Why is it surprising that USGS calls it a gut?  Sloughs are often guts.  There is nothing in this article that indicates it is anything but a gut or slough, not an estuary.  The friends may be a reliable source, who knows, I'm told that if 3 blogs say something in agreement it can be considered reliable information.  If there are no reliable sources that say it's an estuary that  information should be removed.  I would tag it as inaccurate, but I am under threat of permanent community ban if I should question "facts" on Wikipedia in articles.  It's your article, you want to call a slough an estuary, okay.  I would just like readers to know it's unsourced information that appears to have nothing in the article backing it up.  I've done that.  I wash my hands of this inland stagnant water estuary.