The water level at a bridge in the South's border county of Yeoncheon surpassed 1 meter about 3.3 feet on Monday afternoon, government data showed, requiring visitors on the riverbank to evacuate.
An official at South Korea's unification ministry, who handles inter-Korean affairs, said North Korea appears to have released water from its Hwanggang dam.
Yeoncheon county sent alarm messages to nearby residents, warning them to move to a safer area.
It was not immediately clear how many people had to leave after the suspected water release.
The water level is now decreasing after it has hovered over 1.6 meters 5.2 feet, a Yeoncheon county official said. North Korea is stepping up efforts to prevent flooding from recent heavy rains. South Korea has repeatedly urged the North to give notice before releasing water from the dam, as the river flows through Yeoncheon, but Pyongyang has remained unresponsive. In 2019, North Korea released water from the dam without warning, resulting in flash flooding downstream that killed six South Koreans. The North Korean delegate expressed regret for the deaths and condolences to the families affected by the situation, with officials from the two countries meeting to discuss the issue.