This is part of a series on issues facing California’s rivers.

After a very wet winter, California has been declared drought free. But planning for future water shortages has continued. In Los Angeles, Mayor Eric Garcetti recently announced a goal of 100% wastewater recycling by 2035 to help make city supplies drought proof.

While recycling wastewater helps cities adapt to a changing climate and prepare for droughts, it can have unintended consequences for local watersheds. In some cases, the growing use of recycled water could minimize or even eliminate flows from wastewater treatment plants into local rivers and streams and reduce ecological and recreational benefits. The Los Angeles River exemplifies this kind of trade-off: expanded water recycling will reduce the amount of treated wastewater flowing into this increasingly revitalized urban waterway.

The lower stretch of the river, which was converted into a concrete flood channel in the mid-1900s, is changing. Concrete has been removed from large stretches of the river and public parks and bike paths have been built along its banks, encouraging recreational use and increasing public interest in the river’s restoration. The river was declared a navigable water in 2010 and opened to kayaking the following year. It provides a vital refuge to a variety of riparian species that lost most of their habitat to channelization and urban development.

Last year, researchers from the Southern California Coastal Water Research Project (SCCWRP) launched a study to document the effects of reductions of treated wastewater on vulnerable species and habitats along the highly urbanized, 45-mile stretch of the lower river, as well as on recreational uses of the river. One of the project’s goals is to determine how these impacts could be offset by investments in river restoration and upstream mitigation projects.

The SCCWRP researchers plan to develop recommended seasonal flow targets for each section of the LA River. They will consider the balance between protecting the river’s ecological and recreational uses and local agencies’ desire to capture, divert, and recycle more water in the watershed. The study will also help inform a number of planning efforts, including One Water LA and the LA River Revitalization Master Plan.

Eric Stein, principal scientist with SCCWRP, emphasized that the project’s success is dependent on its collaborative nature. “We are bringing together agencies, municipalities, nonprofit organizations, and community groups to help explore alternative future scenarios and find innovative ways to balance potentially competing demands for water in the LA River,” he said.

The LA River watershed is only one example where a conflict between recycled water investments and downstream users is emerging. As the demand for recycled water grows and local agencies consider new projects, other watersheds could benefit from similar efforts to better understand the impacts of water recycling on the local ecology and downstream users.