I was photographing the Pelicans and other birds feeding near the Kerr Dam but I noticed the Pelicans were flying low over the river when they arrived. I moved down river so that I could photograph them skimming the water. It is amazing to see how low they will get.
Why Birds Fly Low
Skimming permits the birds to take advantage of an aerodynamic phenomenon known as “ground effect.” The patterns of airflow around a wing that is operating close to a surface are modified by that surface in a manner that reduces drag, the resistance of the air to the progress of the wing.
Thus, everything else being equal, it is more efficient to fly close to a surface than far from it. But things are rarely equal, which is why birds most often tend to take advantage of the ground effect when the “ground” is water. The ground effect only occurs when the flying object is much less than a wingspan from the surface — and at such an altitude over land a bird would be continually flying among obstacles, through grass, and so on. Only water is sufficiently uncluttered to permit such close safe passage. (Stanford)