RIVER INCISION, THE FLUVIAL SADLER EFFECT, AND GEOMORPHIC INTERPRETATION OF ROCK UPLIFT

Surface process rates have long been known to display a negative power law dependence on the time interval over which the measurements are made. Rates of river incision measured from Holocene fluvial terraces are commonly more rapid by a factor of 2 or 3 with respect to Pleistocene terraces, a characteristic that we here term the fluvial Sadler effect. If not viewed in the correct geologic framework, with multiple chronologic constraints, and integrated across the significant hydro-climatic cycles of the Pleistocene, acceleration in the rate of incision could lead to erroneous interpretations of unsteady rock uplift (base level fall). Recent compilation and analysis of fluvial terrace datasets suggests that unsteady river incision and the fluvial Sadler effect persists on time scales approaching 10⁷ years and this arises because hiatuses in fluvial incision follow a stochastic, heavy-tailed distribution. Here we demonstrate that an apparent fluvial Sadler effect exists only when poor assumptions regarding the datum for measurements are used coupled with poor terrace geochronology. Examination of modern river profiles reveal that they are in various stages of incision or aggradation (disequilibrium) where the modern, Holocene longitudinal profile may not be representative of the mean, graded equilibrium profile. Streams in regions of active base level fall with climatically-driven late Pleistocene strath terraces experience particularly aggressive Holocene incision, resulting in anomalously low-standing channels and a strong Sadler effect. Conversely, streams with more modest rates of base level fall and/or in semi-arid environments experience significant late Pleistocene-Holocene valley alluviation, exhibit long profiles that are closer to the mean graded form, and lack the Sadler effect. The impact of the datum chosen to measure incision is further illustrated by the lack of a Sadler effect for marine terraces where base level (sea level) can be carefully controlled. We conclude that interpretations of unsteadiness in the tectonic processes that uplift rocks can be interpreted from fluvial records of incision, given adequate geologic and chronologic characterization, integration across at least one glacial-interglacial cycle, and consideration of the correct graded profile datum.