Authors: Luna B. Leopold and William B. Bull. Abstract: The influence of local base level and the effects of a change of base level can be separated from the influence of hydrologic and geomorphic processes with the basin under certain conditions: 1) observed changes in the longitudinal profile by aggradation or degradation when a local base level is lowered or raised while the basin remains unchanged; 2) observed changes in aggradation and/or degradation of a valley fill alluvium during periods of unchanging local base level. Originally published: Proceedings of the American Philosophical Society ; v. 123, no. 2 (April 1979).