Forshadowing: Be a warning or indication of a future event.
The dog saying "Oh My Gosh! This river is way to high...." shows that at some point, that river will be the cause of a flood.
Oh My Gosh! This river is way too high....
"What has roots as nobody sees, is taller than trees, up, up it goes, And yet never grows?" (Tolkien 149) This is a mountain, which indicates a mountain (Where he riddles with the dragon) is where he is heading.
This part is when Bilbo and Gollum are riddling back and forth to each other... This foreshadows riddling in Bilbo's future.
And here is when the foreshadowing comes true. This is the part of the book that is when Bilbo is confusing the dragon with his riddles.
"I am the clue-finder, the web-cutter, the stinging fly. I was chosen for the lucky number." (Tolkien 425) This is Bilbo confusing the dragon, with his riddles.