Add a Hue/Saturation adjustment layer and reduce the Saturation value to the maximum.
Add the forest 3 image to our canvas and set the far foggy background to the middle section of the existing forest. Use a layer mask to leave only the foggy area visible.
Create a Hue/Saturation adjustment layer and bring the Saturation value down to -100:
Make a Curves adjustment layer to darken this part of the forest a little.
3. How to Import the Stream
Add the stream image to the lowest area of the scene and flip it horizontally (Edit > Transform > Flip Horizontal). Use a layer mask to mask off the top, leaving the stream with the rocks and lower vines visible.
Create a Hue/Saturation adjustment layer to desaturate the stream, rocks, and vines.
Make a Curves adjustment layer to reduce the lightness of the stream.