The US president told military engineers to open two dams in the state's central valley, claiming it would help put out blazes that have ravaged the city and would also irrigate farmland.

"Everybody should be happy about this long fought Victory!" he boasted on social media last week.

"I only wish they listened to me six years ago - There would have been no fire!"

But water experts say the order opening dams in California's San Joaquin Valley sent water down channels and waterways into irrigation ditches in the same valley -- nowhere near the fires, and at a time when farms there do not need irrigating.

Water scientist Peter Gleick of the Pacific Institute said the release amounted to Trump having "thrown away" billions of gallons of water.

It will "not be used or usable for firefighting, not be used by farmers since this isn't the irrigation season, and won't be saved for the dry season, which is coming," he said, according to the Los Angeles Times.

"California's water system is very delicately balanced among all of the competing interests, and this episode shows that even slight interference in that system can cause chaos."

Blazes that erupted around Los Angeles last month during hurricane-strength wind storms devoured 40,000 acres (16,000 hectares) and destroyed thousands of homes.

The battle to contain them was hampered in the first 24 hours by the fact that winds were too high for helicopters and planes to take to the skies.

With no aerial support, firefighters were dependent on hydrants that in one area ran dry because of the unprecedented demand.

Trump seized on that, claiming it was proof that California officials managed their water supplies badly, resurrecting erroneous claims about water from the north of the state spilling into the Pacific Ocean instead of being diverted to the south.

But his order to the Army Corps of Engineers to "open up the valves" did nothing to improve firefighting conditions and has instead drained water that farmers will likely need later this year, said Democratic congressmen Jared Huffman and Rick Larsen.

"These releases did not meet their stated intent of providing Los Angeles with additional water," they said in a letter to Defense Secretary Pete Hegseth and Interior Secretary Doug Burgum.

They "could reduce the availability of critical municipal and agricultural water supplies later in the year, further exacerbating the fire, safety, and economic risks facing this drought-prone region for years," the letter said.