To pick a random set of citations: http://tahoetowhitney.org/content/sierra-nevada-backpacker-w...

There seems to be a fair amount of disagreement.

Look. I have no idea personally. However, conventional wisdom is that it's prudent to treat water. If I were dehydrated and had no treatment options, I'd certainly take my chances and drink apparently clear, clean water. (And have on occasion.) But I wouldn't just set off on the assumption it wasn't necessary either.

Have been drinking from NZ lakes and rivers for nearly 30 years and never had a problem. Did hear of a friend getting a bug about 15 years ago though.