This was a pretty basic way to simulate water errosion. The basic idea is let some water flow of the terrain and on the way it picks up some sedement and drops it. The way this was create is by seeding waterdrops at random over the whole terrain. From there on it follows the lowest path.If there is no more way down it drops all its sedement. The heigher the height difference the faster the water flows so the more sedement it picks up and the other way arround the lower the differnce the slower the water flows so the sedement starts dropping off. As all no water doesn't stay on one place it evaperates over time so the lower the life of the drop the less sedement it can carry.

This gave some pretty good results, the area in white is where most of the sedement is dropped off.

So the next step was to try and make some rivers. The main difference between a river and the previous is that a river has a continuous source of water. So my idea was just do the same but instead of evaperating the water let if flow until it reaches the sea or the border of the land. This didn't work the main problem is that terrain isn't always going down on atleast one side.

My second thought was store the water data and use this to make the water flow as before. I was even thinking that this water map could be used for the extual rendering of the rivers in later versions(.

interesting links on watersimulation and terrain errosion
http://vertexasylum.com/2010/10/30/gpu-based-water-simulator-thingie
http://graphicsrunner.blogspot.com/2010/08/water-using-flow-maps.html
http://cgg.mff.cuni.cz/~jaroslav/papers/2008-sca-erosim/2008-sca-erosiom-fin.pdf