Bedrock fluvial incision and longitudinal profile development over geologic time scales determined by fluvial terraces

Abstract

Fluvial terraces preserve the history of river incision into bedrock over geologic time scales. In this paper we use terraces and a comparison of terrace longitudinal profiles to stream longitudinal profiles to develop a conceptual model of bedrock fluvial incision in diverse geologic, tectonic, and climatic settings. The conceptual model highlights a distinction between bedrock stream behavior in settings of relatively high versus relatively low tectonic activity. This distinction arises from the fundamentally different ways in which runoff is generated in these respective tectonic settings and the positive feedbacks that exist between topography and climate. The model allows for qualitative predictions of long profile shape that can be directly compared to the longitudinal profiles predicted by the stream power law. Our approach has the advantage of helping understand the geologic (and climatic) constraints on the wide variations in k, m, and n revealed in recent applications of the stream power law. We reconcile diverse longitudinal profile shapes and long-term rates of bedrock fluvial incision by considering how a drainage basin generates fluvial discharge and whether that discharge can produce the necessary stream power distributed across a valley bottom such that the long profile can rapidly accommodate changes in base level, climate, and/or rates of rock uplift. We propose that in tectonically active settings (Type I basins), the entire drainage basin experiences uplift which, in turn, builds steep slopes and concomitant increases in orographic precipitation that effectively generate the high peak discharges and fluvial-system wide stream power necessary to create and maintain concave-up long profiles and rates of incision equal and opposite to rates of rock uplift. Measured stream power for one of these basins is highly correlated to the width of the channel and valley bottom which argues for a conservation of energy along the profile and the apportionment of stream power to vertical incision, lateral incision, and bedload transportation. The stream power used for lateral incision processes periodically widen the channel bottom during transient, hydrologically-driven changes in discharge and sediment load, producing fluvial terraces. In contrast, drainage basins in tectonically inactive settings (Type II) may not have a hydrology characterized by high peak discharges, particularly for those drainage basins which do not receive large, highly seasonal and/or highly-variable precipitation (Type IIa). Streams in the tectonically-inactive setting are more dependent on local changes in stream power, spatially restricted to knickpoints, that require long periods of time to propagate through the system. A change in down stream base level in these settings has a particularly profound impact on long profile shape, especially where the river crosses resistant rock-types. Type II basins located where climate favors highly seasonal and/or variable precipitation (Type IIb) retain minor rock-type controlled convexities on otherwise concave profiles.