Complex sentence: Deokhuri Valley to the south encloses the (west) Rapti River, forcing it to flow west some 130 km between the Dundwa and Dang subranges of the Siwaliks until the Dundwa range ends near Nepalganj and the river turns SE into India.
Split into simpler sentences: Deokhuri Valley to the south encloses the (west) Rapti River, forcing it to flow west some 130 km between the Dundwa and Dang subranges of the Siwaliks. Eventually the Dundwa range falls away near Nepalganj and the river turns SE into India.