Boise River managers want to know why return flows in the river’s drains have been decreasing and how to fix the problem.

Declining drain flows prompted the Treasure Valley Water Supply Project, now entering its second phase. The Idaho Water Resource Board prioritized the project and contributed funding.

The drains collect runoff, including post-irrigation returns, and route it back to the river. Drain flows have been decreasing, according to a recent study by Boise State University researchers. Urbanization and conversions from flood to pressurized irrigation systems are among factors.

“For everything below Caldwell, we rely on drain flows,” said Daniel Hoke, watermaster at Star-based Water District 63. Since drain flows supply all deliveries in this stretch, understanding drains and current flow trends is “imperative to proper management of the river.”

A goal is to keep the water supply from falling short of demand, he said. That almost happened from Caldwell downstream to Notus on June 30, 2022, when flow of about 150 cubic feet per second — an approximately 25% reduction in the volume available historically — was lost due to diminishing drain flows, he said.

Flows were higher in 2023 and this year, “but still well below historical averages, and declining,” Hoke said. If the trend continues, water rights likely would be shorted and the district would be required under state law to administer rights on the Lower Boise River and tributaries.

In the project, “a lot of where we are going to go depends on the data we collect,” he said.

Installing drain data collection equipment at nine sites and starting to compile data for future mass-balance and trend analysis was a primary focus of the first phase. Mass balance reflects all of the factors that contribute to Boise River inflows and outflows.

The second phase, which emphasizes monitoring and analysis, likely will lead to “potential mitigation options” and consideration of “any and every option that helps maximize our water using data collection,” Hoke said.

The water district will measure at all sites, and HDR Engineering will incorporate the data into a dashboard and a hydrologic model. The data also could be used in connection with a Treasure Valley groundwater flow model that the state Department of Water Resources and U.S. Geological Survey released in 2023.

Managing and mitigating drain-related hydrologic impacts are at the center of the project’s third phase, now partially underway, Hoke said. Elements could include surface water supply management and mitigation, groundwater management and mitigation such as aquifer recharge, water conservation incentives, and projects that aim to optimize quantity and timing.

Depending on data collected, drain flows back into the Boise River could increase at Fifteenmile Creek, a major return point, by as much as 50 cfs, he said. A landowner has shown interest in working with the district to move water between the drain and an existing pond for stabilization. Water would move from drain to pond when flows are near daily highs and from pond to drain when flows approach daily lows.

Several other return points also could see flow increases as a result of phase-three projects informed by work in the first two phases, Hoke said.