In Chapter 7, Huck remarks that the June rising of the Mississippi River always brought him luck.
Why is this an example of foreshadowing?
Because Huck meets the Duke and the King while traveling down the river, which is lucky for him since they help him get rich.
Because Huck ends up stopping a family feud between the Grangerfords and the Shepherdsons which is an auspicious event for Huck.
Because Huck ends up meeting the love of his life, Mary Jane Wilks
Because the rest of the book is about Huck's journey down the river, which in the end, brings him good luck in the form of reuniting with Tom, freeing Jim, and being able to master his own destiny.
Actually, it's NOT an example of foreshadowing, as nothing actually lucky happens to Huck, including Jim getting bit by a snake due to Huck's tomfoolery, Huck losing all his possessions to the Duke and the King, Tom getting shot at the end.