(Not sure why this one has gone so dark...)
This is the kind of stuff that most people don't really need to know when they draw maps. But if you think about the map rivers carve their way into the landscape, you begin to understand the terrain a bit better.
The landscape isn't completely random, it is a developing ecosystem. Rivers carve into the continental landmass, creating valleys according to the types of rock they cross. Soft rock might allow for deep gorges (or it might allow for wide floodplains, depending on the speed of the water)...hard rock might push water in other directions as it tends to follow the path of least resistance. Harder rock might push water around it, possibly forming islands in the course of a water-stream before it reforms on the other side.
Consider the type of rock and the speed of the water, using these we can offer hints about the land (in the case of dwarves, there might be an indication of the minerals typically found in the area...in the case of elves, there might be more of an understanding about the plants and greenery).