This is not meant to be critique or asking for side adventure of discovering "what is wrong with the stream", but as someone who actually did used to have to fetch water from local spring when electricity shut down for few weeks back when i was wee little shit, i've got some experience and i can't help myself.
So without further ado.
Pretty easily, the only things that commonly cause you to be ill from drinking water that don't come from the stream itself do so years in the future to deadly effect, usually because there is an overabundance of heavy metals somewhere under the ground. That has virtually no signs until the heavy metal reaches critical mass and you invariably die, at least in these times.
If the water is actually clear and doesn't flow from some sort of swampland, just about the only thing that could cause illness is animal carcass or waste somewhere upstream, and thats something they would know to check for
Most water contaminants that won't outright kill you are products of civilization.