When a smell so foul that locals called it “unimaginable” wafted over Cape Town this week, a search for the source of the stench choking the scenic South African tourist destination led to the city’s harbor.

Nearly a mile from the dock on Monday morning, Terence van der Walt, a local wine distributor, was stuck in traffic when the odor, made worse by the hot summer weather, began to drift into his car. With a smell so enveloping, rolling up his windows felt pointless.

“It was so putrid,” Mr. van der Walt said on Tuesday, describing his experience. “It would have been green if this were a cartoon.”

After the smell hovered over Cape Town for several hours, a team from the local environmental health department discovered the source: a 623-foot-long livestock carrier registered in Kuwait — with 19,000 cows onboard.

The carrier, Al Kuwait, had docked in Cape Town’s busy harbor on Sunday to replenish the feeding stocks during its journey to Iraq from the Port of Rio Grande in Brazil, according to shipping data. The animals had been onboard for more than two weeks.

It was the ship’s first time docking in South Africa, said Jacques Peacock, a spokesman for the national Society for the Prevention of Cruelty to Animals. So the organization had obtained a court order before the vessel’s arrival that allowed inspectors from the group to board the ship and inspect its cargo.