Best way I've found is to use the paintbrush tool, 1 pixel to draw your rivers. If you zoom in to about 300-500%, you can control exactly where you want them to go. To make them wider, retrace just a little to the side with the same brush width. The little gaps and irregularities you'll get this way make t look more natural and give the appearance of current. Then, to taper it off less than one pixel, say, at the source, the a circular transparency gradient where you want it to start and it will look to be very naturally shrinking.
Really the main thing is to zoom in really far to be able to control in perfectly.