What ARE the Powers of a Sheriff vs the Feds?

I've read elsewhere, and even heard it suggested in comments here at WarOnGuns, that the sheriff is the highest law enforcement officer in his county--and that federal agents may not conduct investigations without his cognizance. One of the reasons generally given is that because he is elected, he represents the highest actual authority--the people.

Generally cited to back up these claims is the case of Castaneda v US. The story goes something like INS targeted a family in Wyoming with an Hispanic surname as illegal aliens and recruited the local sheriff to assist in the bust. It turns out they were citizens, and in the resulting legal settlement, the ruling was made acknowledging the doctrine of "Sheriff Supremacy."

Thing is, I can find no such ruling from any court. I found sites that say there was a private and undisclosed settlement, and even sites that quote the court as saying this, and one (but not an "official" one) with a docket showing the case was dismissed. I found sites that say precedent was established and sheriffs can now wield this power if they only knew--if they only would. I found sites claiming the whole thing is a hoax. And a general conclusion among the "naysayers" is that any sheriff who tried to interfere with a federal investigation would soon find himself behind bars and facing charges.

This latter part is believable, regardless of any purported stare decisis, only because my experience has been that the largest, most vicious gang controls the turf. And my sense is that such a doctrine would run afoul of the Constitution being "the supreme law of the land"--and as doors swing both ways, it could allow sheriffs to enforce all kinds of infringements on civil liberties.

My point in all this being, I was going to write an open letter to the sheriff in Twin Falls about the persecution of Red's Trading Post, with the assumption that he has some sort of say in what goes down in his county. But when I started doing my basic fact-checking, I quickly realized I could not back that assumption up.

Feel free to chime in with comments and educate me. I'm not trying to slam the door shut on this. But this is a legal area where I freely admit ignorance, and I need more than unsubstantiated opinions and wishful thinking.