Thinking of islands

My recounting of Strawberry Island got me thinking about islands and a trivia question popped into my head.

What is the largest fresh water, riverbound island in the world?

5 points if you can name it without searching, 2 points if you can find it in a search.

Update: Rob won the points and now I can tell you why this somewhat esoteric trivia bit ended up in my noggin.

Several years ago the officials of Grand Island very expensive billboards proclaiming Grand Island to be "The Largest Freshwater Island in the World!". After all, everybody 'knew' this to be true. Well, shortly after this they were informed (in rather embarrassing fashion by a local news channel) that Grand Island was most certainly not the largest freshwater island in the world. That honorable title goes to Manitoulin Island.

The honorable officials were horrified to have been exposed in such a gaff. A cursory search of great islands would have shown that Manitoulin was the largest freshwater one. They now had very expensive signs that were a constant reminder of their lack of fact checking before spending huge sums of money. They decided to have the signs changed (at no small expense) to lay claim to being the largest river island in the world.

It was pointed out to them that this was also not true (and easily verifyable). Bananal was in fact the largest island formed by rivers. Fortunately this was before the signs had been changed.

They then decided to forgo trying to use facts and had the signs changed to say "Grand Island. Greatest Freshwater Island in the World!" in exceptionally tacky 1960's perma-smile Pleasantville style.

It was then pointed out that this choice of words indicates by over-definition that they felt Grand Island was inferior to at least one saltwater island. The signs weren't changed again.