No. If you wanted to use "truly" to say that the entire sentence is true, you would need "truly" to modify the verb. You would need to say either 'Truly, at this time of year the river is a beautiful sight", or "At this time of year the river is truly a beautiful sight" However, when you put "truly" next to "beautiful", "truly" modifies the adjective "beautiful", so the sentence means that the beauty is true and the sentence is equivalent to "At this time of year the river is a sight that is really beautiful". To emphasize that it really makes a difference where you place the word, we can create a sentence that combines truth and falsity together: "It is false that the river is a truly beautiful sight at this time of year." and that is a perfectly fine sentence.