An adjective or a noun?

Hi teachers,
I wonder why we say educational enviroment, nervous system but say river system, education system, etc. The first ones have an adjective preceding while the second have a noun before another noun.

Hi Englishlover.

It is also possible to use education environment, nerve system, riverine system, educational system. There are no rules for when and whether an adjective is originally an adjective or originally a noun-- it is more a matter of the historical development of their popular acceptance in each individual noun phrase.

Of course, there are many cases when different forms create different meanings, as for instance value system and valuable system.