I wonder about the long-term risk as well. All my life I've seen pictures of the Amazon that show a winding river. Meaning it is relatively slow moving. This also means the river path will change over time. This means you need to add geography to your list.

4) Geography: There must be major urban centers on each side of the river at a place where the river does not change course over time.

A bridge built today on a changing river bed would have to be extended or become partially obsolete when the river changes course by much. I guess alternatively it could be made extremely wide to start, but then it would be even more expensive and less efficient, making it even less economically viable.