Eagles and river dams go hand in hand.  Extremely cold weather freezes all the waters in ponds, lakes, and most of the rivers…except around spillways and dams that churn the water so much it remains open throughout the winter.  The colder it gets the more the ice expands and the open water gets concentrated in very small areas.  This is when hundreds or thousands of eagles might congregate around these open waters.  Of course fish are the other part of this equation.  The waters must have a good amount of fish to sustain such a gathering.  Fortunately most of the big river dams on the Mississppi do indeed have good fish populations.  Shad are a favorite of the eagles.