Last week we read about the havoc created by the Kosi River in Bihar. In this article we will read about yet another, much smaller Kosi, the Nana Kosi of Uttarakhand and how even an insignificant river is eroding and denuding the Himalayas. The process of erosion is the nature's package deal. It can not be avoided, but it can be augmented by our mistakes. Similarly the erosion and denudation processes can be slightly slowed down by not repeating or avoiding the mistakes we made in the past. Here are the details.
The nature is always at work. Natural agencies like wind, water (including ice) are always busy eroding the land surface. Once the Himalayas came up, over a period of time, the slopes were covered with vegetation. Soon tall trees like pine and cedar took hold of the slopes as per the altitude suited for their growth. The roots of trees went deep and also spread. These roots acted as anchors for the trees and binders for the slope forming material of the Himalayas.
Humans as they learnt to create fire learnt the art of cooking food as well. Thus firewood became a necessary source of energy and continues even now in most of the Himalayan villages. There is a total ban on felling of trees, yet surprisingly, the forests of the Himalayas are vanishing. The mountain slopes bereft of trees are eroded rapidly and the natural agencies denude the material fast.
'Eighty percent of the sediments delivered to the world's oceans each year come from the Asian rivers and amongst these the Himalayan rivers are the major contributors' say J.S. Rawat and M.S. Rawat of Kumaon University, Nainital, India in a research paper published in Mountain Research and Development.
Any one would wonder at such a heavy rate of denudation! What could be the causes and what measures are required to save the Himalayas, are being discussed here. Though youngest, the Himalayas are the loftiest mountains of the world. Their average height above mean sea level (msl) is much more than the world's average of 840 metres m s l. Being the youngest; they are made up of soft, easily denuded rocks. Extremes of climate and rainfall help the eroding agencies work faster. The eroded material rolls/slides down the slopes and is easily carried away by the swift flowing rivers.
Prof. K.S. Valdiya, a renowned earth scientist says that the region is vulnerable to environmental degradation, because of the factors enumerated above. The hydrologic regime of the Himalayas is mysterious, because unlike the plains the subsurface or rather say even surface in most of the cases is made of jointed and fractured rocks. How much water goes underground and how much of it is run off no one knows precisely.
Till about 1974, it was estimated that the rate of denudation of Himalayas is 900 mm per 1000 years, i.e. 0.9mm/year. Whereas, a study published by Valdiya and Bartarya proved it to be 1.7mm/year, a much higher rate. The higher rate of denudation has been ascribed to deforestation, road construction and other development works, like large dams, poorly managed agriculture, forest fire and over grazing. Unfortunately nearly all these are anthropogenic interventions and have never been systematically studied from the point of view of their contribution towards erosion and denudation. In case of dams across the Himalayan Rivers, systematic studies are carried out, but more for the purpose of silting of reservoir and damage to the turbines from the silt, than the impact of dam on the acceleration of erosion.
In order to assess the actual erosion and denudation the Rawat duo took a case study of a 55 square km area of Nana Kosi River between Shrinagar in the north to Katarmal on the south in Uttarakhand. The altitude of the area ranges from 1200 to 1900m m s l. and the average slope is 19 degrees.
For the type of study carried out by Rawat and Rawat, uniformity of terrain is essential. It is a Himalayan terrain, where slopes, geological set-up, rock types, etc can vary. Higher slopes and softer rocks would mean more erosion; gentler slopes with fissile rocks and heavy rainfall would also mean greater erosion. Ideal conditions are difficult to meet, that is why they chose an area which was more or less uniform in the enumerated aspects and had more or less uniform rainfall pattern.
The study requires constant monitoring of streams and points have to be identified from where samples are to be collected regularly, preferably at the same time of the day. Apart from collection of water samples, the velocity of stream is also required to be measured. With these factors in mind, they selected 25 localities which were continuously studied for two years.
Deducing results from the data collected requires statistical skills and use of complex formulae. The study showed that the rate denudation under natural, undisturbed conditions ranged from 0.02mm/year to 0.04mm/year during a normal year. Incase of a drought for a year the rate of denudation became almost half, i.e. 01 to 0.02 mm/year. Areas where forest cover has been removed/hacked or where crude agriculture practices are being carried out the rate of denudation rises by about ten times to 0.18mm/year.
The sediment load generated in least disturbed areas, for example, oak and pine forest is 2 tons/kilometer square/month compared to 12 tons/kilometer square/month in areas devoid of forests and used for agriculture.
Lose soils on the hill slopes wash down to the river valley to be transported as dissolved sediment load of the river. In Nana Kosi watershed in the rainy season the dissolved sediment load increased to 70 tons/kilometer square/month from 10 tons/kilometer square/month in dry season.
The pioneering study by Rawat and Rawat is an eye-opener, because the rate of denudation of the slopes and suspended sediment load of the river increases multifold in areas devoid of forest cover. The study though still in its infancy is a pointer towards the significance of the forest cover and proper land use in the Himalayas.
The area selected by them has some of the fast developing towns like Almora and Bageshwars. Therefore, lot more construction activity in the region is expected. Construction means hacking of tree cover. In other words Kosi the sole water provider to urban area at present is likely to be choked with sediments.
There is a dire need for planting trees that hold the soil and save the rivers from choking. Development is of course the need of the hour, it can not be stopped. But for each tree hacked at least ten more should be planted, so that in hundred years the area is again rich with forest. Yes it takes 100 years for the trees to re-grow and effectively control denudation.
Before developing any area in the Himalayas impact of construction activities on environment is required to be done. Usually no one bothers about small construction activities, unless it is a major project. But the study shows that the hill slopes get denuded whether the activity is major or minor.
May be one day we will have urban areas of Himalayas dotted with concrete pillars of apartments in lieu of trees!