T HE FEDERAL government and the state of California seem to love suing each other, and have done so dozens of times in the past two years without causing anyone much damage. But President Donald Trump is now threatening to sue the state over control of water. This could harm a lot of people, because water is the source of the most contentious and enduring battles in America’s largest state.

In October, Mr Trump ordered his administration to speed up (meaning, relax) environmental reviews of Californian water projects and to suspend or dilute rules that prevent water being siphoned-off for farmers. The timing of the memo was an election gimmick. It aimed to please Republican voters in farming districts on the eve of the mid-term elections. But this was not the first action of its kind.

In July the Bureau of Reclamation, the federal agency that oversees water management, threatened to sue California’s water board, arguing that its plan to keep more water in the state’s two largest rivers would unfairly reduce the amount diverted away for agricultural and urban use. The secretary of the interior then threatened to break off relations between the federal government and state water managers unless the state dropped its plan. At the same time, the bureau said it wanted to renegotiate a landmark agreement of 1986 which governs how much water federal and state agencies may pump out of the delta formed by the Sacramento and San Joaquin rivers. The California delta is the largest estuary on America’s Pacific coast and the source of water for 25m people. All this seemed straight out of Mr Trump’s playbook: bluster, threaten to rip up an old treaty and try to impose a new one.

This example might seem doomed to fail for legal reasons. Normally in federal matters, federal law outranks state law. But water is different. An act of 1902 (the Federal Reclamation Act) says that state law precedes federal law on disputes about water, even regarding dams and levees run by the federal government. A federal law concerning California, the Central Valley Improvement Act of 1992, reasserted the primacy of state law and was confirmed by the Supreme Court. Whatever Mr Trump’s administration may think, the feds must abide by Californian law, not the other way around.

But in practice, says Rick Frank of the University of California ( UC ), Davis, “there is a lot of play in the system.” The federal authorities can do a lot of harm if they stop talking to state managers. To understand why, you must go back to first principles.

Three-quarters of the state’s water falls as rain and snow north of the California delta. Three-quarters of the water is consumed south of it. A vast network of dams, canals and pumps shifts water from north to south. The system is an integrated one, but when it was planned in the 1930s (during the Depression), the state could not afford to finance the whole project. The federal government stepped in and built (and still manages) many of the dams and pumps. Operationally, the two sides need to work together.

Over the years demands on the system have increased. There is not enough water to satisfy farmers, city dwellers, fishermen, environmentalists, and so on—and conflicts have to be resolved by two groups of managers, not one. To make matters worse, climate change and a six-year drought that ended in 2017 have increased the stress on the hydrology of the state, forcing a further squeeze on users and ever-tougher choices.

The next two years will be exceptionally important because, in addition to the usual conflicts, the state’s water managers face big decisions in four areas. A breakdown in relations with the federal government, worries Leon Szeptycki, a water expert at Stanford University, could make agreement on each of these matters harder.

The first task is to replenish groundwater depleted by farmers during the drought of 2011-17. Normally, groundwater (in aquifers) accounts for about 30% of urban and agricultural water use. During the recent drought, however, the share rose to over 50%. Worse, according to the Public Policy Institute of California ( PPIC) , a think tank, groundwater reserves have been depleted more during each successive recent drought. A new law called the State Groundwater Management Act ( SGMA ) tries to deal with over-use by requiring water users to organise themselves into groups and asking each group to show how it will stop depleting groundwater in 20 years’ time. The plans, which constitute one of the most ambitious water-conservation projects ever, must all be approved by 2022. They are vulnerable to disruption. Any breakdown in relations between the state and federal authorities could undermine SGMA if, for example, the two sides issued different instructions on how much water may be pumped out of the system.

Second, the state’s water-control board is trying to increase the flow of water in the main rivers of the delta. Early in 2018 it reaffirmed that it wants to increase the amount in the San Joaquin river from its current level of 20% of its natural flow to 40%. The measure is long overdue. Some of the river fish, including the Delta smelt, are on the verge of extinction. Water-users are divided about the plan, with fishermen pitted against farmers, and both fearing for their livelihoods. The state governor, Jerry Brown, has set up a process for settling these disputes. As with the groundwater plan, the threatened rupture between state and federal authorities could wreck the settlement process.

Third, the state is trying to build two new tunnels in the Delta as a more reliable and, its proponents claim, less environmentally damaging way of pumping water out. The $15bn project, called Water Fix, has been in the works for decades and is probably nearer to getting the go-ahead now than at any time since 1980. But this too is vulnerable to a state-federal bust-up because the state may not be able to finance the project on its own. Adding to the uncertainty, the project’s main supporter, Mr Brown, is retiring and it is not clear that the new governor, Gavin Newsom, will throw his weight behind the scheme.

Lastly, the third giant river feeding southern California, the Colorado, is in the middle of an unprecedented 19-year drought. Of its two reservoirs, Lake Powell is half full and Lake Mead barely a third full. In early October, the seven states (including California) that share its waters issued drafts of new agreements to stop water levels falling further and to forestall rationing. The Trump administration’s threat to break off relations with California referred only to the management of water within the state, not to the Colorado. But once disputes start, they are not easy to contain.

Optimists argue that brinkmanship has always been part of California’s water wars and no one has ever careened over the edge. And as Jay Lund of UC , Davis says, “water users all know they have to get along after Trump has gone.”