Mount Tabor Reservoir 6 Structural Repairs

The Portland Water Bureau completed construction on Mount Tabor Reservoir 6. Repairs included removing small sections of the concrete liner in three locations, two in the south cell and one in the north cell. Loose material around the voids was removed, one drain line was repaired, the voids were filled, and concrete was poured to seal the liner. 

Once the concrete was cured for the appropriate amount of time, the reservoir was partially refilled. By filling one cell of the reservoir at a time, it will help to identify potential leaks. Reservoir 6 will continue to be monitored.

A consultant completed a GPR scan of the floor and sloping walls of Reservoir 6. Analysis determined that none of the detected voids would contribute to the failure of the reservoir, but they will require ongoing monitoring and maintenance to avoid cracking and leaking of the liner. Three voids need to be repaired before Reservoir 6 can be refilled and will require ongoing monitoring and maintenance. 

The Water Bureau Dam Safety Group’s investigation showed that there were voids underneath portions of the reservoir. 

​​​​​​This photo, taken in October 2021, shows one of the larger voids underneath Reservoir 6. Ideally, there would not be any space between the concrete underside of the reservoir and the ground beneath it. The concrete of the reservoir is visible at the top of each photo. The earth below the concrete has been washed away, leaving a gap and nothing to support the concrete.

Unlike the reservoirs at Washington Park, the Mount Tabor reservoirs are no longer connected to the water system. Therefore, the cost for maintenance and repairs must come out of the City’s budget rather than the funds the Water Bureau collects from ratepayers. 

The hill that can be seen on the western edge of Reservoir 6 along SE 60th Avenue is an earthen dam. This dam supports the sides of the reservoir and helps hold the water in place. Before the extent of the voids were known, there was concern that water leaking from the reservoir could potentially cause more soil erosion and destabilize the dam. 

Updated analysis in the winter of 2022–23 found that none of the detected voids would contribute to failure of the dam. Ongoing additional monitoring and maintenance of the voids will be needed to prevent cracking and leaking of the reservoir's liner. 

AYes. In 2018, Reservoir 1 was surveyed using the same GPR technology and repairs were made to the voids as necessary. The repairs at Reservoir 1 were much less extensive than those needed at Reservoir 6 because Reservoir 1 is smaller. 

Reservoir 5 has a liner that prevents water from seeping through the concrete and eroding the ground underneath, so the likelihood of voids beneath it is significantly lower.

City Council passed Resolution 37146 in 2015, establishing City’s responsibility for preservation activities at the Mount Tabor Reservoirs and setting aside funding for that work. Since then, the Water Bureau and a committee of partners have completed several preservation tasks and planned for more, all of which are outlined on our Reservoir Construction Projects page. The Water Bureau also conducts regular maintenance and cleaning of the reservoirs, which is outlined on our Reservoirs Management page. 

Water Bureau staff meets with representatives from the Mount Tabor Neighborhood Association and Friends of Mount Tabor for periodic check-ins related to reservoir management and restoration. Updates will be communicated through those organizations and will also be posted to this webpage.