INTEGRATED FIELD AND NUMERICAL TEST OF STREAM EROSION MODELS USING THE TRANSIENT RESPONSE OF BEDROCK RIVERS TO TECTONIC FORCING

Fluvial erosion is fundamental in controlling the relationships between climatic/tectonic conditions, topographic relief and sediment flux across landscapes. Several long-term fluvial erosion models have been published that make very different predictions in terms of landscape response to changes in external forcing (e.g. tectonics or climate). We field test these competing theories using detailed observations of bedrock rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. Our field data include measurements of channel geometry (slope, depth and width) and sediment grain size in active gravel bars. We demonstrate that rivers with drainage areas >10 km² and crossing faults that have undergone an increase in throw rate within the last 1 my have significant long-profile convexities where the channels become steep and narrow. We use the CHILD model to show that the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. The height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault, consistent with predictions of the detachment-limited (DL) fluvial erosion model. The upstream migration rate of the profile convexities, which varies from 1.5–10 mm/y, is a function of the slip rate increase as well as the drainage area. By including in CHILD a hydraulic scaling relationship derived from our field data, expressing channel width as a power function of drainage area and channel slope and with an uplift-dependent coefficient, we are able to explain the migration rates without having to invoke an erosion threshold or a non-linear slope dependence in the stream power model (slope exponent > 1). Features not well-explained by the modified DL model are reach-scale variations in channel slope upstream of some faults, which do not correlate with lithology. One plausible explanation is that large amounts of sediment, supplied to the channels from adjacent hill-slopes where the river is down-cutting most rapidly, are modulating fluvial incision rates either via impact abrasion and plucking, or via inhibiting erosion by limiting the amount of bedrock exposed, i.e., the “tools-versus-cover” effect.