I have brought up this point before, but that was in the official future Fallout game suggestions thread I do believe, and I wanted to put it in its own thread because I think it's interesting.

According to The Pitt DLC for Fallout 3, The Pitt is a place where three irradiated rivers meet. These rivers are the Allegheny, Monongahela, and Ohio Rivers(the Ohio river being created by the merger of the other 2 rivers) What's interesting to note is that these rivers are all fairly long, as I have marked on the below map. The Ohio river even goes all the way to the Mississippi river.

While it's said that it's the specific toxins from The Pitt's industrial areas that causes the Trog syndrome, and likely the bright yellow glow of the water, the rivers themselves are still irradiated naturally. These three rivers alone cover most of western Pittsburgh, West Virginia, and runs along the borders of Ohio, Indiana, Illinois, and Kentucky.

What makes this even more interesting is the possibility that the other tributaries of the Ohio are also irradiated. If they are, that would not only cover almost the entirety of the states listed above, but would also cover Tennessee, parts of northern Alabama, the northernmost tip of Georgia, and the western part of Virginia.

To put that into perspective, I overlayed a rough outline of the area those rivers cover, over a map of the entire US. The black box is the area those rivers cover, and the red box is all the rivers that flow into the Potomac, the river in Fallout 3 that is irradiated. As we can see, the two areas meet up, and form an almost constant area of terrible water across a large section of the American east.

Now, Bethesda might not make all of these rivers irradiated, but it's interesting to see how much of an area they have possibly given themselves already that might be similar to Fallout 3's wasteland.

Not to mention Philly is called a dump, and Great Lanta and the Broken Banks don't seem to be such great spots to move to either, suggesting the wasteland goes all the way south, and into the coast of North Carolina, where the Broken Banks are assumed to be, and all the way up north to around New York city at least, suggesting that most of North Carolina, Delaware, New Jersey, and Pennsylvania are similarly affected.

Pics

https://i.imgur.com/29FxPqa.jpg

https://i.imgur.com/wSbv11Q.jpg

To be honest I think it's a oversight or game gimmick that as them still be that radioactive but it could be leaking nuclear power plants that course it.
I know that's not what you asked so I'll try to answer your question. I think it's possible that all the major rivers have some radioactive waste in possibly over the whole USA but more likely in a few hot spots that had a lot of high value targets near them e.g big cities or major production sites which would explain the north east corner been so heavily hit