masonry Pool, which was created by the Masonry Dam. Built in 1914, the dam diverts water into two large 78-inch penstocks. The penstocks drop water 620 feet to the hydroelectric power plant at Cedar Falls, the birthplace of Seattle City Light.

After the water generates electricity at Cedar Falls, it is released back into the river and continues to flow downstream for 12 more miles to the Landsburg diversion dam. At Landsburg, an average of 18% of the river is diverted for drinking water. It is screened, chlorinated, and fluoridated before being sent to Lake Youngs. At Lake Youngs it is ozonated and exposed to ultraviolet light. Lime and additional chlorine are also added.

Landsburg Dam

82% of the Cedar River's annual flow continues downstream past Landsburg, through Maple Valley and Renton, flowing into Lake Washington, Lake Union, and out the Chittenden Locks in Ballard to Puget Sound.

Water, water everywhere

Only 18% of the annual water flow from the watershed is used for drinking water.

The rest is dedicated to many things, including water flows needed for fish. Water levels in the Cedar River below Landsburg must be kept at healthy levels for proper seasonal spawning, hatching, and rearing needs for salmon. The Cedar River is home to four salmon species; Chinook, Coho, sockeye, and steelhead trout.

Cedar River water is also sent to Lake Washington and the Chittenden Locks. Seawater is heavier than fresh water and can creep into Lake Union through the locks, so a certain amount of fresh water always has to be flowing out. When there are long lines of boats at the locks in the summer, it is because fresh water released up stream is restricted to store anticipated drinking water and for early fall salmon migration.

Balancing act

Predicting the water supply is as difficult as predicting the weather. Managing the water supply is a delicate balancing act. In an average precipitation year, the two reservoirs have just enough water storage for one water cycle year.

If not enough water is released in the winter, there could be flooding. If too much is released, there won't be enough stored for the dry summer months.

Water levels have to be kept low in fall and winter for flood control, yet drought conditions in the spring could prevent the reservoir from filling adequately.

Salmon, lakes, locks, and people all share the same limited amount of water gathered and stored in the Cedar River Watershed.

Related links

Water System History
Water Quality
Landsburg Mitigation
City Light History