On top of the torrential downpours over the Hantan River and Imjin River basins, North Korea opened the gates of the Hwanggang Dam upstream and released water, sending river levels in parts of Yeoncheon County, Gyeonggi Province, surging.
The Han River Flood Control Office raised the flood advisory for the Imjingyo point on the Imjin River in Yeoncheon County, Gyeonggi Province, to a flood warning as of 3:30 p.m. on the 21st.
As of 3:20 p.m. that day, the water level at Imjingyo had already risen to 10.3 meters. With the level climbing rapidly by 0.1–0.2 meters every 10 minutes, authorities expected it to surpass the flood warning threshold of 10.8 meters around 4:30 p.m.
A flood advisory was also issued from 3:40 p.m. for the Biryong Bridge basin nearby. As of 3:30 p.m., the Biryong Bridge water level was 10.5 meters, closing in on the advisory threshold of 11.5 meters.
The sudden surge in river flow stems from the recent downpours combined with the North's surprise release from the Hwanggang Dam. At Pilsung Bridge, located at the forefront near the Southern Limit Line, the water level reached 10.1 meters as of 3:20 p.m., leaving just 1.9 meters before the 12.0-meter "caution" stage in the border-area response levels.
The reservoir level of the Gunnam Flood Control Dam downstream of Pilsung Bridge also rose sharply to 34.8 meters. Water is currently rushing into the Gunnam Dam at 7,298 tons per second, nearing the maximum planned flood level of 40 meters. In response, the Hantan River Dam opened its emergency spillway from 3 p.m. to release water and began regulating levels.
As the situation grew urgent, the Han River Flood Control Office and the Yeoncheon County Office sent disaster text alerts to protect residents. The agencies said, "As the Imjin River keeps rising, traffic on some bridges may soon be restricted," and added, "Anglers or campers with tents along the river, fishers at work, and residents of nearby villages should immediately leave the riverbanks and evacuate to a safe place on higher ground."