Answer:
I believe the answer to be "the river".
Explanation:
The object of a preposition can be a noun, a noun phrase, or a pronoun that comes right after it and completes the preposition's meaning. In the sentence we are studying here, the preposition by indicates location and its meaning is completed by the words "the river" - article + noun.
Note: even though "to" can be a preposition as well, in this sentence it is used to show that the verb "study" is in its infinitive form.
