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The City of Portland, Oregon

Portland Water Bureau

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Customer Service: 503-823-7770

GENERAL INFORMATION: 503-823-7404

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Reservoir Management

Together the Water Bureau and the Mount Tabor Neighborhood Association (MTNA) are developing a maintenance plan for the reservoirs that ensures their cleaning and draining has low impact on water quality; provides annual water use monitoring at each feature; and minimizes water use in accord with City Council requirements for managing water features. 

Background

A remote operated vehicle inspects the reservoirs

In July 2015, the Portland City Council directed the Water Bureau to work with MTNA in managing the quantity and quality of water in the Mt. Tabor open reservoirs. Prior to 2016, the reservoirs were connected and in service as part of the potable water system. They were typically drained, cleaned, and refilled twice per year—once in spring and once in fall. There have been many changes to the Portland Water Bureau operations at Mt. Tabor in the past few years, particularly with the disconnection of the open reservoirs from the water system, and a maintenance plan will ensure the reservoirs meet water quality standards.  

  • Draining: When the reservoirs were connected to the system, instead of draining a large volume of water from the reservoirs directly into the Bureau of Environmental Services (BES) collection system, most of the water in the reservoirs was delivered to customers and to other lower-elevation reservoirs. Now that the reservoirs are disconnected, all of the water will need to be drained into the sewer/stormwater system. We are trying to reduce the amount of water that is drained into the BES system and are working with MTNA to develop new protocols for how frequently we do this.  

  • Cleaning: We no longer have to keep the water clean enough to drink, but we do want to maintain the water at the levels described in the Council Ordinance. Periodic water quality testing was performed for the first three years after disconnection to check that algae, chlorophyll, chlorine, pH, temperature, and turbidity were all within expected ranges. Testing is now done only as needed. 

  • Refilling: We don’t want to waste water, and one way to do this is by reducing the frequency of reservoir refills. We are working with MTNA to develop new protocols for how frequently we do this. 

The cycle of draining, cleaning, and refilling takes several weeks. This work is also weather dependent. If heavy rainfall is expected, the work is postponed so we don’t overburden the sewer system. 

Reservoir cleaning schedule

The reservoirs at Mt. Tabor are drained, cleaned, and refilled on a regular schedule to protect the quality of this nonpotable water and to conduct inspections of reservoir conditions. 

Spring 2020 update

The current schedule for draining, cleaning, and refilling the reservoirs follows. 

  • Cleanings every second year: We purposefully delayed the annual cleaning of Reservoir 5 in 2018 until March 2019 to learn more about what would happen if the reservoirs were  cleaned every two years rather than annually. This reduced the overall amount of water drained and needed to refill the reservoir, and no negative water qualities were observed or reported during that time. The practice of reservoir cleaning every second year will be implemented for all three reservoirs going forward.  

  • Fewer reservoir refills during dry months: During 2019, we explored what would happen if we stopped refilling the water in Reservoir 5 during the dry summer months, when the City was supplementing its water supply with groundwater from the Columbia South Shore Wellfield. We monitored the visual impact to the reservoir water level while the water evaporated to make sure that the historic views were not compromised. The lowest water level in Reservoir 5 was measured at 71.5 percent full on November 18, 2019, which is within an allowable range. During 2020, all three reservoirs will be allowed to partially evaporate. 

The Water Bureau and MTNA are working together to review the management plan annually and make adjustments as needed. 

Reservoir

Date

Reservoir 6

Reservoir 6 is not planned to be drained, cleaned, and refilled until spring 2021.

Reservoir 5

Reservoir 5 was drained and cleaned in in late March 2019. The next cleaning is scheduled for spring 2021.

Reservoir 1

Concrete maintenance on Reservoir 1 was completed in December 2020. Reservoir cleaning is scheduled for spring 2021.