A fork is a stream that drains into a larger stream. Easterners are likely to call these watercourses branches, tributary is also appropriate, and those in west Texas would call smaller forks prongs. They are most often named by their geography—that is, the west branch of the Sacandaga, the North Fork of the Flathead. And then there’s the North Prong of the Little Red River Fork of the Prairie Dog Town Fork of the Red River, in Briscoe County, Texas. Playfair’s Law holds, among other things, that tributary forks almost always join the main river at its level instead of with a sudden drop.

Bill McKibben