The first step is to make a duplicate of the background layer. This is essential to apply a Blending Mode. To accomplish this, drag the Background Layer to the Create A New Layer icon. The copy will appear above the Background Layer.
Go to the Layers Blending Mode pull down menu button and drag the cursor to the Multiply mode. Most likely the word NORMAL will appear in the window – this is where you'll change it to MULTIPLY.
You'll notice a significant global lightening to the original photo. The background now looks great but the waterfall is washed out and void of detail.
Modifications to darken the water will be made via the use of a layer mask. To add the layer mask, click on the ADD LAYER MASK icon at the bottom of the layers palette. A white window will appear next to the Background copy.
The magic begins via the use of the paintbrush. Click on the paintbrush tool to access it. In the OPTIONS BAR at the top of the Photoshop screen, set the hardness to 0% and the OPACITY to 40%. Make sure the foreground color on the tool bar is set to black and begin to paint away the local areas that were rendered too light as a result of applying the SCREEN Blend mode.
Build up the effect in increments as every pass of the brush changes the exposure. Vary the strength of the OPACITY to work more or less aggressively. If you go too far, switch the foreground color to white and paint over the area where you need to reverse the effect. Given the look I wanted, note the density changes of the blend mode as they appear on the layer mask. The shaded gray areas illustrate the sections of the photo I painted.