Thousands of migrants are flowing across the US border in Arizona every day — literally through open floodgates that have made the Tucson post the busiest point of illegal entry into the country, The Post has learned.

US officials have inexplicably welded open 114 massive gates along the Arizona border to allow water to flow freely during the annual monsoon season and for the migration of an endangered species of antelope, officials said.

But the move is also letting an average of 1,400 migrants from as far away as China casually walk into the country daily — with overwhelmed and outnumbered border agents practically helpless to stop them.

“We thought the agents were going to tell us something,” one Ecuadorean migrant said. “But we just walked in.”

Said another from Cuba: “It was so easy to get into the US.

“Nothing like our journey through Mexico. That part was hard,” she added. “I thought there was going to be more security.”

Video taken by The Post shows a group of around 50 migrants strolling through the opened gates and into the US in a matter of seconds, with nobody stopping or questioning them.

Smugglers are capitalizing on the floodgate blunder, driving migrants by the busload to the border and dropping them off as if they were casual tourists. Once across, they turn themselves in to border agents and say they are seeking asylum.

Border Patrol agents call them “give-ups.”

Last month, 42,561 migrants were encountered at the Tuscon border post, a huge jump over the June number of 27,294. Tuscon now tops traditionally busier border spots at El Paso and Laredo in Texas, where 24,352 and 26,627 border crossings, respectively, were reported in July.

“We haven’t seen this many migrants since about 2008,” said Adam Isacson, director for defense oversight at the advocacy group Washington Office on Latin America. “With the end of Title 42, in a way that nobody oversaw, it seems to come back to Tucson.

“What you’re seeing is a lot of large groups who want to turn themselves in,” Isacson said. “Tucson has also traditionally been where smugglers concentrate Mexicans and Central Americans who don’t want to be detected. Now they’re seeing 100 people at a time who are not running away.

“It’s really becoming an epicenter,” he said. “This is big.”

Each of the 114 gates in Arizona, which have been open for nearly two months, has 12-foot doors wide enough for a motorcycle to drive through. Smugglers drive busloads of migrants to the Mexican side of the border, where they get off and simply walk into the US.

Border Patrol sources said the call to open the gates came from several federal agencies, including the National Park Service.

But because the monsoon season started late this year, they remained open for weeks before there was any rain — allowing migrants a dry path into the US.

“We tried to shut the gates but the order came down that we had to leave them open,” one source said. “You wouldn’t leave the front door of your house open in a bad neighborhood.”

The gates run along a 36-mile stretch near Lukeville, Arizona.

Unlike the largely South American migrants who have been nabbed crossing into Texas, the immigrants coming to Arizona are from throughout the globe, including India, Egypt and China.

And unlike many South American migrants, who are typically disheveled, exhausted and weary from a long and treacherous trek across the barren land, the migrants at Tuscon look more like vacation travelers.

Understaffed border agents can do little to stop the flood.

“Three nights ago, a big group of migrants were on the Mexican side,” one source said. “There were two agents on ATVs [all-terrain vehicles] and one line agent trying to stop them from entering. The agents blocked the gates with their quad [bikes]. The cartel guy just started pushing people.

“They rushed the agents. You had people climbing over quads. You had people pushing the agents. Not a single one got charged.”

In response to questions about the open gates across her state from The Post, former Democrat Senator Kyrsten Sinema, who switched to being an independent politician last December, said: “Arizona communities bear the brunt of the federal government’s failure on our border. The Administration must do more — border communities are stretched to their limits and they need increased support.

“I’m pressing for quick disbursement of federal resources to border communities, including local non-profits and first responders, so we can secure our border, keep Arizonans safe, and ensure the fair and humane treatment of migrants.”