"This river is narrow, and the other is wide."
Translation:Ez a folyó keskeny, a másik meg széles.
Sorry, but that is just confusing, what is happening here.
"meg" has generally the same meaning as "és".
But it is rather used to show, that two things or people are related to each other. If two children named Zsuzsa and Lajos are siblings, you use to say "Zsuzsa meg Lajos" instead of "Zsuzsa és Lajos". Last one is more neutral, both could be two people without any relation at all.
Another example, where "meg" is prefered is calculating: "két meg egy, az három".
And finally the sentence "Ez a folyó keskeny és a másik széles." above sould be accepted as correct as well as "Ez a folyó keskeny, a másik meg széles."
The only difference is, that in cases, when "meg" is used as a conjunction instead of "és" it is placed at the second position.
Maybe there might be an additional difference as KAPOS wrote. But without any context - all the sentences here have to be considered that way - both sentences are equal.