The Arun is the largest
trans-Himalayan river passing through Nepal and also has the greatest snow and
ice-covered area of any Nepalese river basin. The Arun drains more than half of
the area contributing to the Sapta Kosi river system but provides only about a
quarter of the total discharge. This apparent contradiction is caused by the
location of more than 80 percent of the Arun's drainage area in the rain shadow of the Himalayas. Average annual rainfall in Tibet is about 300 millimeters.
The river leaves the Tibet region at a
height of about 3,500 metres (11,500 ft) and crosses the main Himalayan
ranges. Leaving their rain shadow, the river's flow increases substantially in
the monsoonal climate of east Nepal. The landscape south of
the border tends to be steep with less than 15 percent of the area having a
sustained slope of less than 15° and is strongly dissected by stream channels.
Many of the hill slopes are structurally unstable, and the region is
seismically active. An earthquake in August 1988, with an epicentre more than
50 km south of the Arun basin, had a magnitude of 6.7 on the Richter scale
and resulted in more than 100 deaths in the basin alone.
via:- en.wikipedia.org
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