It's a man-made lake, and if you examine the local terrain it's pretty easy to comprehend that the natural features are existing both above and below the water surface of FDR. Quite evident in the winter during the draw-downs, and especially every 20-30 years when the Feds need to do maintenance on the Gates. There are some awesome Pre-Dam pics of the Columbia on the Inter-Webzz, where you can see these "sand bars" were actually bluffs next to rimrock cliffs.