Ganga · Hydropower

What do Rivers have to do with Silt??

Rivers are again in the news, though so far only for symptomatic reasons. The new government at the centre has renamed the charge of water resources minister to Minister of Water Resources, River Development and Ganga Rejuvenation. There is fundamental contradiction within this name plate and we have in fact yet to see this nameplate.

There is also a lot of discussion about rejuvenation of Ganga, with the Prime Minister promising the people of Varanasi Parliamentary constituency that he will rejuvenate Ganga.  There is no clarity about how he plans to go about in achieving that. His claim during elections that Gujarat Government’s Sabarmati Riverfront Development provides a model for this is clearly a non-starter. Sabarmati has water only in 10.4 km of the river stretch that flows through Ahmedabad. If you go upstream of this stretch, you will find a dry river in most non-monsoon months and if you go downstream, you will find a river more polluted than Yamuna in Delhi. And even the water that one sees in this 10.4 km stretch is not the water from Sabarmati river basin, but is taken from Narmada River via Sardar Sarovar Canal! Pertinently, Ahmedabad or Sabaramati has no right over that water: the Sardar Sarovar Project has been built and justified in the name of Gujarat’s drought-prone areas like Kutch and Saurashtra.

The nameplate-changing business also extended to Union Ministry of Environment and Forests, its name changed to Union Ministry of Environment, Forests and Climate Change, though here again the new nameplate is yet to be seen. Unfortunately, all the noises that we have heard so far from this front seem to give primacy to growth rather than environment or forests or climate change! The new environment minister has yet to say anything about river protection, but he is already talking about river linking!

So is there a hope for rivers in this new establishment, going beyond the symbolic name changes? Here one is reminded of a meeting, where one of us (HT) was invited a few months before the elections, to discuss the state and fate of Yamuna River in Delhi. When HT started speaking, he started by asking what is a river? Is it just a source of water as engineers see it? Following a sudden change in program, Sushri Uma Bharati was the chief speaker at the meeting and when it was her turn to speak , she actually tried to understand that question and tried to find an answer to it: what is a river? Her becoming the Union Water Resources Minister also raises hopes since she had been campaigning for Aviral Dhara (Continuous flow) of the Ganga and against building of dams and hydropower projects in Uttarakhand. We hope that she will realize that impact of dams and hydropower projects on rivers is similar, if not same everywhere.

Ms Bharati is also minister of river development and Ganga Rejuvenation. The question, What is a river? becomes even more relevant in that context. A river is possibly the most complex ecological entity and we still do not understand fully how to define a river. But here we would like to highlight that river is a lifeblood of the ecology and carries so much more than water. One of the key elements that river carries is silt or sediment (although there is a slight difference between the two, we will use it interchangeably here).

The rivers carry silt from various points in their journey from the hills, to the deltas where most major rivers meet the sea. In this journey, the type, quantity, movement of silt varies with place and time. The silt comes in various forms, from suspended matter to fine silt to coarser sand. It is the transfer of silt from upper catchments to the plains that helps build fertile and alluvial flood plains like the Indo Gangetic plains.

Ganga-Brahmaputra Delta during Monsoons. Pcture from :
Ganga-Brahmaputra Delta during Monsoons. Pcture from :

The flow of sediment through rivers to the delta also protects the deltas, which are very highly productive & biodiversity rich ecosystems, population centers and agriculturally fertile areas. Deltas are constantly facing the threat of erosion by sea. In this fight against erosion by sea, the sediment brought by the rivers helps the deltas in a major way. Sediment flow to delta becomes even more important when sea levels are rising in changing climate.

However, when we build dams, hydropower projects and diversion structures on the rivers, we completely change the silt flow pattern in the river. The dams arrest the silt and hydropower projects and release silt free water in the downstream. The erosion capacity of the silt free water is greater, and the additional erosion they cause in the immediate downstream may not compensate for the silt trapped in the dams. The run of the river hydropower projects may release silt annually or more frequently and also on daily basis from desilting chambers, but the pattern of transport of the silt again completely changes. Moreover the dams and diversions completely change the character of flood-flow in the downstream area, when it is established that floods are the most important sediment-transporting events. All these changes have huge impacts in the riverbeds, in the floodplains and in the deltas. And most worryingly, we do not understand these impacts completely as yet.

REbuilding a flood barrier in Bangladesh Delta, destroyed due to Cyclone Aila from :
Rebuilding a flood barrier in Bangladesh Delta, destroyed due to Cyclone Aila from :

It is only recently that scientists have started work that provides a glimpse of impacts this changing silt flow is causing. For example, our deltas are literally shrinking and sinking, and several independent scientific studies are telling us that dams must take major, about three-fourths of the blame. About 80% of the sediment that rivers bring can be trapped by the dams and this means that dams are annually trapping about 40 billion cubic meter of sediment globally. That is more than five Sardar Sarovar Dams every year! In India, our estimate earlier showed that large dams are trapping at least 2 BCM of silt every year, this figure is likely to have gone up now.

Not all the sediment trapped in the dams would reach the deltas, a significant part would have been left on the floodplains and in the river channels. And sediment trapped by dams is one of the many reasons behind sinking of deltas. However, scientists are estimating that already deltas have been deprived of at least 73 BCM of sediment by the dams. In South Asia, during the past century, Indus delta sediments have been reduced by 94 percent, Ganga-Brahmaputra delta sediments by 30 percent, and Narmada delta sediments by 95 percent.

The Ganga Brahmaputra Delta, formed of rich sediment From: EO
The Ganga Brahmaputra Delta, formed of rich sediment From: EO

In 2007-08, the Ganges, Mekong, Irrawaddy and many other rivers flooded with more than 100,000 lives lost and more than a million displaced. Most of the deltas that were flooded did not receive a significant input of sediment. These major flood events lead to sediment trapping behind mega dams.

The direct impacts of delta subsidence and effective seas level rise include inundation of coastal areas, saltwater intrusion into coastal aquifers, increased rates of coastal erosion, an increased exposure to storm surges, in addition to the threats to food security, livelihood security, water security for millions and a huge loss of biodiversity. These threats impact hundreds of millions of people who inhabit the delta regions as well as the ecologically sensitive and important coastal wetland and mangrove forests.

Sundarban Forests constitute parts of Ganga-Brahmaputra Delta from: Wikimedia Commons
Sundarban Forests constitute parts of Ganga-Brahmaputra Delta from: Wikimedia Commons

As Prof. James P Syvitski, the Chair of the International Geosphere-Biosphere Programme, told SANDRP, “We must learn to do better.” However, decisions surrounding dams in most regions of the world are not even assessing the impacts on deltas. Ignoring sediments when building and operating dams comes at a huge price. Someone else is paying that price right now and this price is steeply increasing. For full SANDRP report on this issue, write to us or see

This article provides a glimpse of the role that rivers play in sediment transport. It goes to show how little we know about the role played by rivers in our lives. We hope we have much richer debate around the role of rivers in our lives in days to come.

Himanshu Thakkar (, Parineeta Dandekar (

For a response to this blog by Dr D K Mishra, see:

Satellite image of Brahmaputra River From: Wikimedia Commons
Satellite image of Brahmaputra River From: Wikimedia Commons

( An edited version of this piece appeared in the Civil Society Magazine: