There's stories all over the country of light-skinned and haired Natives. Then there's the Melungeons and the Lumbees... and the, forget their name, the Red Bones, or something like that in Louisiana region. It was reported that several Natives in different areas either spoke or understood Welsh. Thomas Jefferson was very interested in this as his ancestors came from the region of Wales that Prince Maloc (sp?) came from.
The Mandan were in North Dakota, and were well documented by Catlin, as were notable similarities between their language and Welsh. For all practical purposes they were wiped out by smallpox. Double Ditch or Fort Lincoln/On A Slant Village