|Click on image to enlarge.|
The biggest thing working in favor of making today's image easy for the mind to "go with" is the lighting. Both images used in the composite were taken in the same area, on the same day, within a half hour or so of each other. The way the road twists is happenstance and set up the direction of light being close to the same in each location. This is despite the fact that the bridge was shot "up stream" and the cascade was shot across the flow of the stream.
Both images were adjusted and matched in Adobe Photoshop Lightroom (LR) to get the exposures to sync up. They were then brought into Adobe Photoshop (PS) using Photo/Open in Photoshop as Layers. This saves a step and puts both images into one (soon to be) composited document.
The Opacity of the top image was brought down to about 50% to make it semi-transparent. Using the Move Tool (V) on the upper Layer the stream can be roughly positioned. Once the cascade is close to being where it's needed the Opacity can be returned fully.
Add a Layer Mask (washing machine icon at the bottom of the Layers Panel) to the upper Layer. Invert the Mask color (CTRL I [eye]) to Black. It can be brought in as Black using the CTRL key as you click the icon. Using a soft White Brush (B) "paint" on the Layer Mask to reveal the bridge on the bottom Layer.
The rest is fine tuning. Make the Brush (B) larger or smaller to get into the bank and trees. Spending a little time at this point will make life easier down the road.
This would be the time to do any critical moving of the cascade Layer to match it to the background stream making a smooth transition between foreground and background.
The composite was brought back into LR using File/Save then File Close. Don't do a Save As, don't rename the file, just let PS and LR do their thing and put the image back where it belongs. Flip back to LR (Alt/Tab) and watch it come back. When it's there, go through any finishing techniques you feel the image needs.
I tried a version where I went a little nuts with the Curves Adjustment Layer dodging and burning. The rocks looked like they were HDR and the rest of the image was "normal". Not a good look for this image.