Nothing is simple, but the bare bones from a non-lawyer are as follows:
A phoned-in tip can provide "reasonable suspicion". That gives legal justification for LE to bring in a sniffing dog. If the dog indicates something illegal is present, there is "probable cause" for a more targeted search. If they find something, your trip is likely to develop a notable glitch.
In my contacts with the Lees Ferry rangers, this kind of thing is by no means their primary interest. But they are sworn LE officers, and when information comes to their attention, they have some pretty well- established procedures to fall back on.
Back when one incident like this happened, the story was that a member of the river party was talking about their stash of pharmaceutical goodies in front of some tourists at the ramp, and it was one of the tourists who notified the establishment. The lesson there would seem to be to stay calm, cool, and collected at the ramp, and wait until you're around the bend before you loosen up. But being snitched off by a disaffected former member of your planning group, that's a whole different thing......
A word to the wise is sufficient.