The Lowland River diorama is in the "Explore Colorado" Exhibition the 3rd Floor.  There's an elf in this exhibit.

The Lowland River diorama. Un-zoomed, elf (out of this picture) is near the base of the branch supporting the heron nest, just below the main sign of the diorama.

Elf is resting on a branch extending to the right from the main branch supporting the heron nest.  He's wearing a green outfit and brown cap.

Closeup