Sens. Barbara Boxer and Dianne Feinstein of California appear with other Democrats at a campaign event for Hillary Clinton last year. (Melina Mara/The Washington Post)

A bill that stands to help farmers struggling through California’s epic drought sits on President Obama’s desk, awaiting a signature that would allow water in a river delta where fish stocks are depleted to be diverted for the state’s parched agriculture.

The story of how the legislation reached the White House reflects the drama of water wars that have pitted groups within California against one another in a fight to survive. The battle over the bill has also divided two of the closest friends and Democratic allies on Capitol Hill, California Sens. Barbara Boxer and Dianne Feinstein.

Their split made headlines this month, given their interwoven history: The pair entered the Senate together 24 years ago in the “Year of the Woman” advance that tripled the number of that chamber’s female members. Over the past quarter-century, even as they became two of the most powerful women on the Hill, Boxer and Feinstein have agreed on virtually every issue that came before them.

Then came the water legislation, which the Senate passed a week ago. Boxer had worked on it for two years as a ranking Democrat on the Environment and Public Works Committee. Shortly before its full vote, Feinstein suddenly pushed forward a 90-page rider that would send greater amounts of river water to farms. In the end, Boxer felt she was forced to campaign against her own measure.

[A giant reservoir that supplies a California county’s drinking water is nearly empty]

“It was heartbreaking,” she said several days ago, in the first remarks either lawmaker has offered in how the fight over farmers and fish came between them. Feinstein termed the standoff “very difficult for me.”

For many Californians, few issues are more rancorous than those involving water. It is the state’s lifeblood, more so than Hollywood, beaches or freeways. It sustains one of the world’s richest economies and is all but worshiped. And with the historic drought entering a sixth year, with residents being called on to conserve even more and farms shriveling, water has become even more precious.

The drought is causing an environmental nightmare. There are now forests of dead trees. Thirsty animals have begun emerging from the woods and venturing into towns and cities in search of a drink. Fish cannot reach the Pacific because large swaths of rivers have gone dry.

Young salmon are loaded onto trucks at the Coleman National Fish Hatchery in Anderson, Calif., to be driven to the Sacramento-San Joaquin Delta, an effort forced by the state’s historic drought. (Andreas Fuhrmann/Redding, Calif., Record Searchlight via AP)

Humans enjoy nearly every drop of water in the state, but a fraction is set aside for fish. Guided by biological science that determines how much water anadromous fish need to reach the ocean, federal and state officials release amounts at specific times as part of the Endangered Species Act. A $1.4 billion commercial and recreational fishing industry in part depends on their action.

But Boxer said in an interview that language in Feinstein’s rider would allow officials to ignore the science and send more water to farmers — who already use 80 percent of the state’s supply — than the act allows. With the rider attached, she was forced to speak against her own bill. “It was an out-of-body experience,” she said.

Feinstein countered in a separate interview Thursday that diverting water doesn’t kill nearly as many salmon and smelt as other problems in rivers that the rider seeks to fix, such as water polluted by sewer system outflows and invasive species that prey on scores of young fish.

[Wild animals in the drought stricken West are dying for a drink]

Drought has to be addressed not just for fish but for people in a state that has changed considerably since its water system was established in the 1960s, Feinstein said. “It was built . . . when the state was 16 million people. We’ve got 41 million people now.”

Nearly every state association for fisheries opposed Feinstein, as did just about every major newspaper in editorials. Growers sided with her, saying thousands of jobs and crops that feed much of America are being lost to save fish.

Agriculture has fought court-sanctioned scientific opinion that has restricted pumping more water to farms for nearly a decade, said John McManus, executive director of the Golden Gate Salmon Association. “And with this rider by Feinstein, they’ve finally won.”

McManus said he fears his industry will return to the days when salmon fisheries were shut down to help the depleted stock recover.

“We just lost our protections,” he said.

Tom Nassif, president and chief executive of the Western Growers Association, sees the recent vote differently. When rains fell and the rivers filled, federal officials who control river flows allowed excess water that farms could have used to flow to the ocean. “It’s not our intention to endanger animals,” he said, but “we’ve had to fallow a lot of land, the cost of water keeps going up, and we are losing jobs, crops, and farms are having to consolidate. I think the facts prove they send more water to the ocean than what’s necessary.”

Not surprisingly, environmental groups side with Boxer. Peter Gleick, a scientist at the Pacific Institute, said, “This is really a fight between the old way of dealing with water problems in California and the new way we have to tackle them. The old way was to take all the water we need for humans and screw the fish.”

Circle G Farms owner Christopher Hurd walks through a grove of pistachio trees at his farm in Firebaugh, Calif. Farmers struggling with drought in the state’s Central Valley got none of the water they requested from a federally controlled water system. (David Paul Morris/Bloomberg)

“The little bit of water that legal decisions have set aside for the fish isn’t enough; everybody understands that. It’s true that other things are hurting the fish as Feinstein says; everyone understands that,” Gleick said. “But you can’t take more water from the fish and think that’s going to solve the problem.”

In the interview, Boxer continued to call Feinstein “my friend, my colleague, my pal,” but added, “A poison pill rider that drops on a beautiful bill at the last minute is why people hate Congress. It’s the way people do things when they can’t stand up to the regular order.

“If Senator Feinstein is saying this is the way to do business, I would say I disagree with that strongly,” Boxer continued. The rider “never had a hearing. It never even went to my committee because it’s in the jurisdiction of the Energy Committee. That’s not the way to legislate in my opinion. It’s not right to have not one hearing.”

Feinstein has long been a friend of farmers and delivering water to them is an issue that has separated the two lawmakers before. Aides to Feinstein said she has spoken with bankrupted growers who broke down in tears because their farms no longer produced.

[Showers and toilets went dry when this California county ran out of water]

As Boxer searched for a way to fix infrastructure and clean water, Feinstein looked for ways to deliver water to thirsty fields. She said her staff relied on research and marathon discussions with scientists at the U.S. Fish and Wildlife Service, as well as other agencies in the Commerce and Interior departments, to craft the language they inserted.

Aides to Feinstein said California water issues are difficult to legislate and are often resolved at the last minute. They said the rider includes endangered-species protections for fish that must be enforced by law, and they maintain that fish and the state’s ecosystem won’t suffer for farmers. But the bill also gives the Commerce and Interior secretaries in President-elect Donald Trump’s administration more say in how much water should be diverted to California farmers, who overwhelming supported the Republican nominee.

“That sort of discretion might have been tolerable if entrusted to cabinet members of an environmentally responsible administration,” a recent Los Angeles Times editorial noted, “but it must be seen in a different light with a White House with a decidedly different approach to the environment.”

Boxer is about to retire, but Feinstein’s current term continues through 2018. “I’m going to fight like a lion to see this bill is carried out as intended,” she said.

The two senators agree on one thing. “There are other issues that can be addressed and must be addressed when it comes to protecting fish,” Boxer said. “However, this is the elephant in the room, how much water these fish have.

“This is a clear issue. Does it hurt the fishing industry? Does it injure the environment of California? This isn’t personal,” Boxer said. “This is who’s hurt by this.”