Upper Oso Reservoir

Update for Upper Oso Reservoir, September 2015:

Under the current weather conditions, algae blooms that can cause nuisance odors are to be expected in the reservoir. This process is naturally occurring and will dissipate as the weather cools down. The District has procedures in place to best control the algae growth while continuously monitoring and working to alleviate this temporary issue.


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. 

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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