California recently eased its sweeping restrictions on water use in its towns and cities, signaling that even though much of the state continues to face extraordinary drought, a moderately wet winter has blunted officials’ sense of urgency over water shortages.

Seemingly overlooked, however, is the state’s enormous reliance on the Colorado for its urban water supplies – while the river is approaching its worst point of crisis in a generation.

The Colorado provides much of the drinking water for some 19 million of California’s 39 million residents, making up, as the state’s largest water utility puts it, “the backbone” of supplies for Los Angeles and 25 other cities and municipalities. In San Diego County, water from the river comprises 64 percent of total supplies.

“The importance of the Colorado River is very little understood by Californians,” Felicia Marcus, chair of the California State Water Resources Control Board, told ProPublica in an interview in February. “Folks just really don’t know where their water comes from.”

On May 18 – the same day the control board lifted its restrictions – Lake Mead, the reservoir storing southern California’s share of the Colorado River, reached its lowest point since 1937.

The lake is expected to reach new lows each day until the middle of June, when managed flows from reservoirs upstream will allow the bleeding to be temporarily staunched. However, levels will dive deeper still next year.

California gets its water from the Colorado through 242 miles [390km] of canals and pipelines that begin at Lake Havasu on the Arizona border, and are drilled through mountains into Riverside County, where the Colorado River Aqueduct empties into Lake Matthews, one of the largest reservoirs supplying the metropolitan south of the state. Another system, the All American Canal, runs 80 miles [129km] along the Mexican border and draws more water from the Colorado for California farmland.

The falling levels in Lake Mead will not immediately dry up those canals. The state is promised all of its water even as the Colorado dries up and Arizona, Nevada and other states face critical shortages, thanks to California’s seniority in interstate sharing agreements that stretch back some 94 years.

A boater drifts toward a boat ramp in an area that was once under water at the Lake Mead National Recreation Area, May 19, 2016, near Las Vegas. The lake has a hit a record low water level. (John Locher, AP)

But whether such a divisive posture can be maintained as supplies continue to dwindle is an open question. In a nod to an answer, California, along with those other southern states, has been considering voluntary cuts in its water use, in anticipation of Lake Mead’s supplies getting even worse.

That is almost certain to happen. The U.S. Bureau of Reclamation, which manages Lake Mead and distributes Colorado River waters, predicts the reservoir – now 37 percent full – will reach a new low in 2017, part of a steady decline that began more than a decade ago as southwestern water users continued to draw far more out of the Colorado each year than the river provides. By 2019, the bureau says, there is a 64 percent chance the water level in Lake Mead will drop low enough to trigger a federal emergency provision that mandates further cuts to the states.

California is the single largest draw on this resource – using nearly one-third of the entire Colorado River’s flow.

California’s water planners need to recognize that the Colorado may soon leave California with a significant shortfall. “That may happen sooner, rather than later,” said James Famiglietti, a senior water scientist at NASA’s Jet Propulsion Laboratory in Pasadena.

Meanwhile, the rest of California’s water supplies are far from secure. Though its largest reservoirs filled quickly after heavy rainfall in the northern parts of the state last winter, precipitation in the Los Angeles area was just 59 percent of normal; more than 43 percent of California remains in what the U.S. Drought Monitor describes as “extreme drought.” Furthermore, California pumped deeply into its groundwater reserves as a sort of “piggybank” over the past half-decade, depleting supplies that will take many wet years in a row – if not centuries – to replenish.

So why ease up on water limits now? Cities, Marcus told ProPublica, “thought we were being too paternalistic.” So the state decided to back off.

While California made permanent a set of common-sense limits on car washing and lawn watering, it set towns and cities loose to determine their own water conservation guidelines.

The relaxed posture is not meant to green-light runaway use; it is more of a trial run for local independence.

“It’s really a question of what is the appropriate role of the state emergency regulations,” said Marcus, who also pointed out that the restrictions applied only to municipal water; the state’s farmers, who use the vast majority of the state’s water, fall under a different set of limitations. “I called it a ‘show me the water’ approach. You prove to us you can go through three more lousy years, and show us where the water is coming from and set your own standard.”

But therein lies the risk. California has always faced water shortages. Yet until Gov. Jerry Brown imposed the statewide drought restrictions early last year, few California cities had effectively curtailed water use by their own free will.

This story first appeared on ProPublica.