Diving into a New Water Year

While a calendar year ends a couple of months into the wet season, a water year follows the flow of the season. October 1 marks the start of the new water year, as rainy days become more frequent and our river and streams begin to flow more strongly.

Forecasters with the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service predict a very strong El Niño fall and winter, which likely means warmer temperatures and potentially higher rainfall that could bode well for stream flows in the Tualatin River Watershed.

“We’re keeping an eye on the strengthening El Niño,” says Clean Water Services Water Resources Analyst Jake Dombrowski. “A wetter winter will be a welcome change for the Tualatin River as we adapt to longer and drier summers, which impact the stream flows and water demand in what is already a highly managed river.”

While we have separate stormwater and sewer systems in Washington County, some rain does make its way into sewer pipes and increase flows into CWS treatment facilities. Operations Coordinator Jordan Luevano says staff make sure the system is prepared as the rainy season approaches.

“Overall, the goal is pretty straightforward — go into the wet season with as much equipment availability and redundancy as possible so we can utilize the plant’s available treatment capacity when higher flows hit,” Jordan says. “We’re also working through known equipment issues and corrective maintenance items ahead of the wet season so we can address potential reliability concerns before they become problems during a high-flow event.”

During the 2025-2026 water year — which ends September 30 — the Tualatin River Watershed received 31.5 inches of rain, as recorded at the Hillsboro Airport. That’s less than the average annual rainfall of 34.2 inches. The majority of rain occurred last fall and winter with less rain in the spring. Drier springs have become more common and result in lower stream flows going into the summer, meaning Clean Water Services has been initiating stored water releases in May, which has become more typical in recent years.

The Tualatin River is an essential resource for Washington County, with high demands on its water for agricultural, industrial, and urban use. To keep up with those demands and ensure the small and slow river flows throughout the drier summer months, CWS and our Tualatin River Flow Management Committee partners — Joint Water Commission, Tualatin Valley Irrigation District, and Lake Oswego Corporation — release water from Hagg Lake and Barney Reservoir. Clean Water Services closely monitors releases to ensure healthy water quality, and we also discharge clean water from our water resource recovery facilities all year. That combination of released and discharged water makes up 50% or more of the flow in the Tualatin River in the summer.