A thriving river, teeming with wildlife.

A future in which the city meets its own water needs without importing extra from elsewhere.

Can Los Angeles have both?

That’s the challenge facing a city that aspires to live within its environmental means.

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If local agencies follow through on their most ambitious plans to capture more stormwater and reuse treated wastewater, the 51-mile-long Los Angeles River will probably dry up for a few months of the year, according to research from UCLA.

That, in turn, would kill fish and make the river less hospitable to the plants, birds and other wildlife that call it home.

“It’s going to be a tough tightrope to walk,” said Mark Gold, the environmental policy professor who led the UCLA study. “If we go all in on water recycling and stormwater infiltration and capture, then there’s not going to be enough water left for a thriving river.”

Steve Appleton, who co-founded LA River Kayak Safari five years ago, said it’s an issue he and others who use the river recreationally have been thinking about for some time.

Elementary school students kayak down the Los Angeles River in 2012. (Genaro Molina / Los Angeles Times)

“The UCLA study brings forward a really obvious set of contradictions,” he said. “We can’t come to the right conclusions if we don’t have a better idea of all the inputs and outputs.”

The L.A. River begins at the western end of the San Fernando Valley in the Santa Susana Mountains. From there, it cuts east until it reaches Glendale and then turns abruptly south, ultimately ending in Long Beach. It empties into the Pacific right near the Queen Mary.

As most Angelenos know, the river is not particularly scenic.

Most of the natural riverbed was paved after a massive flood in 1938 that killed 115 people. In the aftermath of that tragedy, the Army Corps of Engineers led the effort to create the deeply channeled, mostly concrete waterway that we have today.

“They built it for getting floodwater into the ocean as soon as humanly possible,” said Gold, whose research focuses on the environment and sustainability.

It worked.

Each year from 2003 to 2014, the river efficiently shunted an average of 274,000 acre-feet of water out of the metropolis and into the San Pedro Bay, the researchers said. That’s a bit more than half of the city’s water requirements for a year, the study authors noted.

An acre-foot of water is enough water to cover an acre of land one foot deep. A typical California household uses one-half to 1 acre-foot of water a year.

Sources: Mapzen, OpenStreetMap, LADWP

When it rains, the L.A. River is fed by an 825-square-mile watershed that encompasses parts of the Santa Monica and San Gabriel mountains, the San Fernando Valley, Vernon, Compton and Long Beach.

But when there hasn’t been a recent storm, more than 90% of the water flowing along its concrete bottom comes from three reclamation plants.

These plants take all the wastewater from apartment buildings, houses and commercial operations and cleans it.

Some of that water is used for irrigation at city parks and golf courses. Some feeds Lake Balboa.

But most of it — more than 30 million gallons each day — ends up in the river.

That might change.