Mission Trails FRS II water project construction

FRS II: Mission Trails Water Project Takes Shape

When looking from the right vantage point towards Mission Trails Regional Park, a brown hill can be seen. This is the San Diego County Water Authority’s Mission Trails Flow Regulatory Structure II Project, or FRS II, under construction. Once complete, the dirt hill will be leveled to its previous contours and revegetated with native plants – many seeded from plants within the park itself.

An underground reservoir is being constructed within the park to improve the Water Authority’s untreated water system in the northwest area of the park. The upgrade will increase reliable water delivery to treatment plants that serve the central and south sections of San Diego County.

Mission Trails FRS II construction area

From a vantage point on a trail in Mission Trails Regional Park, construction equipment can be seen working on the underground water tank that will help balance regional water flows. Photo: Jim Madaffer

FRS II will help balance regional water flows

Work efforts on the new FRS II are beginning to take shape with many of the wall sections installed.

The FRS II will be an underground concrete water tank that will store slightly less than five million gallons of water and be used to balance flows in the aqueduct system. FRS II will be the second underground water tank in the park – both work to efficiently move water through the region. Construction is also underway on a new flow control facility, which is part of the project.

Drone aerial view of Mission Trails FRS II project

Crews captured an aerial view of part of the project site using a drone, which helps to visualize project areas that are not easily accessible. Photo: San Diego County Water Authority

Contruction closure information

Some trails in the northwest area of the park are closed during FRS II construction hours (Monday through Friday from 7 a.m. to 7 p.m.), with portions closed 24 hours a day, 7 days a week.

The Water Authority created an interactive map showing the location of the closed trails. Visit to view the map and learn more information about the project. For questions about the project call a toll-free project information line at (877) 682-9283, ext. 7004, or email

Construction Begins on Essential Water Project in Mission Trails Regional Park

The San Diego County Water Authority is making progress on the construction of a new 5 million gallon underground reservoir in Mission Trails Regional Park. The underground reservoir is also known as a flow regulatory structure.

Classified as a “critical or essential” infrastructure project during the COVID-19 response, the project is moving forward to stay on schedule. Water Authority and contractor staff are taking heath protection precautions to maintain public safety by following COVID-19 safety guidelines, including wearing face masks, using hand sanitizers, and disinfecting fencing, work tools and equipment.

Construction preparation underway for water project

Recent work includes the installation of temporary office trailers at the east end of Clairemont Mesa Drive in the City of San Diego, delivery of materials, installation of eight-foot fencing around the major construction sites and placement of silt fencing around environmentally sensitive areas.

Topsoil is being removed and saved where digging is planned. This topsoil will be placed back in its original location after construction is done to encourage plant re-growth, help hold water and prevent soil erosion. Construction crews are working Monday through Friday between 7 a.m. and 7 p.m.

Trail closures to ensure safety of public and essential workers

Some trails in the western portion of the park are closed to ensure the safety of the public and essential workers on site. Several trail closure signs with maps have been strategically placed to help park users navigate the trails and take detours to avoid the construction area.

The project is in the western part of Mission Trails Regional Park near the Tierrasanta community. It includes building the new underground covered reservoir, a flow control facility and pipeline interconnections to upgrade the system that delivers water to treatment plants serving the central and southern areas of San Diego County. The project is anticipated to be complete in early 2022.

Several sensitive species of small animals, such as this western spadefoot toad, live within Mission Trails Regional Park. Photo: Water Authority

Wildlife Protected for New Underground Reservoir Project

One by one, small mammals and amphibians living within a construction zone in Mission Trails Regional Park are being relocated to safe areas. Protecting sensitive species is one part of the Mission Trails Project.

A team of biologists from the San Diego County Water Authority, AECOM, and the San Diego Natural History Museum began surveying for and relocating the wildlife in preparation for a new underground reservoir. The reservoir will be constructed in the western portion of the park. The habitat surveys and wildlife relocation program span 15 acres of the park and are designed to protect sensitive species in the project area from construction activities.

Biologists move, monitor sensitive wildlife

Several sensitive species of small mammals and amphibians will be encountered and moved.

Biologists will focus on four sensitive species that are covered under the Water Authority’s Natural Communities Conservation Plan and Habitat Conservation Plan: northwestern San Diego pocket mouse, Dulzura pocket mouse, San Diego desert wood rat, and western spadefoot toad. Biologists will continue to monitor for these and other sensitive species during construction.

Biologists took special care to find burrows or covered areas for the small animals, such as this kangaroo rat, so that they could begin to build their new homes in the safe areas of the park. Photo: Water Authority

Biologists took special care to find burrows or covered areas for the small animals, such as this kangaroo rat, so that they could begin to build their new homes in the safe areas of the park. Photo: San Diego County Water Authority

Protecting sensitive species, environmental stewardship ‘paramount’

For about two weeks, more than 500 traps will be placed in a grid pattern in the evenings and the sensitive species will be moved one at a time to safe areas within the park early each morning. Special care is being taken to help the animals find new burrows or covered areas as quickly as possible. Each animal is carefully marked, and data is logged to track the relocated animals. This tracking process helps ensure that the wildlife is not returning to the construction zone.

“We are conducting sensitive species surveys and habitat management before the start of construction to allow the animals to find new habitat and build homes safely away from the upcoming work,” said Summer Adleberg, a principal water resources specialist at the Water Authority. “Environmental stewardship is paramount to the Water Authority, and we always aim to minimize impacts to the surrounding land and communities while we improve regional water infrastructure.”

New covered reservoir will improve regional water delivery system

The reservoir, also called a “flow regulatory structure,” will store up to 5 million gallons of water underground and help regulate untreated water flows in the regional water delivery system. When completed, the reservoir will be covered with soil and vegetation. Above-ground access hatches and vents will be constructed to allow air to move in and out of the reservoir.

Once the project is completed, the area will be restored to its original condition and monitored over the next several years.

Mission Trails Project protects wildlife

The structure is part of a suite of infrastructure improvements, called the Mission Trails Project. The underground reservoir, pipeline tunnel, and removal of existing blue vent stacks are part of the project.

A new concrete crossing over the San Diego River will also be constructed to allow Water Authority vehicles to access pipelines and other infrastructure more efficiently. The pipeline tunnel, river crossing, and removal of most above-ground vent stacks have already been completed.

Construction activities on the covered reservoir and removal of the last two vent stacks are scheduled to begin in March and last approximately two years.

For trail closures and more information about the project, go to