A little read from wiki.

"In the 19th century, six major floods were recorded: 1842, 1858, 1871, 1875, 1885 and 1892. Eighteen major floods occurred in the 20th century. Those of 1987, 1988 and 1951 were of catastrophic consequence. More recent floods include 2004 and 2010.

The catastrophic floods of 1987 occurred throughout July and August[4] and affected 57,300 km2 of land, (about 40% of the total area of the country) and was estimated as a once in 30-70 year event. The seriously affected regions were on the western side of the Brahmaputra, the area below the confluence of the Ganges and the Brahmaputra and considerable areas north of Khulna.

The flood of 1988, which was also of catastrophic consequence, occurred throughout August and September. The waters inundated about 82,000 km2 of land, (about 60% of the area) and its return period was estimated at 50–100 years. Rainfall together with synchronisation of very high flows of all the three major rivers of the country in only three days aggravated the flood. Dhaka, the capital of Bangladesh, was severely affected. The flood lasted 15 to 20 days.

In 1998, over 75% of the total area of the country was flooded, including half of Dhaka. It was similar to the catastrophic flood of 1988 in terms of the extent of the flooding. A combination of heavy rainfall within and outside the country and synchronization of peak flows of the major rivers contributed to the river. 30 million people were made homeless and the death toll reached over a thousand.The flooding caused contamination of crops and animals and unclean water resulted in cholera and typhoid outbreaks...

...The 1999 floods, although not as serious as the 1998 floods, were still very dangerous and costly...The 2004 flood was very similar to the 1988 and 1998 floods with two thirds of the country under water.

In early October 2005, dozens of villages were inundated when rain caused the rivers of northwestern Bangladesh to burst their banks.

Floods also occurred in 2015 and 2017."

That little narrative reveals how flooding from rivers in Bangladesh has significantly increased in frequency, and, arguably, in intensity, over a couple of hundred years. I could highlight bits but we can all read. It also reveals the unreliability of "return period" predictions. I suggest that increased river flooding may be caused by heavier rainfall, by silted river beds, by rapid runoff due to deforestation and overgrazing and by rapid melting of glaciers and snowfields, or, very likely I should think, by a combination of factors. Yer man who wrote that piece is keen to exonerate mountain dwellers but he explains nothing. Very odd.