Water, or the lack of it, has emerged as one of the greatest sources of stress for California, its people and its native species. Fish populations are declining in the Sacramento-San Joaquin Delta, while farmers are facing short supplies. Urban dwellers have come under pressure to use less water, and underground reserves are being rapidly depleted. Making matters much worse is the ongoing drought, which shows no sign of ending. In fact, forecasts for less annual rainfall in years to come have cast uncertainty on the very future of California and its rapidly growing human population.

But state officials have proposed a solution – a massive hydroengineering project dubbed California WaterFix. Its two giant tunnels will divert water from the Sacramento River toward Silicon Valley, Los Angeles and farms in the San Joaquin Valley. The project is billed on its website as “a real solution that provides reliable, clean and safe water to California businesses, farms and residents … while protecting our environment.”

There are firm believers in the project who see it as the only road forward in an age of insecure water supplies, and there are firm opponents who warn the tunnels will irreversibly destroy the Sacramento-San Joaquin Delta. There may be some truth to both arguments. And there is also a lot of confusion and misinformation about the project as a whole.

To counter that, we answer key questions about California WaterFix, with help from Erin Mellon, communications and outreach adviser with the California Natural Resources Agency, and John McManus, executive director of the Golden Gate Salmon Association. We consider the pros and cons of the tunnels to better understand what WaterFix means for Californians.

Why exactly do we need a new water supply system?

Currently, two large pumping stations at the south edge of the Delta divert millions of acre-feet of water each year from the estuary. This causes serious problems in how the estuary functions. Specifically, having the pumps at the south end of the Delta can reverse the westward flow of freshwater, usually moving toward San Francisco Bay, and, instead, move the Sacramento River’s water southward, toward the pumps themselves.

This draws salmon and steelhead off their migration courses and into remote sloughs and backwaters, where millions become hopelessly lost and may be eaten by predators. Laws meant to protect fish limit when the pumps can be operated – and how much water they can remove from the Delta. This leads to supply problems at times for farmers. So the current system isn’t good for people or for fish.

How will WaterFix change this?

WaterFix could alleviate the problems associated with pumping from the south Delta by withdrawing the water more than 20 miles (32km) upstream, near the town of Hood. The idea here is that, after a certain amount of water is diverted into the tunnels, the water left in the river will move through the Delta from east to west, every day of the year, like it always did prior to the installation of the state and federal water projects, Mellon says. The Delta, she explains, will once again function as it historically did, fish numbers may rebound and farmers who need water on a daily basis can reliably expect to receive it.

But if it’s really a good plan for everyone, why are fishermen and environmentalists generally opposed to the tunnels?

Scientists have said that the tunnels could benefit native fishes, such as Chinook salmon, for the reason described above. That, however, will be the case only if the tunnels don’t withdraw too much water from the Delta, and many WaterFix opponents believe the tunnels may actually increase the amount of water being taken from the Sacramento River and, ultimately, worsen the plight of Chinook salmon and other Delta fishes.

How big will the WaterFix tunnels be?

The two proposed tunnels will each be 40ft (12m) wide and will run about 35 miles (56km) from the three intakes to the south edge of the Delta, where the tunnels will join with the existing water pumps that currently supply the San Joaquin Valley and cities to the south and west.

How much water will they be able to take from the Sacramento River?

The tunnels, which will be 150ft (46m) underground, will have the total capacity to move 9,000 cubic feet (255 cubic meters) of water per second. That’s more than the entire volume of the river for most of the year, though it’s just a fraction of the total flow during flood stages.

Are we sure they won’t take too much water?

No, we aren’t sure. In fact, no one really knows what the Delta’s maximum sustainable water export limit even is. What we do know is that the current export rates, of about 4.5 million acre-feet (5,500 million cubic meters) each year on average, seem to be exceeding the sustainable limit, since nearly all fishes native to the Delta, as well as striped bass, are in steep decline.

Currently, the State Water Resources Control Board is working on determining the Delta’s minimum flow needs and how to balance that figure with the minimum export needs of cities and farms. “But until they make their flow determination, you cannot responsibly size a water tunnel project like this,” argues McManus, of the Golden Gate Salmon Association.

But state officials have promised the tunnels will not exceed sustainable river diversions. Shouldn’t the public trust them?

While state officials promise that the intakes will never be opened to full capacity except during flood periods, McManus and other critics don’t believe that. “They’re telling us to trust them because they’ve got their hand on the valve,” McManus says. “But we can’t trust them.” That, he explains, is because of what has happened over the past two years on the Sacramento River.

In the spring of 2014, after two years of drought, the water level in Lake Shasta was very low. The U.S. Bureau of Reclamation – the agency that will help operate the Delta tunnels – promised it would leave enough water in the reservoir so that Chinook salmon would have the cold flows they need during their summer and fall spawning period. Water warmer than about 58F (14.4C) is lethal to fertilized Chinook eggs.

“But they didn’t fulfill their promise,” McManus says. “They let too much water out, and we lost 95 percent of the winter-run spawn.” A year later, spring conditions were similar. “And they said, ‘OK, you can really trust us this time,’ and they did the same thing again,” McManus says. At least 95 percent of the Chinook’s endangered winter-run was destroyed for the second year in a row, as well as much of the commercially valuable fall run. “There’s not a salmon fisherman in California who should believe [state and federal water officials] again,” he says.

What other options do we have besides building the tunnels?

Biologists and environmentalists have long argued that simply curbing the amount of water leaving the Delta via the existing pumps could alleviate the estuary’s ecological havoc. Farmers and their lobbyists say otherwise – that no amount of reduction in water diversions via the Delta pumps will help revive salmon or smelt numbers because, they claim, pumping is not the source of the ecological problems. Instead they point to pollution, overfishing, cycles in ocean productivity and a suite of other factors as explanations for declines in fish numbers.

How do climate change and sea-level rise factor into the tunnels debate?

The ocean is rising as the polar ice sheets melt, and this could, within several decades, put the intakes of the existing pumps at or below sea level. If this happens, it will be very difficult and expensive to keep saltwater out of the state’s main water supply system. But the town of Hood, where the intakes to the tunnels would be, sits at about 7ft (2m) above sea level – enough, officials believe, to eliminate the risk of saltwater intrusion.

How will Delta residents be affected by the construction of the tunnels?

Mellon says 34 homes will be affected. Residents of half those homes, she says, will need to be permanently relocated. The rest will be temporarily relocated. Those who lose their homes to the project will be given new ones by the state and will be fairly reimbursed for the inconvenience, she adds. There are other concerns, too. Downstream from the diversion points, Delta residents have expressed serious concerns that the Delta will be all but sucked dry by the proposed tunnels, leaving not enough water for Delta farmers and communities to live on.

How much will WaterFix cost?

The state estimates the bill will run to $17.1 billion. Other estimates, especially from opponents, have run as high as $50 billion.

Who will be paying for the tunnels?

Public water agencies, which deliver water to urban users and farms, will be paying the bill. This ultimately means their customers – that is, the California public – will be paying through increased rates.

Will the public be able to decide whether the project goes forward or not?

Not directly. Mellon says the elected board of directors for the state’s public water agencies will eventually decide whether or not they want to fund the project, but it also needs to pass some regulatory requirements.

At what stage is the project now?

State officials are currently working on a final environmental impact report and other documents required by the California Environmental Quality Act and the National Environmental Policy Act. Also underway now are public meetings, held by the State Water Resources Control Board, reviewing the potential impacts of adding the three new water intakes that will feed the tunnels. In early 2017, the board will hold meetings that specifically address how the tunnels will affect fish and wildlife, recreation and other public trust resources.

How long will construction take?

Mellon says that, if the project is approved, construction could start in 2018 and would take roughly a decade.