Here's what happens when you take a hierarchical Poisson disc stipple field, assign a rainfall level of 0 or 1 based on rank in the field and a precipitation surface (in this case (sin x*cos y)^1.5), assign the nodes around the edge to a starting set (effectively the ocean) and then randomly add nodes adjacent to the connected set in the Delaunay triangulation of the whole set to the connected set until you've connected them all, and then display the graph with edges weighted by the sum of upstream rainfall.

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