The river leaves the Tibet region at a height of about 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 . The August 1988 Nepal earthquake , with an epicentre more than 50 km south of the Arun basin , had a moment magnitude of 6.9 and resulted in more than 100 deaths in the basin alone
1988 Nepal earthquake
1988 Nepal earthquake
Arun River, China–Nepal