word order - When to put "River" before or after its name and why?

Unlike mountain names, where "Mount" always precedes its name, e.g. Mount Everest, I've noticed that some rivers have "River" before its name, e.g. the River Nile but others have it after, e.g. the Colorado River. How does one decide where it should be and how did the difference come about?

Answer

There is no such rule, and for that matter, there is no rule about the position of Mount either; there's Rocky Mount in my part of the world, and many Appalachian peaks are known as Nnn Mountain. It is a matter of convention. The same goes for many other geologic or hydrologic features: the Leyte Gulf but the Gulf of Mexico, Loch Lomond but Alemoor Loch, the Isle of Wight but Portsea Island.

That said, Nnn River is the far more prevalent form in the U.S.; it would be quite rare to hear of the River Missouri or the River Columbia in prose. This is also true of other American terms for streams, e.g. branch, brook, run, kills (though not so much for lakes). This format may then carry over to foreign names, unless the entire foreign name is borrowed in whole: thus Americans usually known the Rio Amazonas as the Amazon River, but it is the Arroyo de la Laguna east of San Francisco Bay and not the Laguna Arroyo.

As JeffSahol noted, you can somewhat sidestep the question for rivers by referring to them with the definite article: the Hudson, the Platte, the Potomac.