I just saw this link below posted on ycombinator about calculating voroni patches and wondered if this would be useful for this process. What I particularly liked was that you can rapidly get the distance from the points. Surely that has great implications for calculating the river flow. I can see that this could be useful for other things too like city mapping where the probability of putting stuff down depends on its closeness to a resource. I was doing something similar to this when trying to find the closest route from person to resource in my mapping with economics thing and I used a similar but not nearly as effective way of doing it.

http://rykap.com/graphics/skew/2016/...onoi-diagrams/