By Colin Green of Middlesex University
Two popular explanations for floods are climate change and deforestation.
There is little doubt that climate change has occurred and that, even with drastic action to reduce emissions of greenhouse gases, the climate will not stabilise for another 50 years.
Floods are also frequently blamed on deforestation - floods in India and Bangladesh often being said to be caused by deforestation in Nepal.
Deforestation is not a cause
There are many reasons why deforestation is both socially and environmentally damaging.
The loss of trees means that women have to spend hours each day collecting fuel wood.
The absence of trees can also increase both soil erosion and make slopes less stable, making landslides more likely.
Natural forests are also rich in biodiversity and capture carbon dioxide, one of the main greenhouse gases.
However, the loss of forests in Nepal is not the explanation for the flooding of the Ganges river in India.
Over the last 20 years, only 3% of Nepal has been deforested - not enough to cause significant changes in the intensity of flooding.
Some of the wettest places on earth
A far greater proportion of Nepal is bare rock mountainsides, where every drop of rain runs off into the rivers.
The problem is that India gets most of its rainfall during the monsoon season and these rainfalls are very intense.
Apart from one or two islands, north-east India is the wettest place on earth.
Cherrapunji, in Meghalaya state to the south of Assam, has the world record for rainfall.
The rainfall on the catchment of the Ganges river is much less intense, but the area the river drains is much larger.
In terms of what could happen, the floods of both the Ganges and the Brahmaputra are reported to be quite small floods, with a chance of about 1 in 20 occurring in any year.
The effects are so dramatic because of the size of the floodplains and the number of people living on them.
They are densely populated precisely because they are floodplains.
They usually have very fertile, alluvial soil, deposited by the river and plenty of water with which to irrigate crops.
The flood plains are heavily populated
Bangladesh can support the highest population density of any country in the world because it is a river delta where up to three crops a year can be grown.
The Ganges and the Brahmaputra are two of the three great rivers that drain through Bangladesh.
The floods in India may be a sign that severe flooding will again affect Bangladesh in a week or two's time.