I like to measure the total gradient for a trip which gives an idea of how fast the river is relative to other prospects.
So to this end, I measure the elevation from the put-in and compare it to the take-out.
I use google earth for this which is handy...but for narrow rivers and streams, it is extremely unreliable. It frequently can be off significantly with strange bumps in the rivers and the center of the river often "higher" in elevation than the shoreline.
Are there online resources that are more accurarate than google in determining proper elevations of river segments?