As heavy rain pummeled Japan for a second straight day, the Kinugawa River broke through a flood berm, sending a wall of water into Joso, about 50 kilometers (30 miles) northeast of Japan's capital.
National broadcaster NHK showed aerial footage of rescuers lowered from helicopters and clambering onto second-floor balconies to reach stranded residents. There were no immediate reports of deaths or injuries.
Nearby a man clung to a utility pole as the waters rose, before being taken up by a rescuer who had to be first lowered into the water so her could make his way over to the man.
Others waved cloths from their deck as torrents of water washed away cars and knocked buildings off their foundations.
Japan's Kyodo news service reported that 39 people had been rescued by Japan's Self-Defense Forces, as its military is called, and rescue work was continuing.
The Transport Ministry estimated that 6,900 households have been affected by the flooding, Kyodo said, adding only about 2,500 of the city's residents had been evacuated beforehand to shelters. The floodwaters reached at least 8 kilometers (5 miles) from the breach.
Tokyo was drenched with rain, but the hardest-hit area was to the north in Ibaraki and Tochigi prefectures. One woman was missing hours after a landslide hit houses at the foot of a steep, wooded incline. Bullet train service was partially suspended.
The Fire and Disaster and Management Agency said 15 people were injured by Etau, two seriously, both elderly women who were knocked over by strong winds.
