Translation:The elephants are standing by the shallow river and drinking.
Would this translation work? "The elephants are standing at the shallow river and drinking"
It's not really a preposition you'd use in English, but it's not wrong either.
At was marked wrong. But I think, AT is nearer to drink from the river, than By.
can i express this sentence using the postposition mellett? is there an implied difference in meaning or context?
"Mellett" means exactly "beside of something", while "-nál, -nél" is less definite.
In this particular context "mellett" would be fine as well.