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Wednesday, January 26, 2022

How are our rivers moving over time? Global database documents 40 years of change

The Ganges river delta is a good example of a system where a combination of migration-enhancing and migration-dampening factors balance each other out.

Written by Ritvik Chaturvedi | Bengaluru |
December 3, 2021 5:01:12 pm
River flowAs river deltas sustain the economies of millions of people, it becomes necessary to document the changes delta systems have undergone in the recent past. (Pixabay)

In order to understand how their course may change in future, a new study has examined 48 river delta systems across the world from a variety of climatic and socioeconomic contexts.

As river deltas sustain the economies of millions of people, it becomes necessary to document the changes delta systems have undergone in the recent past. This data can help governments manage population density and plan future city development.

The researchers identified four aspects that determine the movement and migration of river delta systems:
*the interplay between the effects of rivers, tides, and waves
*the amount of sediment that the channel carries (aka sediment flux)
*the frequency and magnitude of floods that occur
*the average size of the channel.

Also, high tides increase the input of the saline seawater in the delta and interact with the river discharge. The researchers hypothesised that increases in the sediment flux will cause greater changes in the delta channel and, thereby, cause it to migrate more.

The study found that all deltas exhibiting large migration rates, in excess of three metres per year, are dominated by river action rather than tides.

During high tides, there is increased input of seawater in the delta and the sediment that has already flowed out of the channel is pushed back into the delta, ‘acting as a stabilising force’. However, there are deltas that have a low migration rate but are dominated by river discharge.

What would explain this anomaly?

The reason could lie in the sediment being transported by the river (fluvial sediment flux), the study argues. Merely because the delta is river-dominated and has a significant tendency to change its channel does not necessarily mean it will do so, the paper cautions.

Sediment flux is a key driver for channel migration, as the course of the water naturally changes when sediment gets deposited/discharged at the mouth of the delta.

An oft-overlooked, but obvious factor is the biome classification. River deltas in frigid zones of the Earth will naturally have permafrost balance out all ingredients for a high migration rate.
The combined effects of all these factors, especially flood forcing, were also considered. The juxtaposition of all these shows that when there is high sediment flux, high flood frequency, and high degrees of river forcing, the systems have the highest rates of channel migration.

The Godavari river, India, and the Yellow and Yangtze river deltas in China are prime examples. At the other end of the spectrum are deltas with low flood frequency, low sediment flux, and even low river forcing. They naturally tend to have lower rates of channel migration; examples include the Vistula, Poland; Ebro, Spain; Rhine, Germany and Tone, Japan.

The Ganges river delta is a good example of a system where a combination of migration-enhancing and migration-dampening factors balance each other out. The river has a large volume, large levels of sediment flux, and high flood frequency; but, experiences almost fifty percent tidal forcing, acting as a stabiliser.

Since many of these delta systems sustain human economies, what role would human activities play?

It is natural that humans would like to encumber the natural ‘baseline’ migration patterns of a river delta in order to protect adjacent infrastructure. The Mississippi River delta exemplifies this. It has all the ingredients for a high migration rate: strong river forcing, high flood frequency, high sediment flux, and yet has a median migration rate of less than a meter per year, thanks to an intricate system of man-made levees.

This might not necessarily be a good thing for the delta system, however.

In a contemporary study on the Ganga-Brahmaputra-Meghna system titled ‘Stable ≠ Sustainable’, published this year, the team argued that the inherent mobility of the delta system is key to its sustainability.

This, they admit, can nonetheless be either mischaracterised by human communities as ‘degradation’ or simply detrimental to long-term economies. In fact, this may just be a reflection of the delta’s normal life.

– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)

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