I wouldn't think this is a rare find at BK since its so close to the river
Some facts about Hooded Merg from Cornell http://www.allaboutbirds.org/guide/Hooded_Merganser/id
Hooded Mergansers are fairly common on small ponds and rivers, where they dive for fish, crayfish, and other food, seizing it in their thin, serrated bills. They nest in tree cavities; the ducklings depart with a bold leap to the forest floor when only one day old.
Look for Hooded Mergansers on small bodies of freshwater. In summer, these small ducks nest in holes in trees, often near freshwater ponds or rivers