River networks would be nice, but they would be quite complicated. I haven't read that PDF yet (it's too long), but I'd say that the easiest method to create a river would be to place a water source (or multiple sources) somewhere and specify their water output, the speed at which the river naturally grows (from water dripping from soil and rain and other stuff), and the age of the river (for erosion purposes). Terragen could then work out where the water would flow and, based on the size and age of the river and the speed at which the water is travelling in it, it could create the path of the river as well as any associated features (such as ox-bow lakes). However, to get the most out of that system, Terragen would need different rock types, which would affect erosion speed at different places and the sediment load of the river...
