a tributary region or tributary areas closely links to .... (in this case the Mediterranean sea)

ref. logic, Babylon confirmed

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Note added at 2002-11-11 00:47:22 (GMT)
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if one expands the concept of Mediterranean (sea or basin) to the hinterland/to the adjacent interior areas

e-rich's comment is correct but in the U.S. we would prefer back country, meaning the area away from the main road. Basically, a rural or mountainous area where one is "in nature". In the case of the Mediterranean, I would say away from the coast where most of the action was taking place.

Exploring the back country around the Nice region. Here the foothills of the Alps stretch right down to the Mediterranean Sea giving a great variation in the ...


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These are the 4 possible translations given by Termium.

The Harrap's gives "hinterland"

The Grand Dictionnaire Terminologique 2000 gives "hinterland" and "backland"

So the two previous answers you already received are both valid

I think it can just be translated like this.

It is usually the beginning of the mountainous area next to the shorelines and their plains. In France, it is often used in reference to the olive tree covered hills behind the riviera.

might fit?

Is how I would refer to the land away from the coast

Or "(adjacent) inland areas" as someone else has suggested.
To my mind "back country" is too specific, referring to land bewteen mountains and plains as someone else has said, and also has connotations of rural backwardness (which may or may not be the case in your text).

And for me "hinterland" has strong industrial connotations relative to a port or urban centre.