B: the countries involved, the people concerned, the rivers navigable
What's the difference in meaning between A and B?
-Ed verb forms do, however, often post-modify, because they can be construed as elliptical clauses: the countries (that are) involved, the people (who are) concerned. Aside from the fact that some -ed forms do not normally pre-modify (*the found keys, *the built house) and others do not naturally post-modify (*love unrequited, *dancer tired), I see difference only in style and not in meaning. I suppose that premodification would suggest more of a done condition than the event, and vice versa for post-modification.
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the involved countries, the concerned people, the navigable rivers.
To me these mean that the nouns are actively 'doing' the verb. The countries are involved in something, the people are feeling concerned about something.
B: the countries involved, the people concerned, - this suggests to me that the countries and people could be quite passive here - someone else could be talking about or taking action on the countries involved, the people concerned.
The reaction to the Tsunami - concerned people (in other countries) donated money to help the people concerned (who were suffering in the aftermath). Likewise the involved countries (who were taking part in the rescue and aid) were working to assist the countries concerned (who were hit by the Tsunami)
The river part is a little different somehow. I can still see a difference in meaning though. The navigable rivers is pointing out which rivers from many are navigable. 'The navigable rivers are the River Wet, the River Wild and the River Windy'. The rivers navigable sounds more like a description or recommendation overall for the region - 'the fruit is sweet, the people friendly and the rivers navigable.'