HOOVER DAM, AZ - MARCH 30: A 'bathtub ring' surrounds Lake Mead near Hoover Dam, which impounds the Colorado River at the Arizona-Nevada border, on March 30, 2016. The white ring shows the effects of a drought which has caused the level of the lake to drop to an historic low. The ring is white because of the minerals which were deposited on the previously submerged surfaces. (Photo by Robert Alexander/Getty Images)

LOS ANGELES — California remains a holdout on a drought emergency plan for the Colorado River that is due next Tuesday by all seven river states. Holding up the plan has been a fight between two powerful water agencies in Southern California.

The drought contingency plan is designed to produce voluntary cuts that would keep the river and Lake Mead from reaching critically low levels. If the plan doesn't get finalized, the federal government could step in and force mandatory cutbacks instead of voluntary ones for a river that serves 40 million people and some 5 million acres of farmland.

Drought in the past decade has stressed the Colorado River to the limits and contributed to Lake Mead reaching perilously low levels. Lake Mead, located by Hoover Dam and a vital water source for California, Nevada and Arizona, had dismal levels last year that raised alarm.

"We have a supply crisis that has been developing due to drought on the Colorado River," said Jeffrey Kightlinger, general manager of the Metropolitan Water District of Southern California, a water wholesaler serving 19 million in six counties, including Los Angeles and San Diego.

The need for water led Metropolitan last year to approve funding to help pay some of the costs for a delta water-delivery tunnel project in Northern California that would bring more water to Southern California. But Gov. Gavin Newsom last month said he would scale back the $10 billion plan championed by his predecessor.

Newsom appoints members to an agency's board that represents the state in discussions and negotiations on river issues: the Colorado River Board of California.

Earlier this month, Imperial Irrigation District, a water district in California's southeastern Imperial County with senior rights to the Colorado River, announced a plan with water cutbacks. But it required that the federal government pay $200 million for restoring the dwindling Salton Sea, California's largest lake.

"Our job here is to protect the people and the environment of the Imperial Valley," said Robert Schettler, a spokesman for the Imperial district. He said the Salton Sea has become a health and environmental concern due to dust exposed by the receding lake.

However, the Imperial County agency's demand for federal funds as part of its plan didn't go over well with Metropolitan. The Los Angeles-based agency contends Imperial's condition for money would be unlikely to be met before the federal deadline.

"We gave [Imperial Irrigation District] time and space to continue to work on their Salton Sea issue, but at some point we felt we needed to move on," said Kightlinger.

The U.S. Bureau of Reclamation asked governors or their representatives of the river states of Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming to turn in the emergency drought plan for the Colorado River no later than March 19. Approvals from a handful of water agencies in California and Congress are what's left for the plan to become fully implemented.

The Colorado River typically accounts for about 25 percent of the water needs of Southern California, but during periods of severe drought the river can represent more than 50 percent of the region's water supply. The Las Vegas area gets nearly 90 percent of its water needs from the river system, and Arizona roughly 40 percent.

On Tuesday, Metropolitan's board of directors agreed to bear most of the state's water cutbacks as a way to prevent Lake Mead from reaching critically low levels. The agency could contribute as much as 700,000 acre-feet of water, or the equivalent of enough water for 2.1 million households for a year.

California is only required to make contributions if Lake Mead's elevation drops to 1,045 feet above sea level, or 44 feet below its current level of 1,089 feet.

Lake Mead, which supplies water to cities and agriculture areas, has a more than 50 percent chance of reaching dangerously low elevations in the next few years, according to Metropolitan. If that happens, it said "severe delivery cuts could be triggered, hydropower generation would be threatened and Metropolitan could be prevented from accessing conserved water it has stored in the lake."

"The key part for California is that once a shortage is declared, they still have some access to taking water out of Lake Mead," said Michael Cohen, senior researcher at the Pacific Institute, an Oakland-based think tank that studies water issues.

Added Cohen, "If Metropolitan didn't sign on to the agreement, when a shortage is declared (which could happen next year), Metropolitan could not access that water. So by signing on, they still have the ability to withdraw water that they've stored previously."