This is the 10832nd most frequent Turkish word.
dereye
"To the stream" or "to the creek."
The word 'dereye' in this sentence is used in the locative sense, indicated by the suffix '-ye', to show the direction or target where the books were dropped, namely the stream.
Kitaplarını dereye düşürdü.
He dropped his books into the stream.
Here, 'dereye' again takes the locative suffix to imply 'towards the stream,' indicating the fishes' destination or direction.
Balıklar dereye doğru yüzüyor.
The fishes are swimming towards the stream.
In this sentence, 'dereye' with the locative suffix signifies the direction of the action, showing that the stones were thrown towards or into the stream.