Tista, a swift flowing river of northeast India and Bangladesh originates from Chitamu Lake in Sikkim Himalayas at an altitude of 7,200 m and descends to Darjeeling Plain and flows through Sivok Gola, the magnificent gorge, to Bangladesh. As per the Hindu mythology, the river originated from the breast of Goddess Parvati. Tista is notorious for changing courses. The excessive rains of 1787 caused heavy floods in Tista valley and the river instead of following the original course to meet the Ganga River directly, changed its original course and captured a minor river, Ghaghat. 

Rivers often change their courses then what is so special about this phenomenon in context with Tista? Sikkim the mountain kingdom is established on the rocks of the yore, which happen to be severely jointed in this area. South Sikkim enjoys a sub-tropical humid climate, whereas the north has a tundra type of climate. Under extreme cold conditions water enters the joints and freezes during the nights. During bright Sunshine it thaws. Alternate freezing and thawing of water contained within the rocks develops weak planes and sometimes even tears them apart. 

How powerful could be the action of freezing and thawing can be understood by an example. While working on Barashigri Glacier in H.P. during a traverse to Chotashigri, a hanging valley on the left bank of Chandra and Shigri Rivers sudden a sudden explosion made us freeze on our tracks. This was followed by a hail of rock boulders. The floor of the hanging valley was strewn with such boulders. A daring rock climb to peep into the gaping joints at higher elevation revealed the cause. The frozen water on thawing expands. It was this expansion that was tearing the rocks apart.

Thus rocks weakened from inside by water action and also a heavy rainfall make the mountain slopes vulnerable to the forces of nature. Tista River being a high gradient river cuts the toe of the mountain. Consequently the angle of repose of the slope forming material is disturbed and the mountain starts to slide down.

As long as the river gradient and the mountain slopes are in a state of balance, the incidents of landslides are minimized. However, whenever due to natural or anthropogenic reasons if this precarious balance is upset a landslide starts. A river that changes course frequently is bound to erode the toe of the mountains along the new course and lead to landslides.

One of the notorious landslide of Sikkim popularly known as the '20th mile slide' has been a pain in the neck since early seventies for the National Highway 31A near Singtham in East Sikkim. In 1996 this slide was reactivated. This time the slide measured about 150 m from crown to the toe at the Tista River level. At the road level the slide was about 85 m wide. It is worth noting that the toe of the mega-slide is located t a point where the river makes a concave curve. A river always scours the bank on the concave side and deposits material on the convex bank. N.C. Bhattacharyya and his team from Geological Survey Of India (GSI) investigated the landslide in detail.

They found that the hill slope is formed of rocks with a soapy texture. Such rocks are generally less compact and break into thin flakes. This part of Sikkim experiences a heavy rain fall. Average being 500 mm, though locally it can even exceed 3000 mm. Thus rain saturated rocks on this hill slope, lying at an angle of 450 are prone to slide down even with minor disturbance. During the monsoon the discharge of Tista becomes quite high and the mighty river scours faster. Increased pore pressure inside the slope forming rocks due to rainwater seepage and erosion of the toe led to the devastating landslide.

This story is repeated every year in Sikkim and at several places. 

Infact the entire northeastern region of the country faces the brunt of landslides. Landslides are difficult to predict, though in some cases warning signs are there, but people prefer to ignore them. Factors that trigger landslides in the mountain terrains of north western and north eastern Himalayas are 1) Geomorphology; 2) Rock types; 3) Geological structure; 4) Climate and rainfall; 5) Drainage; 6) Seismicity and 7) Human activities.

Out of theses the first six are beyond human control but the last factor is the contribution of man himself. A study conducted by the GSI in Mizoram reveals that out of the 505 slopes 83 susceptible to very high landslide hazard; 177 to high landslide hazards, 236 to moderate hazards and nine to low landslide hazards. It goes without saying that despite this information if the state government or the people of Mizoram decide to start development/construction activity on slopes falling in very high or high landslide zones then engineering measures will be of no help to protect mass destruction. 

The first and foremost condition to start any construction activity on a hill slope is the provision for adequate drainage. It is unfortunate that the society in these mountain terrains has closed its eyes towards this important factor and the investigation reports of most landslides point to slope failure due to heavy precipitation and lack of drainage.

The Kalka-Simla Rail line is a classic example of proper drainage. It is this measure taken by the British engineers that has made the slopes stable along more than a century old rail track winding its way to Simla. The British had developed similar drainage for Nainital and also for the Darjeeling rail track. Alas wherever the drainage has been blocked due to human apathy, nature has taken over and landslides are abundant.

Landslides and mountains are synonymous. It is impossible to imagine a landslide minus a mountain. The only way to avoid them is to know about them and take precautions and remain always prepared. Not only the Himalayas, landslides are a problem in all the highland terrains like western and eastern Ghats, the Nilgiris and the Vindhya mountain ranges of central India. In the Nilgiri hills the laterite layers sandwiched between the hard rocks get eroded and during monsoon, the lose laterite material slides down. The major landslides in the Nilgiri hills are the Runnymede landslide, the Glenmore slide, the Coonoor slide and the Karadipallam slide. In the recent times, casualties and damage due to landslides have increased in the Nilgiri hills. During October-November 1978, 90 people died. Similarly a landslide at Amboori in Kerala buried 23 people alive.

The mountain slopes governed by the laws of Gravity and lubricated by water will continue to slide. We can only avoid mishaps by taking precautions. Landslides can be considerably reduced in numbers and magnitude, if each individual resident plants a tree on the slope. Alas!

Landslides and heavy rains lead to flash floods and even normal floods in the valleys. How the rivers get flooded, why do we have more floods despite less rains etc will be topics of some of the articles in the coming weeks.