An estuary, says Wikipedia, is "a partly enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea." It's also a word that has a lot of vowels in it ("... and sometimes Y!"), with each one pronounced individually. I'm not sure whether that fact adds or subtracts to its likelihood as a typo, but an A-U combo is certainly more common than a U-A one. As a matter of fact, I just met a young man named Raurri who tells me that his Irish name was originally supposed to be Ruarri, but the hospital staff wrote it down wrong on his birth certificate. So apparently, it's an easy switch to make, but at least he got to go home with the right parents. For our part, we got four hits on Estauri* in OhioLINK, and 125 in WorldCat (along with 26 for Estaury).
(The Hudson at the Tappan Zee, by Francis Augustus Silva, 1876, courtesy of the Brooklyn Museum and Wikimedia Commons.)