Noun:a picture of something showing it as a dark shape on a light background; especially : such a picture showing a person’s face from the side

http://www.merriam-webster.com/dictionary/silhouette

The dark was coming on hard now, and off to the west, I could see the mountains rising in silhouette, all the mysterious and unknowns.

I understand that the author is saying he could see the mountains reflection off the river.