Nah, that’s wrong. Traditional methods could only reach a maximum of about 8 to 15% alcohol before the yeast kills itself and becomes safe to drink, or by brewing in the literal sense of heating the liquid. Diluting with riverwater would either keep the ale unfinished or the riverwater unsafe as the alcohol content gets diluted.

In the old days, the knowledge that boiling water made it safe was not widespread, many cultures got their fluid content completely from potable rainwaters, brews, and soups or stews.

Those who relied on drinking untreated riverwater were doomed to low population and constant sickness related mortality.