Why is the usage of whose correct in the following sentence:
In the foothills of that large mountain range are the sources of a river whose course was not fully mapped until this century.
I was under the impression that you have to use which or that when referring to things. When is it okay to use who/whose to modify things?
I thought the sentence would have to be something like "...are the sources of a river, the course of which was not fully mapped..."