Campaign Development: Adding a history to your campaign

Okay...so you decided to give your campaign setting a history...

There are Two kinds of events in history:
1. Events that happen and cant be changed. These are volcanic eruptions, asteroids from space, earthquake, tidal waves.
2. Events that can be prevented. The murderous rampage of a serial killer. The invasion of the kingdom of karameikos by thyatis.
Where a serial killer will have little geographic effect an asteroid from space will leave a crater that might well mean mountain ranges and or a lake exist in the future. The migration of humans into a forested region may result in deforestation, extinction of giant wombats, and global warming. Deserts where no deserts existed. A mountain pass might be the result of dwarves quarrying building stone for their fortress that now fills the pass.
Marko the Arsonist sets fire to a forest in the summer and turns thirty thousand acres of forest to ash in twenty four hours. Years from now that forest might be meadow with bogs and remnant forest.
History leaves a mark ond the landscape.