Halliday & Matthiessen (2014: 382n):
Adjectives and nouns can occur after the Thing in wordings borrowed from languages where Epithets and Classifiers follow the Thing, e.g. professor emeritus, salade Niçoise; and this may be used to give a person a foreign flavour, as with Agatha Christie’s linguistic portrayal of Hercule Poirot: a crime most horrible. However, certain types of adjective also appear after the Thing as part of the pattern of the nominal group in English; they often include a sense of potentiality. There are reasons for interpreting these as Qualifier rather than as Epithet or Classifier. Bolinger (1967) drew attention to contrasts such as a navigable river and a river navigable, and his suggestions have been followed up in the literature (see e.g. McCawley, 1988: Ch. 12; Blöhdorn, 2009). The sequence with Thing: noun ^ Qualifier: adjective is much less common than the sequence with the adjective before the Thing, but it does occur, e.g.:Throughout the U.S., there are more than 30,000 miles of waterways navigable by small boats.;
So did the idea of making the Danube a river fully navigable by large vessels.;
It was the best solution possible.
The sense is that of a Qualifier rather than that of an Epithet or Classifier, and it would be possible to interpret such adjectives as Attributes in reduced intensive attributive relative clauses, as in a river that is fully navigable by large vessels, and, by a further step (‘unpacking’ the potential form of the adjective), a river that can be fully navigated by large vessels.
Note that these reasons are not provided.