Ok looked up the Trumpeter Swan and yes, definitely that is the species. They were almost extinct with less than 70 swans known to exist in 1933. An aerial survey of Alaska found a large population of Trumpeters and they were gradually re-introduced to N America and the population exceed 40,000 now.
Magness Lake in Heber Springs, Arkansas was specifically mentioned in Wikipedia as one of their large winter habitats. There are actually 3 lakes - Magness and 2 smaller ones and we went to one of the smaller lakes.