A rivernetwork object, with a new list element, $distlookup, a
list of three matrices. Element [i,j] of each matrix corresponds to
the route between segment i and j. The
distlookup$middist matrix gives the total distance of the "middle"
of each route (between the starting and ending segments"), and the
distlookup$starttop and distlookup$endtop matrices have value
TRUE, FALSE, or NA if the segments at the beginning or
end of the route are connected to the rest of the route at the top of the
coordinate matrix, bottom of the coordinate matrix, or if the route is
contained to just one segment, respectively. (See rivernetwork.)