[1] An Alpine river in character, its source lies in the Trenta Valley in the Julian Alps in northwestern Slovenia, at an elevation of 876 metres (2,874 ft).

In turn, this is probably based on the substrate name *Aisontia, presumably derived from the PIE root *Hei̯s- 'swift, rushing', referring to a quickly moving river.

[6] In 585, a landslide cut off the upper part of the Natisone riverbed, causing its avulsion and subsequent stream capture by the Bontius River.

The original subterranean discharge of the Bontius into the Timavo River became obstructed, and another avulsion returned the new watercourse into the bed of the lower Natisone.

The river inspired the poet Simon Gregorčič to write his best-known poem Soči (To the Soča), one of the masterpieces of Slovene poetry.