His post on the topic is a worthwhile read for anyone interested in cartography. In addition to a preview of his river system maps, Daniel does an excellent job navigating us through his design process. Daniel has produced four of these maps so far, all of which are available for sale on Zazzle.
Side note: Over at Mapdiva they’re putting final touches on the next update of Ortelius. One enhancement to anticipate is direct label rotation – making this style of map easier to produce. Stay tuned!