The issue was raised recently that there is no central source for information on rivers, how they behave, and how they are drawn on a map. The purpose of this thread is to gather advice, guidelines, and styles for river mapping into one place. Comments, corrections, criticism, and contributions are welcome.
Rivers do not generally fork as they travel downstream. Since this is the most common "river violation" we see, it goes at the top. Unless there is some kind of intervention, it is very unlikely that a river will split naturally. It may happen from time to time, but one of the forks will eventually erode low enough below the other that it will become the sole channel. Lakes behave the same way; there will generally be only one outflow from a lake, although there may be numerous inflows.
Rivers always flow downhill. We all know that water always runs downhill and takes the path of least resistance. There are some ramifications to this in mapping. On a contour map, a river will always cross a contour line at right angles. Rivers will usually move away from mountains, and they will very rarely move through a pass between two mountain ranges because that space between the ranges, though lower than either mountain chain, is usually still higher than the land on either side. Rivers will almost always run toward the coastline of an ocean, sea, or lake, since those bodies of water form where the terrain is at its lowest.
Natural rivers never connect one ocean to another. In most worlds, sea level is the same worldwide. Since water flows downhill, there is no force that will cause water to leave an ocean, flow across land, and back into an ocean. Anywhere that appears to happen is not really a river, but an ocean channel. The tides will usually cause the water to reverse its direction of motion through such a channel. Rivers don't reverse their motion in this fashion.
The steeper the slope, the straighter the river. Mountain streams tend to run straight and fast. Rivers moving across relatively level plains will meander and may frequently change their courses over time.
Lakes with no outflow are often salty. If the only way for water to leave a lake is through evaporation, the lake will tend to accumulate salt and other minerals dissolved in the water. If the water seeps into the earth at a significant rate, though, the lake will probably remain fresh.