Tokyo faced crisis last Saturday, with water levels in the Tama River quickly climbing as heavy rains and winds from Typhoon Hagibis inundated the Kanto region on an unprecedented scale.

Hakone, in Kanagawa Prefecture, saw a staggering 922.5 mm of rain that day alone — three times as much as the total for the month of October of an average year.

Levees all along the Tama, which stretches over 138 kilometers between Tokyo and Kanagawa Prefecture, were designed to withstand precipitation levels seen only once in every 200 years. But at the Ishihara observation station in the capital’s Chofu area, water levels had hit their highest-ever record of 6.24 meters by 11 p.m. on Saturday, far exceeding the 5.9 meters threshold the levees were built to withstand.

Since the levees were designed to have a safety margin of 1.5 meters, making their total height 7.4 meters at the Ishihara observation station, the riverbanks withstood the storm, but only barely.

Any failure of levees along the Tama River could have brought devastating flooding to areas of Tokyo and Kanagawa. For the first time ever, the city of Kawasaki issued an urgent warning, for 915,770 local residents to evacuate by 7 p.m. that night.

“Yes, the situation was very tense,” said Kenichi Ito, who heads the initial crisis management response team at Kawasaki Municipal Government.

That tense night for Tokyo and Kanagawa residents has underscored the risks Japan faces in the age of climate change, predicted to increase the number of powerful typhoons like Hagibis.