Image caption Stones and boulders are collected from rivers across Nepal

The unchecked and unregulated collecting of boulders, stones and sand from Nepal's rivers has placed many of the country's crucial bridges in danger, experts say.

They say it has also increased the risk of flooding and other threats.

The bridges are on the country's most important highway, running across from the eastern-most part of Nepal to the west.

If they collapse, the East-West Highway, the transport lifeline of the country, will be severely disrupted affecting large section of the population.

Officials say a few of them have been damaged and at least one has already collapsed.

Under particular threat are those bridges that have seen boulders, stones and sand removed from right under them around their pillars.

"The foundations of some of these pillars are becoming scarily visible," says Hari Pokhrel, the chief of the eastern division of the road department.

"As a result, the bridge on the River Beding, to cite one example, could collapse anytime," he said of a bridge in eastern Nepal.

By the truckload

The same is true for many bridges in the western and central part of the highway.

During a recent field investigation by the BBC in western Nepal, at least six people were seen at a time digging and collecting boulders and pebbles from around the pillars of the bridge on the Khutia river.

To the north and south of the bridge, along and on the shallow river, these were being loaded on tractors and trucks.

Many people were seen breaking the stones beside the river before they were transported elsewhere.

"We take these boulders and stones to the crushers in nearby towns," said one tractor driver emerging on his vehicle with pebbles and boulders some of which he had collected from around the pillar of the bridge.

Image caption The collection of stones and boulders is threatening to change the courses of some rivers

"We would not know where the crusher operators deliver these stones and pebbles."

Stone crushers were mushrooming along the East-West highway across the country until a recent court ruling banned export of such resources to India.

The court decision came following a parliamentary investigation that found uncontrolled collecting of stones and boulders was threatening the normal courses of some rivers.

Most of them were exported to India where, experts said, they were used in the construction of infrastructure like roads and dams.

As exports soared, hydrologists say, rivers had begun to change their courses, inundating human settlements and reaching unexpected parts of forests.

Changing flow

"Stones and boulders serve as a regulator for the river's flow as water percolates through them and maintains the course," said Ananta Bhandari, an expert with the United Nations Development Program's Ghodaghodi wetland conservation project in western Nepal.

"When they are removed, waters in the river can go wild and can behave weirdly and then wetlands like this can also be affected."

Monitoring has been a challenge in all areas in Nepal at present Jiva Prakash Sitaula, Local development officer, Jhapa district

But there are also some who believe that boulders and stones need to be collected from those rivers that have too much of them because of heavy silt-yields.

Experts say massive deforestation in the Churia and Bhawar ranges below the Mahabharata range to the south of the Himalayas is adding to the silting process.

"The Chure and Bahavar ranges are so sensitive that even if you scratch the soil there with your own fingers, you will see that some erosion will happen then and there just like that," said Keshav Kandel, former secretary with the Ministry of Soil Conservation.

"In such places, you can imagine what happens when trees are chopped on a massive scale: more and more silts fall into the rivers that carry them downstream."

Aggravating the situation is the erratic pattern of rainfall. Meteorologists in Nepal have noticed that many places in recent years have begun to get a huge amount of rainfall in a very short span of time that triggers flash floods and landslides.

Most flood-prone rivers from where stones and boulders are being collected are tributaries to the four major river systems of Nepal that join the Ganges downstream in India.

Getting serious

Although district officials say the collecting of these resources has decreased following the ban on export, local people in different segments of the East-West highway that host the bridges say that is not the case.

They say the district-level authorities may have stopped issuing new licenses for the collection of boulders and pebbles, but contractors who had obtained permission before the court ruling carry on with the business.

Image caption Experts are worried about the increased risk of rivers flooding

Many of them had united to protest the government's ban and argued that they had invested heavily in the industry that was employing thousands.

"Perhaps it is a major source of income for the district development committees (district-level local authorities), therefore they continue to permit the removal of boulders and pebbles from places we have prohibited in the rivers," said Madhav Kumar Karki, chief of the bridge division with the roads department.

"That is why we are having this problem, the district authorities need to be serious."

Following the court ruling, the government had issued notice prohibiting such collections from within 500m of both sides of the bridges.

"We did forward the notice to all quarters at local level," said Jiva Prakash Sitaula, the local development officer in Jhapa district in eastern Nepal.

"But as you know monitoring has been a challenge in all areas in Nepal at present, we have formed a monitoring team and will be discussing on this."

Prolonged political instability in Nepal has rendered the country's administration increasingly weak.

And the fear is that it may contribute to making the crucial bridges on Nepal's major highway even weaker.