This is the 16344th most frequent Russian word.
реку
"Реку" is the accusative form of "река," meaning "river" in English.
The word "реку" in this sentence functions as the direct object of the verb "увидел," indicating the river being seen.
Я увидел красивую реку.
I saw a beautiful river.
In this instance, "реку" is used after the preposition "на," showing the direction or object of the act of looking.
Мы стоим на берегу и смотрим на реку.
We stand on the shore and look at the river.
Here, "реку" serves as the direct object of the verb "переплыть," indicating the river as the object to be crossed.