Alright I don't remember where I heard this and if it was even related to Fallout but I'll go with it anyway.

There is a theory that the Mississippi River has created a barrier between the East and West coasts of the United States due to it high radiation levels. Now this would make sense because all the radiation that seeped into the ground and then drained into a waterway would eventually end up in the Mississippi. So all the radiation would be gathered in the Mississippi essentially creating a heavily irradiated barrier similar to what we find outside of Vault 87. So what do you guys think does it sound plausible or no?

I don't think its that plausible since Fallout 3 established that transcontinental travel is possible. Furthermore a giant river of death in the middle of America seems a little contrived and limiting. Either its a huge radiation field that kills anyone unprotected crossing it or its just a radioactive river and it can be crossed by boat or bridge.

As far as cross-continental travel barriers go (and I would like to see one too because the idea is fun) I would prefer the idea of Midwest having become a giant radioactive dust bowl with massive seasonal storms choking off travel for months at a time. Think it was Van Buren that threw it in. Seems pretty cool. Would also still allow for trans-continental travel without making it too easy or simplistic and allow for good new ideas for cultures and lifestyles (tribes and traders having to constantly move for resources and to avoid danger or migrate from one side of the continental divide to the other before the storms hit or a few people who live out the storms deep in pre-war cities etc.). Would be cool if something had happened to the Mississippi massively expanding or changing course or some other big thing to illustrate the drastic effects the Great War had and the bold new world of post-war America.

The Fallout series could stand to do more with how deeply the world has changed other than "deserts everywhere."

Nuclear Fallout doesn't remain static in one location. Especially in water. The Mississippi river runs to the Gulf of Mexico, and with it, it would carry all radiation away. Eventually, the water would be clean.

If you're scratching your head wondering why the river, the Potomac, was irradiated in FO3, don't bother scratching it any longer. Bethesda didn't know what they were doing and created a horrible idea for a MQ, that being dirty water 200 years after nuclear war. If the earth was that irradiated that water ways remained irradiated for 200 years, there wouldn't be a single living thing on the planet.