This link is dead:
Ehlschlaeger, C. (1989). Using the AT Search Algorithm to Develop Hydrologic
Models from Digital Elevation Data, Proceedings of International Geographic
Information Systems (IGIS) Symposium '89, pp 275-281 (Baltimore, MD, 18-19
March 1989).
URL: http://faculty.wiu.edu/CR-Ehlschlaeger2/older/IGIS/paper.html
Rich