Background
In 2015, Mount Tabor’s original uncovered reservoirs were disconnected from the drinking water system to comply with state and federal drinking water regulations. In response to the disconnection and in collaboration with the Mount Tabor Neighborhood Association, City Council passed Resolution 37146 calling on the two entities to work together to explore sustainable plans for maintaining the reservoirs.
Priorities include:
- Keeping the reservoir water safe
- Maximizing the number of days the reservoirs are full
- Preserving the iconic views of the reservoirs and park
- Maintaining historically significant reservoir structures
- Conducting efficient and sustainable maintenance
The reservoirs are aging, and more substantial repairs will be needed in the next 10 years.
Cleaning the reservoirs
The Mount Tabor reservoirs are emptied and cleaned every two years. This keeps the reservoir water clean and allows the Water Bureau to inspect reservoir conditions. Reservoirs 1 and 5 were cleaned in spring of 2025. Reservoir 6 will be cleaned in 2026.
Where does the water go?
When the reservoirs were connected to the drinking water system, water in the reservoirs was delivered to customers and other lower-elevation reservoirs. Now that the reservoirs are disconnected from the drinking water system, the water is drained directly into the Bureau of Environmental Services sewer and stormwater collection system.
How clean is the reservoir water?
While water in the reservoirs isn’t clean enough to drink, it is maintained at levels specified by City Council to protect public health. For the first three years after disconnection, staff tested the reservoirs’ water quality periodically to check that algae, chlorophyll, chlorine, pH, temperature, and turbidity were all within expected ranges. Today, staff test the water as needed.
How long does it take to clean a reservoir?
The cycle of draining, cleaning, and refilling takes several weeks.
- Draining takes up to two weeks.
- Cleaning and inspection take about one week, depending on how much debris has accumulated.
- Refilling takes about one week.
Weather affects our ability to do this work. If heavy rainfall is expected, we postpone work so the sewer system does not overflow.
Why are the reservoirs cleaned every two years?
One of the goals in City Council Resolution 37146 is to keep water in the Mount Tabor reservoirs as many days as possible. Cleaning the reservoirs less often helps us meet this goal. It also reduces the amount of water needed to fill the reservoirs and reduces the volume of water sent to Bureau of Environmental Services for treatment, resulting in more cost-effective and sustainable management of the reservoirs.
When the reservoirs were connected to the drinking water system, they were cleaned twice each year. After they were disconnected, we delayed cleanings to study what would happen if the reservoirs were cleaned every two years. No negative water quality was observed during that time. We still clean the reservoirs every two years to protect public health and to inspect reservoir conditions.
How often are reservoirs refilled?
Reservoirs 1,5, and 6 are kept 50 to 85 percent full, within the range accepted to maintain the site’s iconic views. Water is added to the reservoirs between fall and spring, when water is abundant in the Bull Run Watershed. Before the dry months, the reservoirs are filled one to two feet below the overflow line and the water level is allowed to slowly lower through the summer.
Leak maintenance
Along with inspections during routine cleaning cycles, the Water Bureau monitors the rate of water loss in the reservoirs to identify potential leaks.
Reservoir 1
Ongoing monitoring of Reservoir 1 water levels identified possible leaks. In 2018, a consultant surveyed Reservoir 1, and repairs were made to the voids and cracked concrete basin liner.
Reservoir 5
Reservoir 5 has a liner that prevents water from seeping through the concrete and eroding the ground underneath. In fall 2022, after noticing increases in the water loss rates, we drained the reservoir so we could inspect the liner. Staff found tears in several locations, so the reservoir remained empty until the repairs were made. Reservoir 5 was refilled in April 2023.
Reservoir 6
In 2021, investigations showed that there were voids underneath portions of Reservoir 6. The Water Bureau kept Reservoir 6 drained, performed a full analysis to identify the extent of the voids, and completed necessary repairs. Reservoir 6 was refilled in 2024. Ongoing monitoring and maintenance are required to avoid further cracking and leaking.
Dam maintenance
Earthen dams at Reservoirs 5 and 6 support the sides of the reservoirs and help hold the water in place. Both reservoirs are inspected and maintained under the jurisdiction of the Oregon Water Resources Department to ensure they are safe to operate.