a). Rivers in the USA are beautiful.
b). (The) Rivers of the USA are beautiful. [. . .]
1. Which variant is more idiomatic?
2. Is there any difference in meaning between them?
3. Should I use "the" in (b)?
Hi, Alexey—Sentence (a) is more idiomatic than (b) without "the." With "the" included in (b), that sentence is just as idiomatic as (a); however, there will be a difference in meaning: (b) will pick out all the rivers, and (a) will not. Of course, you could add "the" to (a) as well, bringing about synonymy and idiomaticity.