About Ashokan Farewell:
Ashokan is a reservoir in upstate New York, not a lake.
The "Ashokan" in Ashokan Farewell refers not to a body of water but to a nearby camp.
Its composition had nothing to do with the Civil War.
It is not classical music. If "White Christmas" were played by the Vienna Philharmonic, that would not make it classical music.
Ashokan Farewell is a folk tune composed by a folk fiddler, Jay Ungar, celebrating the camp experience. His inspiration was Celtic fiddle tunes of a similar nature.
As for the Civil War connection, it was adopted, many years after its composition, for the Ken Burns PBS documentary series on the Civil War. It was played during the entire series and was not focused upon New York soldiers.
I don’t know which experts think it is the "one of the best pieces written in America." That would seem to bypass a great many American classical composers, not to mention Jazz and "American Song Book" composers. It is a good tune, no doubt. I knew Ungar and am confident that’s what he thinks.
This is the internet and people just dump their notions here. Be careful what you read.
Enjoy: