Nepal can be divided into three major river systems from east to west: the Kosi River, the Narayani River (India's Gandak River), and the Karnali River. All ultimately become major tributaries of the Ganges River in northern India. After plunging through deep gorges, these rivers deposit their heavy sediments and debris on the plains, thereby nurturing them and renewing their alluvial soil fertility. Once they reach the Tarai Region, they often overflow their banks onto wide floodplains during the summer monsoon season, periodically shifting their courses. Besides providing fertile alluvial soil, the backbone of the agrarian economy, these rivers present great possibilities for hydroelectric and irrigation development. India managed to exploit this resource by building massive dams on the Kosi and Narayani rivers inside the Nepal border, known, respectively, as the Kosi and Gandak projects. None of these river systems, however, support any significant commercial navigation facility. Rather, the deep gorges formed by the rivers represent immense obstacles to establishing the broad transport and communication networks needed to develop an integrated national economy. As a result, the economy in Nepal has remained fragmented. Because Nepal's rivers have not been harnessed for transportation, most settlements in the Hill and Mountain regions remain isolated from each other. As of 1991, trails remained the primary transportation routes in the hills.
The eastern part of the country is drained by the Kosi River, which has seven tributaries. It is locally known as the Sapt Kosi, which means seven Kosi rivers (Tamur, Likhu Khola, Dudh, Sun, Indrawati, Tama, and Arun). The principal tributary is the Arun, which rises about 150 kilometers inside the Tibetan Plateau. The Narayani River drains the central part of Nepal and also has seven major tributaries (Daraudi, Seti, Madi, Kali, Marsyandi, Budhi, and Trisuli). The Kali, which flows between the Dhaulagiri Himal and the Annapurna Himal (Himal is the Nepali variation of the Sanskrit word Himalaya), is the main river of this drainage system. The river system draining the western part of Nepal is the Karnali. Its three immediate tributaries are the Bheri, Seti, and Karnali rivers, the latter being the major one. The Maha Kali, which also is known as the Kali and which flows along the Nepal-India border on the west side, and the Rapti River also are considered tributaries of the Karnali.
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