The Lake Champlain Basin Project has a great series of pages on the history of the lake. From their site:
pproximately 10,000 years ago, the glacier’s retreat allowed marine
waters from the St. Lawrence estuary to flood the Basin, forming the
Champlain Sea, an arm of the Atlantic Ocean. The extent of this salty
sea is shown on the Champlain Sea Map [182 KB].
Many marine animals, including Beluga whales, Atlantic cod, seals, and
blue mussels lived in the Champlain Sea. In 1849, railroad workers
found a Beluga whale skeleton in Charlotte, VT, which is now on display
at the University of Vermont. Many other fossils of the Champlain Sea
time period have been found in Canada.