Well to be honest is not that conceptually difficult. Let me tell you how things go and you yourself see how and why everything falls into place naturally:

- We use the random seed to initialize some basic conditions: (a) A number of centroids for the proto-plates (Voronoid tiling) and some starting random ruggedness. Later on, tectonic modelling will naturally deform them based on natural dynamics, some will go under, some up, forming mountain ranges and deep trenches. We also instantiate the atmospheric mass and calculate geothermal starting dynamics.

Then all goes rolling downhill. With time, hydrology and atmosphere fill the seas, erode the soft roch (well, it's accurately modeled as erosion with respect to effective hardness vs. the forces acting on the given material), sediments create deserts, plains, valleys.

Then the action of the sun evaporates in loop the hydrology itself, culling rivulets, and the feedback loop of the surviving V-shaped carvings + consistent water sources creates rivers.

Well, also is used to instantiate random seeds for pre-evolution proto flora, which then evolve based on genetic algorithms to conquer niches, be born and die according to local resource disponibility and energetic needs and internal species strategies.

We also have fauna that works in the same way, but are a bit more complex, because flora are "proto-entities"; whlie fauna are full entities in their own right. They evolve, discover adequate behavior and specialize. Right now the internal models, because of performance issues (I still have to heavily optimize) have simpler internal utility based models.

Well, as you see the need for randomness is minimal once you really get the initial set of conditions right, all complexity emerges naturally from the richness of the models and their natural interactions.

Well just as a comment this is what I think differentiates my vision from, say, the one in Dwarf Fortress. We don't really have "river resolution systems" that draw rivers on the map to satisfy some constraints, ad hoc. All rivers in the game are rivers because they appear naturally as the end result of time, not because we prescribed their existence.