There have been many questions in the past on tapering rivers in Gimp... I stated in this tutorial on making rivers with a mouse in photoshop that Gimp didn't have tapered strokes
SO I wrote a little script-fu to do it!
This isn't so much a tutorial, as a posting of a new script. It ain't too fast, but it works
Just unzip the attachment at the end, and save the script into your normal spot.
Draw a path using the pen tool, then switch back to the paintbrush tool (the script strokes with the active tool, so you could even, for example, erase tapered strokes). The script registers under the Edit menu as "Tapered Stroke Path..."
The dialog allows you to select the start width, end width, spacing (20 percent is a good default, but by setting it to 100 you get a string of pearls effect), and a parameter called "Taper Exponent". This control the rate of taper. At 1, the taper is linear from the start to the end. At values less than 1 the taper changes faster, and greater than 1 the taper changes slower (nearer the end). I set the default at 5, which seems good for rivers.
Here is the result after stoking in black with the defaults. I have turned on the path visibility to show how it lines up:
And since I have one more attachment, here is the image:
Couple of things... It will only stroke the first segment of a multi (broken) segmented path. If there are no paths, the script will run but error out.
Hope people find this one useful!