Undue concern about safety of back country drinking water is traced to the Federal "Safe Drinking Water Act" of `1970s.

This law mandated that surface water used for public water supplies be filtered unless special exemptions were obtained.

Dutiful federal employees, naturally, obeyed their direct boss, and advised backpackers accordingly in national forests, BLM and NP properties, without regard to any known risk.

Rockwell, it is true, has Ph.D in physics. He admirably distilled a great deal of science for the lay reader, and cites something like 60 sources, many of them peer-reviewed articles in medical and public health journals.

If you can deal with statistical analysis and jargon, find direct links to many of these studies in the source list at end of Wikipedia article titled "Wilderness Diarrhea."

Among these scientists, Derlet is a particular authority on measurement and has been widely quoted in lay press. Zell, and also S.C. Welch and Timothy Welch (separate works by unaffilated authors) are worthy of attention. I also like TP Gardiner and D.R. Hill's joint work on the Long Trail in Vermont.

These are many of your basic scientific sources that are available beyond Ye Olde Woodsy Woodsman's sage advise.

Personally I filter main-stem, valley river water. In back country, I'd avoid livestock and seek tributary brooks, but filtering is, based on available evidence, "water purity overkill" and it clutters the backpack.