Find nearest downstream value of a river network

  If the FROMJCT and TOJCT numbers define nodes of a graph, and the
presence of a row with FROMJCT=a and TOJCT=b defines an edge from a to
b of the graph, then you could use something like R's igraph package
to build a graph object. Once you've done that it should be easy to
use the graph algorithms in that package to wander down the river
system at your leisure...

It was my first look at the igraph package, and I dont think I found the 
optimal set of functions. So far I ended up with the following, although 
I am sure there are better solutions:

rndf = data.frame(FROMJCT = nres$FROMJCT, TOJCT = nres$TOJCT,
                OBJECTIT = nres$OBJECTID, pred = nres$pred)
igr = graph.data.frame(rndf)
igrs = topological.sort(igr, mode = "out")
rndf$to = match(as.character(rndf$TOJCT), V(igr)$name[igrs+1])

while (TRUE) {
   lcon = which(rndf$to == min(rndf$to[is.na(rndf$pred)]))
   if (length(lcon)==0) break()
   while(is.na(rndf$pred[lcon[1]])) lcon = c(neighbors(igr,lcon[1]-1)+1, 
lcon)
   rndf$pred[lcon] = rndf$pred[lcon[1]]
}

The best with this solution is that it is extremely fast compared to my 
first suggestion, about 20-30 times faster for 1200 segments. And it 
also seems to scale better with larger data sets. So I think I will 
either try to simplify this further, or just make a function of the 
lines above.

  It might get complicated if you are modelling on the Orinoco where it
bifurcates, or a river delta :)

Complicated or more interesting, depending on how you see it and how 
much time you have ;-)
For the second reason I am happy that the river system I am interested 
in behaves rather "normal"...

Thanks!
Jon