Rivers of Nepal and their features

Types of rivers in Nepal

1. Antecedent (Himalayan) rivers: the Koshi, Gandaki (Narayani) and Karnali, whose sources lie in the snow of the Himalaya or on the Tibetan plateau. They are fed by snow and glaciers, cut deep gorges through the rising Himalaya and carry water all year.
2. Rivers from the Mahabharat range: the Bagmati, Kamala, Kankai, East Rapti, West Rapti and Babai; fed by monsoon rain and springs, with some flow in the dry season.
3. Rivers from the Siwalik (Churiya): small, seasonal and flashy; dry in winter, with severe floods in the monsoon and a high sediment load.

By pattern, rivers are also divided into straight, meandering and braided.

Features developed by the rivers

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: formed by vertical (downward) cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: form where hard rock lies over soft rock; a plunge pool forms at the base.
• Potholes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: formed by lateral (side) cutting in the old stage.
• Interlocking spurs: ridges of the valley sides that project alternately into the winding valley of a young river.

Depositional work occurs when the velocity and carrying capacity fall (on gentle slopes, at the river mouth, and on the floodplain during floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad, flat valley floor built by flood deposits.
• Natural levees: low ridges of sand and coarse silt built along the banks during floods.
• Braid bars and point bars: sand and gravel bars in the channel (braid bars) and on the inner bank of bends (point bars).
• Delta: fan-shaped deposit at the river mouth, e.g. the Ganges-Brahmaputra delta.
• Ox-bow lake and meander scars: cut-off bends.

  Mountain      Foothill        Terai
   gorge  ->  braided fan  ->  meanders
  V-valley     (Bhabar)        ox-bow, levee

Engineering significance

• Hydropower: gorges, high gradient and large discharge in the hills; narrow bedrock valleys suit dams and give a high head, but the high sediment load wears turbines and fills reservoirs.
• Bridges and roads: terraces are good sites for roads and bridges; braided Terai rivers need long spans and guide bunds.
• Flood control: levees, embankments and spurs on the meandering and braided reaches, e.g. the Koshi barrage and embankments.
• Construction materials: river-bed sand, gravel and boulders.
• Water supply and irrigation: alluvial groundwater and canals.
• Hazards: landslide dams, GLOF, bank erosion and shifting channels.