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.

The new watersystem is based on the idea that every vertices is connected by a virtual pipe in which the waterflows. Using this virtual pipe system we can create a realistic flow of water over the terrain.

New rendering methode for the water tests.

Using the waterFlow of the terrain we can calculate a velocity of the water streaming downhill. Then using the velocity of the water and the angle of the terrain we can mimic the push of dirt downhill by the water.

As you can see on the following comparison pictures the terrain has been carved out by the flowing water. One of the advantages of the velocity map is that the water tends to streamfaster in bigger bodies of water and there for the main rivers are carved out dieper than the others. There are still some artefacts on the water system especialy when water starts to build up in diep pools of water. This causes to make the water stream much faster and therefore carve even deeper and you got a avalanche effect.

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