What is the 7 rocks rule?
Pretty much what Mr Smith said.
Surprisingly, there is some science there.
I cleaned shitty water for over 30 years.
Any idiot can making drinking water from river water. Clarify, balance the ph, disinfect. Simple process.
Without getting too in depth....
River rocks have growth on them. Biological film, otherwise known as zoogleal mass.
As the water flows across this zoogleal mass "critters" use nutrients from the water. This is a simple biological process you'll see in trickling filters and other "fixed film" biologic processes.
You create a controlled biologic process, so the receiving stream doesn't suffer total oxygen depletion, through BOD or Biochemical Oxygen Demand.
Unfortunately it takes a lot more rocks, and "detention time" than 7 rocks, to make life miserable for many pathogens. Even then, if the water source can be polluted at any point, all bets are off.
Clear doesn't mean pure always.
I've seen properly collected well samples pop positive on a bac-t test lots of times.
Contaminated wells are more common than you'd think, even today.