Brahmaputra leaves a desert behind

As the floodwaters of the Brahmaputra recede for now, farmers of Assam find their fields covered with sand on which they can grow nothing

In a village about 20 kilometres from Lakhimpur – the main town on the north bank of the Brahmaputra in Assam – Kushal Das stands on the road and strains his eyes to locate dry patches in his farmland. All he sees is muck and water. Although there has been some respite from the constant rains, and the floodwaters have started receding, the farmer knows better than to smile in relief. “The river has taken away my land’s fertility,” he says, unsure of what the future holds.

Das is not alone in his predicament. As floodwaters of the Brahmaputra start receding – at least for now – the farmers are not celebrating. Instead, they are staring at what the waters have left behind – more sand than silt. Their farmlands are now infertile.

“Aagote enekua naasil (It wasn’t always like this),” says Lakhi Gogoi, another farmer in Lakhimpur. “Earlier, after the floodwaters receded, we could still plant our paddy, we could still hope for a decent harvest. But in the last few years things have changed. The soil left behind is very sandy and coarse, unfit to grow paddy.”

With the South Asia summer monsoon barely halfway, Assam has already had three floods this year. At least 49 people were killed and 4.1 million affected, Pallab Lochan Das, minister of state for disaster management, told legislators last week. Of the 35 districts in the state, 29 are flood-hit. The minister said 41,426 hectares of cropland had suffered “massive damages”. The Kaziranga National Park was among the worst-hit. Nearly 80% of it was inundated; animals were killed or swept away. See: From drought to floods in 45 days

Floods damage, but they also used to bring a boon. For millennia, the annual flooding of rivers that flow down from the Himalayas – especially the Brahmaputra – has replenished the soil of South Asia, South East Asia and East Asia with silt. This material – in size the particles are somewhere between clay and sand – is largely mineral-rich rocks from the Himalayas that the flowing river waters have broken down into granules. The deltas of Asia – of the Indus, Ganga, Brahmaputra, Irrawaddy, Salween, Chao Phraya, Mekong, Yangtze or Yellow rivers – are essentially made up of silt that the floodwaters left behind when they receded. This regular replenishment of silt has maintained soil fertility in this intensely farmed continent.

But the Brahmaputra has not been doing this for the past few years, in Assam or downstream in Bangladesh. “The river’s waters have started leaving behind more sand than silt,” Partha Jyoti Das, head of the Water, Climate and Hazards Programme of the Guwahati-based think tank Aaranyak, told indiaclimatedialogue.net.

Effect of climate change

There are various reasons for this change of behaviour, he said. Climate change is one of them. “It has always rained a lot during monsoons, and floods have always happened in Assam. But over the last few years, the intensity of rain has increased. So now you have heavy spells of rains over a handful of days instead of a stretched-out period, resulting in flash floods. This is because of climate change.”

When it rains hard, the water flows down the hills faster – from Bhutan and Arunachal Pradesh to the Assam valley. It also dissipates with more force. Scientists have measured that major tributaries of the Brahmaputra, such as Ronga Nodi and Hingora, have faster water flows than before.

“The cumulative effect of this micro seismic activity or vibration that takes place every day, combined with the monsoon, makes the Himalayan hills erode more easily,” Das explained. “This sediment goes to the river channels. Then again, with more development activities taking place in the hills, there is more digging of the soil for building dams, roads and factories. This sediment again goes to the rivers.”

“As a result of all this, the river gets laden with an abnormally large quantity of sediment. Now, normally, the river would break all this down into tiny particles, which is silt. But now the sediment load is too high. So much of the sediment remains as large coarse particles, which ultimately gets left behind in the fields when the floodwaters recede.”

Effect of dams

Some experts blame the building of a spate of dams on the Brahmaputra — more specifically, the run-of-the-river projects — for the change in the composition of what the floodwaters carry.

In a run-of-the-river project, a dam is built in the river and the water diverted to a tunnel. The silt-laden water is first taken to a settling tank, where the silt settles at the bottom so that it does not damage the turbine blades when the water turns the blades to generate electricity. The silt-free water is then led back to the river.

Policymakers who approve these hydroelectricity projects know the importance of silt downstream, so they have ensured the silt gets back to the river through an exit at the bottom of the settling tank. The trouble is, this silt then accumulates behind the dam wall, where there is not enough water flow to carry it along. Most of the water is led back to the river up to 10 kms downstream, after it has done its job of turning the turbine blades. See: Brahmaputra dams hold back silt, not water

But Partha Jyoti Das is not sure to what extent the dams are responsible for the deposition of sand rather than silt. He points out that the dams are a “fairly recent” phenomenon, while the silt-to-sand change by the Brahmaputra has been gaining momentum over the last two decades.

Effect of embankments

Himanshu Thakkar, coordinator of the South Asia Network on Dams, Rivers and People (SANDRP), has another explanation. He blames the embankments that line the Brahmaputra to protect people from floods.

“Where there are embankments, sand (from the river water) accumulates while the silt flows downstream. Embankments provide temporary flood protection, and, as seen in several cases, they are then breached. The breach starts with a hole in the embankment and water flows through it, taking along the sand that has accumulated. This is also called sand casting,” Thakkar said.

Comparing the situation with other places in India and with other rivers, he added, “I have seen similar cases of sand casting in Gorakhpur (Uttar Pradesh) in the early part of this millennium. It has happened in the Koshi flood plain.”

Das, the scientist from Aaranyak, agreed, and added that the concrete embankments hold back sediments till they breach, and then everything mixes with the floodwaters. “Since this sediment has more sand than silt, it is sand that gets deposited in the farms,” he said.

“In some areas on the north bank of the Brahmaputra, the river is taking away the top fertile layer of soil, and started leaving behind five-six feet of sand. It will take at least five years, may be even seven or eight years, to reclaim this soil. Until then nothing will grow on it,” Das said. “There are places like Jiadhol in Sonitpur district where the river has left behind 10 feet of sand.”

Adaptation through new crops

Where there is an appreciable amount of silt mixed with the sand, traditional rice farmers are now growing other crops on the advice of experts – watermelons, peanuts, kidney beans, sugarcane.

Akhil Sharma, a local farmer activist, however feels that these measures are not enough. “Even when he is growing watermelons, a farmer here keeps hoping that very soon his land will be fertile enough for rice, which he will harvest in quantities large enough to both sell and consume for the whole year. And where the soil has become too sandy, the farmer can grow nothing. Until special measures for soil reclamation are introduced, the farmers are staring into an uncertain future.”

Experts also say that there is an urgent need for a scientific study of this phenomenon.