You can use the make-rivers
method to add rivers to the world.
Rivers are basically water features that depend on terrain elevations,
so they won't be generated unless both a river terrain type (either
border or connection) and elevation data is available.
You get them by specifying a nonzero chance for some type of
terrain to be the location of a headwater (river-chance
).
Xconq doesn't have any intuition about the behavior of water;
it will happily trace rivers all the the way down to the bottom of the sea.
Use the liquid
property to tell make-rivers
what types that rivers cannot touch.
The method still traces the river's course, and resumes modifying
terrain when possible, which means that the river can appear
as both the inlet and outlet from a lake.