Water Reservoir Levels Improving, Continued Conservation Needed

Yesterday, the City pushed out urgent messaging asking the Mercer Island community to immediately reduce water use. This was due to declining water levels in the City’s reservoir tanks. Thank you for the fast response! The levels in the City’s reservoir tanks are improving, but we need to continue conserving water.

Between midnight and 6:00am this morning, the Island consumed 787,000 gallons of water, with use peaking at 5,200 gallons per minute. This was a considerable decrease from the same period yesterday, where 1.2 million gallons were consumed, peaking at an unprecedented 7,000 gallons per minute. Please continue conserving water to keep this trend going in the right direction.

We know you have questions about how the City’s water system works and why these levels matter. Here’s what you need to know:

Why isn’t there enough water to meet demand?

The Island’s reservoirs are currently being fed by a backup water supply line that brings a maximum flow of 2,500 gallons per minute of water to the Island. Normally, the reservoirs hold enough water to meet high daytime demands and are then refilled overnight when demand is low. A combination of really hot weather and high demand, coupled with reduced supply, meant the reservoirs could not “recover” enough water overnight on Sunday, and reservoir storage levels started to drop.

Why do these levels matter?

The reservoirs must have enough water in them to supply adequate pressure and volume to meet community demand while also being resilient in an emergency. Critically important: the reservoirs must maintain high enough levels to ensure adequate fire suppression response. Mercer Island’s water infrastructure also supplies the fire suppression system in the I-90 tunnel.

To put this simply, our water reservoirs must have enough water in them to meet peak demand even if there was a major fire, a water main break, or another unexpected draw. For example, a water main break last year caused the reservoirs to drop two feet in two hours, draining more than 500,000 gallons. With incoming water limited to 2,500 gallons per minute right now, it would take multiple days to recover the water lost from that type of incident on top of high demand household and business water use.

There seems to be a lot of water in the reservoirs, though?

It’s important to understand what these levels actually mean. The reservoirs are considered full when they reach 29.5 feet. The “fireband” (the level below which the reservoirs may no longer provide enough water pressure for fire suppression) is about 19 feet. That means we can only draw down the reservoirs about 10 feet before we begin experiencing reduced pressure in parts of the water distribution system. At this stage, the City’s ability to pressurize fire hydrants may be compromised. Yesterday, the reservoirs were drawing down and not recovering, which is why the City issued the urgent alert to conserve water.

What should I do, and how can I help?

Keep conserving water! We have reduced water use significantly since yesterday and maintaining this reduction will help us refill the reservoirs to the maximum capacity overnight.

Please also help spread the word! Tell your friends and neighbors on the Island to make sure they’re aware.

How long do I need to keep this up?

City staff are carefully monitoring water use and will adjust water conservation requests based on usage data and the levels in our reservoirs. We are hoping that your continued hard work over the next few days will allow the reservoirs to continue refilling. If we do that, we will avoid mandatory water conservation measures.

We will keep the community updated each day to let you know of progress and any changes. You can also monitor the City’s Water Usage Dashboard for real-time updates.

When will the main water supply pipeline be repaired?
The repair on the Seattle Public Utilities main water supply pipeline is currently underway. A new liner was installed at the end of June to address the leak. Work is underway now to connect the liner to the main pipe. Once that is done, the liner will be sanitized and brought back online. The current timeline to have the pipeline fully operational is the end of July.

How can I stay informed?

All Islanders are encouraged to sign up for City emails, including urgent alerts.

You can also sign up for ALERT, the King County emergency alert system.

Stay updated on Let’s Talk.

Thank you for your continued efforts to help conserve water.