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Water Supply & Treatment

Our shared waters

The Cedar River Municipal Watershed is 90,638 acres of land owned by the City of Seattle. The watershed is carefully managed to supply clean drinking water to 1.4 million people in the greater Seattle area as well as downstream water flows for salmon, lakes and locks.

99.9% of the lands within the watershed’s hydrographic basin, from the Cascade Crest along the north and south ridges and downstream to Landsburg diversion dam are owned and protected for water supply, flows and habitat protection.

Quality water

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

To that end the City restricts public access and management is guided by a Habitat Conservation Plan. The Cedar River Watershed is an unfiltered surface water supply which 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
  • Ultraviolet disinfected 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 created by the Masonry Dam. Built in 1914, the dam diverts the water into two large 78 inch penstocks. The penstocks drop water 620 feet to the hydroelectric powerplant at Cedar Falls, the birthplace of Seattle City Light.

The water is released back into the river, and continues flowing 12 more miles to Landsburg diversion dam. At Landsburg, on average 22% of the river becomes drinking water and it is screened, chlorinated and fluoridated before being sent to Lake Youngs. At Lake Youngs it is ozonated, exposed to ultraviolet light and lime and additional chlorine are added.

Landsburg Dam

78% of the Cedar River's annual flow continues past Landsburg down the Cedar River into Maple Valley through Lake Washington, Lake Union and out through the Chittenden Locks in Ballard to the Puget Sound.

Water, water everywhere

Only 22% of the annual water flow from the watershed is used for drinking water.

The rest is dedicated to many things, including flows needed for salmon. 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.

Other uses include Lake Washington and the Chittenden Locks. Seawater is heavier than fresh water and will creep into Lake Union through the locks, so a certain amount of fresh water always has to be let 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 flows.

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