Where the big rivers are
The biggest river deltas are flat and that’s why the cities which occupy some of the have expanded so much, so quickly. The last 50 years has seen a big population expansion on deltas – cities like Dhaka in Bangladesh. Twelve megacities on deltas have expanded in terms of populations from 62 million in 1975 to 153 million in 2010, an expansion that is not slowing.
‘Global Change’, which is the magazine of the International Geosphere Biosphere Programme (IGBP), has brought out a special number of deltas and the risks borne by city administrations that occupy deltas. The IGBP, in its own words, “coordinates international research on global-scale and regional-scale interactions between the Earth’s biological, chemical and physical processes, and their interactions with human systems”.
Flooding both from rivers and the sea is increasing. There was a storm surge in the Irrawaddy in Myanmar in 2008 when 200,000 people were killed. But people are still living on the delta. However, the estimate is that two million people have left the Indus delta in Pakistan to move to higher ground as salt water has invaded the farming zone. [A larger version of the graphic above can be found here (1.4MB). The original IGBP infographic which I have modified can be found here – caution, big file (12.7MB)].
The Po delta (near Venice in Italy) subsided largely because methane was being pumped from underground. They stopped the pumping and the delta is sinking 10 times less fast than it was. But the land surface is not actually rising, and it’s still below sea level. The Chao Phraya River Delta (along which Bangkok is built) subsided because of groundwater being pumped out to supply Thailand’s thirsty capital. So they introduced a tax on water use, such as showers. In Shanghai, the local government slowed the rate of pumping water out of the ground.
However, when countries set up commissions to look at the natural environment, it’s often water/river courses they’re concerned about, like with the Rhine. There is not so much focus on the delta. Where countries have tried geo-engineering, they can scarcely bear the prohibitive costs. It is estimated that China in the 15th to 18th centuries used 12-15% of its historical GDP in attempts to control the Yellow River from spilling out into its floodplain, but these gigantic efforts were never really successful.