I think I remember Photoshop having a Text to Curve feature (not sure), I know vector apps do this easily. Again, if you can get Inkscape, a free vector program, it lets you put Text on a Curve easily, plus it allows you to make diminishing river lines. In vector you draw a line parallel to the river curved to same shape that is longer than the text line. Then select both the line and text and apply Text to Curve and you're done. I think you can do this in Photoshop, but doing this in vector is easier.