I did a quick search for this on here and didnt find one for PS (if it exists then admins feel free to nip this thread).

Any, creating tapered rivers in photoshop quick tip (no pun intended):

1. Draw the river with the brush tool
2. Create a new layer, set the brush shape dynamics to Control: Fade
3. Draw the end of the river which will taper nicely
4. Select the original river layer, earse the old end.
5. Merge the two river layers.

(Note: you can easily skip the earsing and merging by just changing your shape dynamic and then finishing the river, but I like to rough everything in first.)

Again if this has been posted before, feel free to cut it. Otherwise, enjoy.