"Shannandore" is how it's spelled on a land deed of 1746:

http://ftp.rootsweb.com/pub/usgenweb/va/augusta/deeds/f5400002.txt

On a formal map of 1760, it's "Shanedore":


https://www.loc.gov/resource/g3880.ar143600/?r=0.152,0.138,0.55,0.201,0

The original English name of the river, discovered by Virginia Governor Alexander Spotswood in 1716, was "Euphrates."