Canada Geese + Interloper

Stopped by Lake Fairfax after the Riverbend Walk this morning. Spotted a couple of Warblers (Magnolia and Black-throated Green), but the most interesting/confusing find was an interloper swimming with the Canada Geese. The mostly-white bird appears to be just a tad smaller than the Canada Geese, and it should be noted that none of the CGs were bothered at all by the unidentified bird's presence.

I need help with the ID. Please post comments below or to the VaBird Listserv.