That number is nowhere near accurate as we learned during the South Carolina floods back in October. The state estimates there are between 10,000 and 20,000 unregulated dams in South Carolina alone: http://www.thestate.com/news/local/article41315256.html

Those are the dams that pose the greatest threat. You can't plan for something you don't know about and the threat posed by such an extensive system of "invisible" dams is immense. A small earthen dam on a farm may not pose much of a threat by itself but its failure can cause a cascade of failures further downstream if it breaks when those dams are already at their limit.