Open up this start image of the waterfall. Because the movement is only going to be applied to the water and nothing else, drag the Background layer onto the Create a New Layer button in the Layers palette. There should now be two layers, one locked and one editable.
We need to make a selection of just the water in this image. Choose the Magic Wand tool and head to the Options bar. Set the tool’s Tolerance value to 50. Tick the Anti-alias box but make sure Contiguous is deselected.
Now the Magic Wand tool is set up, click once over the brightest part of the water. To select more of it, hold down Shift and then click on a slightly darker part. Try to avoid selecting the rocks and don’t worry if the fainter areas of water are left out of the selection, as we can bring these back later.
Press Q to enter Quick Mask mode to reveal the selection as a temporary mask. Areas in red are outside the selection. You may have parts such as the sky and the rocks that are still inside the selection. To remove these, press D and then select the Brush tool. Paint over these areas to add to the Quick mask.
Press Q once more to return to the normal image mode. The selected parts should now be ready to blur. On the duplicate copy of the Background layer, add a new layer mask. This applies the selection to a mask, shown by a black and white thumbnail on the layer.
Click on the thumbnail of the image on this layer, as we don’t want to mask the selected object. Now go to Filter>Blur>Motion Blur to open up the controls for the movement effect. Set Angle to 81, Distance to 112px and hit OK. You should now see the water areas blur in a downward motion.
Blurring will likely need to be added to other areas. You can do this by selecting the mask’s thumbnail on its layer, and then selecting the Eraser tool. With black as the Foreground colour, brush over parts of the water to increase their blurriness. Lower brush size for more accuracy.
[this post syndicated from Photoshop Daily]