River's Name as an Adjective
Solution 1:
Those are completely different things. You use an adjective when you need an adjective. Use a predicate test to know which is which.
In the Danubian corridor, you have an actual adjective derived from a noun: the corridor is Danubian. Hence Danubian is an adjective.
In the Danube delta, you do not have an adjective at all: the delta is not Danube. It is the delta of the Danube, using a prepositional phrase to connect one noun to another. Hence Danube is only ever a noun. It is not an adjective.
You could think of Danube delta as a compound noun.