give a short introduction to the three grades of rivers in nepal
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Rivers in Nepal can be classified into three broad groups on the basis of their origin. The first group of rivers is snow fed-types such as the major rivers systems: the Koshi, Gandaki, Karnali, and Mahakali. They originate from snow and glaciated regions in Himalayas and their flow regimes are mostly governed by the melting of snows and glaciers. As a result, flow in these rivers is perennial and sustain flow during the dry season. These rivers are reliable source of water and also provide potential opportunities for hydro-power generation and irrigation in the downstream. The second group of rivers originates in the middle mountains and hilly regions. Their flow regimes are affected by both monsoon precipitation and groundwater. Contribution from groundwater yield maintains the minimum flow level and prevents from drying during non-monsoon periods. The Bagmati, Kamala, Rapti, Mechi, Kankai, and Babai rivers fall into this group. The third group of rivers originates in Siwalik zone. Tinau, Banganga, Tilawe, Sirsia, Manusmara, Hardinath, Sunsari and other smaller rivers are examples of rivers falling in this group. The flow in these rivers is mostly dependent on monsoon precipitation and their flow level could deplete significantly low during the non-monsoon period. Summer monsoon is important period when about 60-85% of annual runoff of all river systems in Nepal occurs during July to September.