I’ve already entered my river but my imagery was in 2D so… was it well acquired?… First of all, I import my shape (shp) file into Global Mapper (what a great sofware!) and export my vector file as a KML.
Import my river.kml file into my brand new Google Earth 5.2 or just double-click it in his location… easyest impossible!.
Select it and use ‘show elevation profile’, a very useful new tool for these purposes…
And that’s it, constant altitude, almost perfect!.
Hope you have considered it interesting:-)