Rivers of Nepal descend further and faster than any other rivers in the world. As a result, many have cut sheer-sided valleys thousands of metres deep, creating enormously unstable hillsides. Any external action - an earthquake, severe rainfall, the construction of a road - can trigger catastrophic earth and rock slides which sometimes create natural dams impounding large lakes, only to burst open and flood the valleys downstream.
The most of the big rivers are drained from the higher himalayas. Rivers of Nepal can be broadly divided into three categories in accordance with their origins. The first category comprises the three main river systems of the country-the Koshi river, Gandaki river and Karnali river systems, all of them originating from glaciers and snow-fed lakes.
The Koshi river system consists of the Tamor river, Arun river, Dudhkoshi river, Likhu river, Tamakoshi river, Sunkoshi river and Indravati rivers. Of these, the Arun and Sunkoshi originate in Tibet. The confluence of these rivers is at Tribeni (near Dharan) in Sagarmatha Zone. Flowing for almost 10kms through a narrow gorge before entering the plains, the "Sapta Kosi" or he "Koshi" swollen with the waters of the seven riversÕ finally merges into the Ganges.
The Gandaki river system in central Nepal consists of the Kaligandaki, Budhigandaki, Marsyanghi, Trishuli, Seti, Madi and Daraundi rivers. The Kaligandaki is the longest river and the Trishuli, the main tributary of this system.
The Kaligandaki river originates in Mustang and converges with the Trishuli at Deoghat in Chitwan. The river is then called the Narayani and goes on to meet the Ganges. The Karnali river system in western Nepal consists of the Humla Karnali, Mugu Karnali, Seti and Bheri rivers and is the longest river system in the country. The Humla Karnali, which rises in Tibet, is the main tributary. After entering India, this river assumes the name Gogra.
Rivers like the Mechi, Mahakali, Bagmati, Kamala, Rapti, etc., most of which have their origin in the Mahabharat range, constitute the rivers of the second category. The Bagmati, which rises at Bagdwar and drains out through the Chobhar gorge, is the principal river of the Katmandu Valley.
Streams and rivulets originating mostly from the Chure hills make up the third category; these rivers rely on monsoon rains and are otherwise dry.
Nepal's mountain rivers has become increasingly popular for Rafting and Nepal has become almost as well known for its Whitewater Rafting as for its trekking. The rafting trips mainly take place on the Sun Kosi,The Trisuli and The Kali Gandaki Rivers.