CARLSBAD, CALIFORNIA (REUTERS) - The reservoir created by Hoover Dam, an engineering marvel that symbolised the American ascendance of the 20th century, has sunk to its lowest level, underscoring the gravity of the extreme drought across the US West.

Lake Mead, formed in the 1930s from the damming of the Colorado River at the Nevada-Arizona border about 50km east of Las Vegas, is the largest reservoir in the United States.

It is crucial to the water supply of 25 million people, including in the cities of Los Angeles, San Diego, Phoenix, Tucson and Las Vegas.

As at 2pm on Thursday (June 10), the lake surface fell to 327m above sea level, dipping below the previous record low set on July 1, 2016.

It has fallen 42.7m since 2000 - nearly the height of the Statue of Liberty from torch to base - exposing a bathtub ring of bleached-white embankments.

The drought that has brought Lake Mead low has gripped California, the Pacific North-west, the Great Basin spanning Nevada, Oregon and Utah, plus the south-western states of Arizona and New Mexico and even part of the Northern Plains.

Farmers are abandoning crops, Nevada is banning the watering of about one-third of the lawn in the Las Vegas area, and the governor of Utah is literally asking people to pray for rain.

Firefighters are facing worsening conditions this summer - after nearly 10,000 fires in California alone during the last wildfire season burned 1.7 million hectares, an area nearly as large as Kuwait.

Droughts are a recurring natural hazard but made worse recently by an accumulation of extremely dry years for most of this century.

Scientists say human-influenced climate change has exacerbated the situation.

The rain that deluged the West at the end of 2015 - before the previous low-water mark was set at Lake Mead - was a mere respite from what is now a 22-year drought, the driest period in 115 years of record-keeping by the US Bureau of Reclamation, which manages water resources in the western states.

"In some states, especially parts of California and parts of the south-west, it's really quite extreme drought conditions,"said climate scientist Ben Cook from the National Aeronautics and Space Administration's Goddard Institute for Space Studies.

In his decade of farming in North Dakota, Mr Devin Jacobson has never seen it this dry.

His 1,416ha of mostly durum wheat, canola, peas and lentils near Crosby have seen little rain this season beyond 50mm in late May and 6mm this week.

"Another couple inches would put is in a pretty good spot, but there's nothing like that in the forecast right now," the farmer said.

Officials across the west are enacting emergency measures.

On Wednesday, Arizona's governor declared an emergency after two fires burned more than 58,679ha and triggered evacuations.

Arizona is "in a completely unique situation relative to our historical records", said climate scientist Michael Crimmins from the University of Arizona.

"We're just desperately looking to the forecast to see when the monsoon might show up."

The Bureau of Reclamation is likely to declare Lake Mead's most extreme shortage condition for the first time, which would cut water supplies to Arizona, Nevada and Mexico, spokesman Patti Aaron said.

Arizona could have its supply cut by 320,000 acre-feet, Ms Aaron added.

That is a year's supply for nearly 1 million households, according to the Arizona Department of Water Resources.

California Governor Gavin Newsom, facing a recall election, has issued a drought emergency proclamation for 41 of the state's 58 counties, empowering the state to take greater control over water resources.

But he so far has stopped short of measures taken by his predecessor Mr Jerry Brown in 2015, when California ordered mandatory water use reductions that affected voters.

For now, water management mostly concerns agricultural businesses, which consume up to 80 per cent of California's water.

Some farmers are switching to less thirsty crops or letting land go fallow.

The Regional Water Authority, which represents water providers serving 2 million people in the Sacramento area, is recommending providers drill more wells for now, a short-term solution, and is asking customers to voluntarily reduce consumption 10 per cent.

Professor Jay Lund from the University of California in Davis and director of its Centre for Watershed Sciences, warned that some of the more dire predictions were hyperbolic, saying Californians generally comply with mandatory and voluntary reductions in water usage, enabling the state to survive until the rains come again.

"There's going to be a lot of pain in this drought," Prof Lund added.

"It'll be catastrophic for some communities and for some local industries. It'll be catastrophic for some fish species. But it is not going to be catastrophic statewide."