"One of the sources of water in our country are the rivers" in this sentence, wasn't it supposed to be 'is' instead of 'are'? Normally we say "One of my friends is responsible for this," don't we?
Yes, you're right. Is is correct. It should state, "One of the sources of water in our country is the rivers."
The conjugation of the verb to be should be in the third-person singular present indicative in order to agree with the third-person singular subject one. Sometimes people get confused by prepositional phrases that come between the subject subject and verb, particularly when they involve plurals. However, prepositional phrases are never part of the subject and never change its number.