I'm glad to see some work with my canyon. Very good use of it!
Like Frank said the sense of scale is amazing because of the trees especially.
Very simple and effective work on creating the river, it really works here.
Did you use the painted canyon or the standard-canyon?
I'd suggest to indeed add some vegatation to the canyon walls and perhaps some fake stones as well.
If you need help where and how to place them in the shader-network then just let me know.
Furthermore you could add a second tree-population for variation.
Don't forget to use enough size-variation on populations in any case, in this image it could use a bit more of it.
Looking forward to see your next iteration

Cheers,
Martin