We are accustomed to thinking of river drainages, like trees, as having a directional structure — branching as one moves upstream, but converging into larger streams as one moves downstream. Deltas, like the root systems of trees, are boundary cases.
There is a remarkable exception to this structure, however, that occurs at Two Ocean Pass in northern Wyoming. Two Ocean Creek drains the plateau northwest of the Pass and rushes down the mountain straight into a ridge line that forms part of the continental divide. There the creek splits into two parts: one of which becomes Atlantic Creek and flows north into the Yellowstone, and thence into the Mississippi; while the other becomes Pacific Creek, flowing southwest into the Snake, and thence into the Pacific. Here is a map:
View Two Ocean Pass in a larger map
Cutthroat trout used this route to migrate from the Snake River into Yellowstone Lake which is in the Mississippi drainage. Parting of the Waters describes the hike back to Two Ocean Creek, and has some nice photographs and maps.