Water Releases Make Tualatin River a Year-Round Resource 

Stored water from Hagg Lake and Barney Reservoir is essential to keeping the Tualatin River cool, healthy, and flowing

The arrival of warm and dry weather and a hot forecast ahead prompted Clean Water Services (CWS) to begin releasing cool water on May 23, 2025.  

The Tualatin River is a vital source of water for Washington County communities, crops, and companies. To keep the Tualatin River flowing and healthy in dry months, CWS releases cool water from Hagg Lake and Barney Reservoir. Scientists closely monitor the river to determine when more water is needed to sustain water quality and the ecology of the watershed.

“The 2024-25 winter was moderate with cooler temps, moderate snow, and moderate precipitation. Hagg Lake and Barney Reservoir are nearly at capacity, and in good shape as we start flow releases in response to the forecasted warmer temperatures for the remainder of May,” said Water Resources Analyst Katie Thomas. “This summer is anticipated to be warmer than in past years, so starting the season with a full supply of stored water to release to the Tualatin River is great.”

In an average year, CWS releases about 30 million gallons of water per day to maintain sustainable base flows, cool temperatures, enhance water quality, and provide habitat for fish and wildlife. By late summer, more than 70 percent of the flow in the lower Tualatin River is from stored water releases from Hagg Lake and Barney Reservoir, and from CWS’ water resource recovery facilities in Forest Grove, Hillsboro, and Tigard. See the Tualatin River Flow Diagram (PDF). 

“We manage our stored water releases based on weather forecasts, stream flow, and anticipated demand of the river. With the possibility of a hotter summer on the horizon, we will see higher water demand for irrigation and drinking water at the same time stream flows are naturally lower,” Katie continued. “We always plan to manage flow releases to minimize the impact of those demands on the river.” 

Among the ways CWS is responding to the anticipated impacts of climate change is through long-term strategies such as expanding water reuse, increasing the Tree for All shade program, and optimizing existing water supplies. 

Knowing when and how much water to release is an intricate balancing act based on stream flow, irrigation and water supply withdrawals, weather, water quality conditions in the Tualatin River, and the amount of water in the reservoirs. A network of continuous monitoring data from the United States Geological Survey, Oregon Water Resources Department, and CWS helps determine when and how much water is released. CWS coordinates with the Joint Water Commission (which provides drinking water to several cities in Washington County including Hillsboro, Beaverton, and Forest Grove), the Tualatin Valley Irrigation District, the Oregon Water Resources Department, and the Lake Oswego Corporation to maintain stream flow.  

CWS was one of the original investors that built Hagg Lake’s Scoggins Dam in the early 1970s for agricultural irrigation, drinking water, and flow enhancement. With the Joint Water Commission, it expanded Barney Reservoir in 1998 to secure additional stored water. CWS has access to 24% of the stored water in the Tualatin River Watershed. The stored water is used to enhance stream flow, provide habitat, and improve water quality in the Tualatin River Watershed.   

Everything we do at Clean Water Services aims to protect public health, while enhancing the natural environment of the Tualatin River Watershed. Combining science and nature, we work in partnership with others to safeguard the river’s health and vitality, ensure the economic success of our region, and protect public health for more than 600,000 people in urban Washington County. Although CWS maintains a close working relationship with Washington County government, it is separately managed and financed.