An infant flushed this perfect pool day down the toilet at one Gowanus swimming spot.

“Somebody didn’t put a Pampers on their kid,” a staffer at the Douglass and DeGraw Pool told The Post about the Friday evening accident.

Patrons who showed up Saturday morning hoping to beat the heat with a refreshing dip were livid to learn about the unsanitary conditions.

“What the f***? On the hottest day? They closed the pool?” fumed Suegeil Mercado, 43, who brought her 6-year-old daughter to the Gowanus pool. “And the mayor said he was going to open the pools early for the heat. We got up for this too!”

Around 11 a.m., a line began to form outside the gates. Workers were on hand to inform people of the closure.

“Red Hook it is!” one disappointed pool-goer exclaimed upon learning of the shut-down.

Pool staffers attempted to placate another disappointed kid with snacks.

“He’s very upset,” said Zina Franklin, 55, of her 11-year-old grandson, Jeremiah.

“We’re gonna suffer out here,” she said, noting that temperatures will be too high for a beach day. “The sand is going to be too hot. They said on the news you’re going to get blisters [on your feet].”

The pool is expected to reopen Sunday at 11 a.m., according to a posting on the city Parks website.

Parks has extended hours at all city Olympic- and intermediate-sized pools to 8 p.m. Saturday and Sunday.

Temperatures are expected to top 100 degrees on both days, with a heat index as high as 111.