I paid more attention to Lief and his backstory this time around when I was rereading. I started looking for floods that occurred around a century prior to the book's publishing in 2006. I figured any flood that would've killed Lief would've had to have been close to New York as that's where the dead forest is. It didn't seem like he had wandered for that long. I don't think Lief would've had the fortitude for a cross country travel as a ghost. Thus, that eliminated most of the floods in the early twentieth century.
The most likely alternative is the Pittsburg, Pennsylvania flood in March of 1907 . However, the death rate of this flood was very low, with only about twelve people being killed. Additionally, I haven't been able to find anything to indicate that an eleven year old boy, let alone an eleven year old boy called Travis, was killed in said flood. .
If, however, Shusterman were to look for a famous flood that was about a century old, and was somewhat close to upstate New York where the dead forest is located, the most likely one to select would be the Johnstown flood in 1889. Not only did this flood take the lives of over two thousand people , but it also was the first of two other historic floods in the town's history.
I did check the names of every one of the dead or missing people listed on the linked genealogy website. I didn't find any poor souls named Travis, but tragically there were several eleven year old boys listed. Regardless, I don't think the absence of a Travis decreases the likelihood. I think both Lief and Travis were picked as names for the name themselves and certainly not a link to any real person. As is mentioned in Lief's page, Travis is a name whose meaning has to do with "crossing." I find it hard to believe that it is coincidence that had the two main characters meet someone's whose name had to do with the subject of crossing or traveling to another place immediately after they had made such a crossing. Lief of course is the name Allie chose simply because it sounds like leaf.
I wonder how much research was put into the backstory for Lief and whether or not Shusterman saw the Johnstown flood and decided it'd be what killed Lief, if it's just a coincidence, or if he had another flood in mind entirely. Now I have something to ask if I ever meet him!