I have tought about doing lakes and rivers in a triangle mesh generated from a height map. I would like to have realistic size for every river and lakes filling every hole in the terrain. To make it complete i would like to include see level. This is the algorithm I have been thinking about:
For every triangle set some initial flow for the lowest vertex. Then sort vertices by height and start with the highest vertex. For every vertex add the flow of every sorrounding vertex that flows into this vertex. When the flow exceed the limit defined to start a river, a river can be created. This will generate rivers ending in every local minimum. For every minimum a process for createing a lake has to be started.