Aciravati, Achirvati or Airavati is the ancient name for a river has been identified with the modern Rapti, flowing through what is now Nepal and the northern portion of Uttar Pradesh. The Chinese pilgrim Xuanzang knew it as A-chi-lo. Jain texts mention it as Eravai. The ancient city of Sravasti, once capital of Kosala Kingdom, stood on the western bank of the Achirvati. The river was a tributary of the Sarayu. It was one of the five great rivers that constituted the Ganges group of rivers and one of the sacred rivers of the Buddhist midland.
Geography
The Rapti rises south of a prominent E-W ridgeline midway between the western Dhaulagiri Himalaya and the Mahabharat Range. A summit on this ridgeline marks a triple divide. North of the triple divide the Karnali and Gandaki basins are adjacent; south of it the Rapti and similar but smaller Babai River separate the two larger basins. After crossing into India, the Babai and Rapti separately join the Karnali's continuation called Ghaghara. The Ghaghara ultimately joins the Ganges.
Hydrology
The Rapti's headwaters descend south from rugged highlands populated by Kham Magar. The western tributary Mādī Kholā rises in northwestern Rolpa and is joined by Lungrī Kholā draining northeastern Rolpa. The Mardi then crosses into Pyuthan. It is joined by east-flowing Arun Kholā at Devithān where it enters a gorge through the Mahabharat Range. Jhimruk Kholā -- east of the Mardi—mainly drains Pyuthan. Below the upper highlands, an alluvial valley opens where Bahun and Chhetri rice farmers irrigate paddy fields. At Cherneta, Pyuthan the Jhimruk approaches within 1.5 km of the Mardi and a 12 megawatt hydroelectric plant exploits the Jhimruk being 200 meters higher. Below Cherneta the Jhimruk loops east, becoming the border between Pyuthan and Arghakhanchi District. Its valley narrows and steepens as it enters the Mahabharat Range. Partway through it joins the Mardi and the combined flow is then named the Rapti. The main river emerges from its gorge into the lower Siwalik Hills and Dang District. At Bhalubang Bazaar Nepal's east-west Mahendra Highway bridges the river. Below Bhalubang, Inner TeraiDeukhuri Valley opens between the Dang and Dudhwa Ranges, both subranges of the Siwaliks. Valley, following the WNW trend of the Siwalik hills for 100 km. Although the land is fertile, before DDT came into use in the 1950s Deukhuri was so malarial that only the Tharu people who had genetic resistance could be confident of surviving the warmer months. The river crosses from Dang into Banke District. Approaching Nepalganj—largest town in Nepal's western Terai—the Dudhwa Hills fall away and the river turns SE, crossing into Uttar Pradesh, India and flowing through districts Shravasti, Siddharth Nagar, Basti, Sant Kabir Nagar and Gorakhpur, passing Gorakhpur city at about 135 air miles from Nepal. Just west of the city it is joined by the smaller Rohini rising further east in Nepal's Nawalparasi and Rupandehi Districts, draining 794 km2 in Nepal then 1892 km2 in India. 60 km beyond Gorakhpur the Rapti joins the Ghaghara at Rajpur. About 120 km further on at Chhapra, the Ghaghara reaches the Ganges.
Resource utilisation
The Rapti's flow has great seasonal variation because the river lacks sources in high elevation glaciers and snowfields to buffer pre-monsoon drought. Average monthly flows at Jalkundi in Deukhuri Valley vary from 17.6 m3/s in pre-monsoon April to 451 m3/s at the peak of the monsoon in August. Maximum recorded flood was 7,390 m3/s on 10 September 1981. 100-year flood flows are predicted at 10,100 m3/s. Over in Uttar Pradesh are at risk of floods every year. Flood control projects under study include a dam at Jalkundi that would inundate of farmland in Deukhuri Valley. An alternative dam site is upstream at Naumure on the Pyuthan-Dang district border. This would be an earthen dam 169 m high with 351 million cubic meters live storage capacity, storing excess monsoon flows for irrigation use during the following dry season and generating up to 207 megawatts. Impoundment would mainly be in gorges through the Mahabharat Range, inundating less farmland than the Jalkundi alternative. Plans are also underway for three irrigation sub-projects – Kapilvastu District involving interbasin water transfer to the southeast, Deukhuri Valley, and Banke District.
