Painting a Landscape
Adding Water for a lake or river

"Layers" - "New Raster Layer". Call the layer 'Water'

You should already have the horizon selected from the previous page

"Selections" - "Invert"

"Edit" - "Copy Merged"

Activate the "Water" layer and
"Edit" - "Paste As a New Selection"

"Image" - "Flip"

Drag the selection to the bottom of the canvas.

In the "Layers" palette drag the "Foothills" layer above the "Water" layer.

With the Water layer active

"Blur" - "Motion Blur"

Next : A painting I did using these methods