It seems very logical that base-level lowering (Figure 1.2) will rejuvenate a drainage network, deliver large amounts of sediment downstream, and cause major channel change. Support for these conclusions comes from experimental studies (Holland and Pickup, 1976; Schumm et al., 1987) and from observations of the headward incision of arroyos, gullies and channelized streams (Schumm et al., 1984). Fisk (1944) concluded that Pleistocene sea-level lowering caused excavation of alluvium and bedrock incision by the Mississippi River for a very long distance upstream. On a smaller scale even a single meander cutoff can cause local steepening and upstream channel degradation (Winkley, 1977). Lane (1955) an eminent river engineer concluded that, when a river is affected by a base-level rise or fall (Figure 16.1), the river will degrade or aggrade to restore its equilibrium profile. After all, the channel must continue to carry its load of sediment with a given water discharge, and this requires a given gradient that must be restored either by deposition (Figure 16.1a) or by erosion (Figure 16.1b).

In contrast to Lane, Leopold and Bull (1979) concluded that base-level changes affect the vertical position of a river only locally and to a minor extent. They argued that not only is stream gradient change important, but that channel pattern, roughness and shape (Rubey, 1952) can also adjust, in order to absorb the effect of base-level change.