3.7 SURFACE HYDOROLOGY

Hydrological networks and units are the spinal cord of the entire natural system. Rivers, streams, springs, aquifers, lakes, ponds and ground water are interlinked to each other and though the link is not prominently seen they function in close association. If we discern the different determinants of natural systems, especially geology, climate and vegetation, these links will be revealed to us. Understanding these links and the functioning of these hydrological units is an important factor in planning and resource management.

As was discussed earlier, water is the prime resource for any habitable place. The entire world has only 3% of its hydrological wealth as potable water, out of which major volumes are in the form of frozen ice, or as ground water. So it is very important for us to manage and protect the available water resources efficiently.

The Wang Chhu and its tributaries are the hydrological unit of the valley. The river is wild and has created the rich fertile valley floors of Thimphu over a period of time. The river has its watershed in the northern part of Thimphu Dzongkhag and Gasa. Its major tributaries are the streams, which drain in from the east and west of the valley. Chhubar Chhu, Ola Rong Chhu, Ngabe Rong Chhu, Samteling stream (Silikha Chhu), Taba Streams and Dechencholing streams are the major tributaries apart from the small mountain tributaries and rivulets. These major streams are fed by their own network of numerous minor streams and rivulets through the gullies running down the hill slopes. These streams are responsible for the Alluvial Fan Deposits of Taba, Motithang, Samteling, Dechencholing and Simtokha. These streams originate from natural springs and lakes at higher altitudes.
The Alluvial Fan Deposits and River Terraces formed by the Wang Chhu and its tributaries provide rich soil for cultivation.

The river and streams support a variety of plant species within their Riparian Zone (The ecosystem in the immediate surrounding of the watercourse, which is influenced by it). In turn this eco-system protects the environment of streams and rivers.

Though there are no detailed maps of aquifers and aquifer recharges the number of springs along the valley indicates the presence of close ground water level, thus indicating the presence of ground water flow and seepage of water from lakes at higher altitudes.

3.7.1 ISSUES
• The Wang Chhu is the most polluted river in Bhutan. The pollution is especially visible at the stretch between Thimphu City and Khasadrapchu. The cobbles that were earlier naked are now coated with muck. The clarity of the water has also become poor due to dirt suspension. The report on Faecal Culiform count survey by RSPN (2000 Sep - Dec) shows that most of the sample collected from streams, river and tap water of some public places and restaurants, are well above safety norms (50cfu/100ml) and in most of the cases it is above the counting limit. The safe limit is only 10cfu/100ml.

• The main cause for the pollution is lack of a sewage disposal network. The present system serves only limited areas of the city. Lack of public awareness is another cause. The streams within the city limit also act as solid waste disposal sites. Most of the streams that run within the Core area of the city are diverted, canalized, polluted with domestic sewerage and solid waste.

• The Riparian Zone within the city area is damaged to the maximum due to unplanned urbanization, thus affecting the quality and environment of streams and river. This will accelerate the erosion along the riverbanks. A cycle of deterioration has begun.

• Though it is mentioned in the Forest and Conservation Act of 1995 that felling of trees is not permitted along the river and streams within 30 meters from the edge of the watercourse, this Act is not respected within city limits. Uncontrolled road construction and other developments in the recent past have severely damaged the river course.

The Wang Chhu.

MAP NO. 3.8 SURFACE HYDROLOGY(Click to view the map)