Four men have drowned in a US reservoir, after a prank turned deadly during a weekend boating trip.

On Sunday evening, police received a mobile phone call from a young child who was adrift with four other children on a boat in the middle of the American Falls reservoir.

The children, between two and nine years old, said one of the four men they were with was standing in the boat when another pushed him into the water as a joke.

"But, apparently, he couldn't swim, so he was immediately in distress," Power County Sheriff Jim Jeffries said. "The second man jumped in, and so then there were two of them in distress in the water, so the third jumped in and there were three in distress."

The last adult on the boat managed to grab a life jacket before jumping in to try to save the other three, Sheriff Jeffries said, but he was not wearing it and may have only hooked an arm through the device.

Meanwhile, the breezy weather caused the boat to drift away from the men.

Authorities have not released the names of the missing men in order to give their families time to notify other relatives. All were described as being in their 20s and 30s and from Pocatello, Idaho. Two were brothers.

Alcohol was found on the boat, but it was not clear if the men were intoxicated, Sheriff Jeffries said

All the children were wearing life jackets. There were enough jackets on the boat for the adults, but none were wearing them, Sheriff Jeffries said.

The boat also had a square flotation device that could have been deployed but was not, he said.

"I guess panic is a contributing factor," Sheriff Jeffries said.

The children called emergency workers soon after the men went overboard, Sheriff Jeffries said. It did not take marine patrol deputies long to find them and get them to safety.

Dive teams started their search that evening focusing on an area where a ball cap was found floating on the water.

Stormy weather and nightfall forced searchers to break until Monday, when they used the GPS co-ordinates from the mobile phone at the time of the emergency call and wind speed data to focus efforts.

The water was muddy, and visibility was low, Sheriff Jeffries said.

"The water's about 45 feet [13.7 metres] deep, and it's hard to see - you can see your hand, and that's about it," he said. "There's a lot of trees and rebar sticking up in the bottom of the water, and the divers are getting hung up in those sticks and things. It's a slow, tedious process."

The men's families were in shock and struggling to comprehend the tragedy, Sheriff Jeffries said.

"They have a pastor there with them," he said.         

Searchers found the bodies on Monday evening.

The Associated Press

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