"The island is over there, so perhaps the stream is somewhere over here" should be allowed?
“maybe” is underlined in the correct solution; “perhaps” is not accepted. These two words are interchangeable and either should be accepted.
What's the context for joten here?
It's the speaker deducing the position of the creek based on the position of the island. For example when looking at a map. "So" could be replaced by "hence" or "therefore".