Members of the local swiftwater team have all been treated for giardia, some of them more than once. (you can't filter the river to make it safe to recover the kid...) They've pulled kayakers, boaters, swimmers, etc out of waterways all the way up to 8,000 feet.

Members of the SAR team have been treated once or twice over their careers as volunteer rescuers.

Still tabulating the members of my own hiking group who've had it. Apparently it occurs in dogs regularly enough that the vet recognized and treated one case in a group member's hiking pooch. I told one old fella who lives in the foothills and has a lifetime of hiking in the mountains that people think they don't need to filter or treat their water up there - he gave me the biggest WTF double take.

What do researchers do? Sample the water. Maybe a couple times? One pool in the river? Not enough to get a good idea of what's really going on. Show me a study that spans a good long time, repeatedly samples various locations on the river, throughout the year, and you might have something worthwhile. Some little study that doesn't do a thorough job ain't cutting it. Cause there's plenty of locals that think you all are nuts. Being sick isn't worth it.

You do your thing in the Central Sierra, don't say I didn't warn you....