Consider the d-dimensional lattice ℤd where each vertex is “open” or “closed” with probability p or 1−p, respectively. An open vertex v is connected by an edge to the closest open vertex w such that the dth co-ordinates of v and w satisfy w(d)=v(d)−1. In case of nonuniqueness of such a vertex w, we choose any one of the closest vertices with equal probability and independently of the other random mechanisms. It is shown that this random graph is a tree almost surely for d=2 and 3 and it is an infinite collection of distinct trees for d≥4. In addition, for any dimension, we show that there is no bi-infinite path in the tree and we also obtain central limit theorems of (a) the number of vertices of a fixed degree ν and (b) the number of edges of a fixed length l.
"Random oriented trees: A model of drainage networks." Ann. Appl. Probab. 14 (3) 1242 - 1266, August 2004. https://doi.org/10.1214/105051604000000288