Explain xkcd #2838.

Similar: Minnesota has a tri-point where water flows to the Atlantic (via Lake Superior), Gulf of Mexico (via Mississippi River) or up to the Hudson Bay (via the Boundary Waters)

There's a Half as Interesting video about this topic:

The Only River That Flows Into Both the Atlantic and the Pacific

That’ll definitely be it. It’s like the “Cornwall is an island” thing, as there’s a pond there with a river going to the north coast and another to the south.

none of these are true. that's kind of the joke. it's an oddly specific hydrology/geology joke