Complex sentence: As they crossed the Brahmaputra river some of their compatriots were swept away down river and came to be called Dimasa ("Dima-basa", sons of the great river Dima, the Dhansiri river).
Split into simpler sentences: As they crossed the Brahmaputra river some of their compatriots were swept away down river. Later they came to be called Dimasa ("Dima-basa", sons of the great river Dima, the Dhansiri river).