An expert on river disasters says what's called the "backwater phenomenon" may have been the cause of fatal flooding in northeastern Japan during Typhoon Hagibis.
Five people were killed in the town of Marumori in Miyagi Prefecture, which is traversed by the long and wide Abukuma River.
Tokyo University of Science Professor Yasuo Nihei analyzed satellite imagery of Marumori shot by geospatial information company PASCO.
He says no levees were breached on the Abukuma River, but banks collapsed in several areas along its tributaries.
Nihei says rising water levels in the Abukuma River may have prevented tributaries from flowing into it, in what's called the backwater phenomenon.
He notes that the rain continued in a region that usually gets relatively little rain. He says new flood control measures may be necessary.
Nihei stresses the importance of getting information on rainfall, as well as water levels. He also says people who live near smaller tributaries that flow into large rivers should obtain data on the mainstream to assess conditions more accurately.
The backwater phenomenon was blamed for extensive flooding last year during heavy rains in Okayama Prefecture, western Japan.