Unbridgeable is about distance between people — never about rivers. Its home is the fixed phrase 'an unbridgeable gap', and it also goes with gulf, divide, difference and chasm. It says the space between two sides is so wide that nothing will ever cross it: no talk, no compromise, no agreement. You use it for politics, for arguments between countries, for two people who want completely opposite things, for a difference in wealth or values that keeps growing. The metaphor is doing all the work: a bridge is what you build over a gap, and this gap defeats every bridge. So it carries a feeling of finality and a little sadness — it is a giving-up word. It is formal, mostly written, and you will find it in newspapers and serious discussion far more than in speech. It appears in front of a noun nearly every time; you rarely hear 'the difference is unbridgeable', though it is possible.

無法彌合的、無法跨越的（差距、分歧）。常見搭配 unbridgeable gap，中文說「難以跨越的鴻溝」。