Upper Oso Reservoir

SMWD’s Upper Oso Reservoir, one of the largest recycled water reservoirs in Orange County, has been in operation since 1979.  It is located near the 241 Toll Road in the cities of Mission Viejo and Rancho Santa Margarita.

Upper Oso Reservoir Cone
Installation of the ECO2 Speece Cone was
completed in 2010.

The reservoir holds up to 1.3 billion gallons of recycled and runoff water used for outdoor irrigation in the surrounding communities, therefore conserving over a billion gallons of drinking water each year. The benefit received from Oso Reservoir will continue to increase as the cost of water purchased from the Metropolitan Water District of Southern California continues to rapidly rise.

In mid-2010, equipment was installed in Oso Reservoir to keep the water well-oxygenated.  The equipment utilizes an ECO2 Speece Cone that was placed underwater and is used to mix the lake’s cold, deep water with a stream of oxygen. Because the equipment is underwater, there are no aesthetic impacts to the surrounding neighborhoods. SolarBee equipment is also used to circulate and oxygenate the reservoir’s surface water.

SMWD performs regular testing of the lake’s water quality and oxygen demands to help ensure the reservoir’s ecosystem remains stable despite seasonal swings in air and water temperatures. 


Update for Upper Oso Reservoir, Week of July 22, 2013:
Duckweed, an aquatic plant which floats on or just beneath the surface of bodies of water, is visible at the Upper Oso Reservoir.  SMWD anticipates the plant will be present in the reservoir through summer, disappearing in the early fall months. Nearby residents may notice the reservoir’s water reflect a green color due to the surface growth of the Duckweed.

Duckweed is an important, high-protein food source for waterfowl (it is also eaten by humans in some parts of Southeast Asia) and the plant also provides cover for many aquatic species. Duckweed is often touted as a water purifier, as the plant naturally removes nitrates in bodies of water and absorbs excess mineral nutrients.

In addition, due to warmer summer weather and extended daylight, we anticipate algae growth in the reservoir during the month of June through October.   This process is naturally occurring  and the District has procedures in place to best control the algae growth within its natural cycle.   



The District monitors and manages for the natural biological and ecological processes and cycles that occur with changes in the weather and seasons. We augment those natural processes by introducing oxygen at the lower depths of the reservoir so that when it naturally turns over a few times a year as temperatures change there's plenty of oxygen to manage the organics. During warmer weather and when there's more sunlight, naturally-occurring algae blooms occur. The District anticipates and manages the reservoir for these events. Customers who desire more detailed information, including histology, water chemistry and other technical aspects of the District's reservoir management program, can fill out the contact form and SMWD will respond to your questions as soon as possible.

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