Rachel Walcott

Using hilltop curvature to derive the spatial distribution of erosion rates

[1] Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high-resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr � 1 ). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between gradient and sediment flux. Having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, we show that hilltop curvature continues to increase with rising erosion rates after gradient-limited hillslopes have emerged. Hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient, providing soil production on hilltops can keep pace with erosion. Finally, hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels.

Tectonic controls on the evolution of the Clutha River catchment, New Zealand

Abstract A synthesis of published information on mountain uplift and river capture in Otago suggests that the Clutha River catchment has evolved westwards and expanded since the Pliocene. River capture events that facilitated catchment expansion are indicated by sediment provenance, drainage geometry and freshwater fish genetics. The catchment has been partly confined by NW-trending ranges and the Southern Alps to the west, and drainage geometry was disrupted by subsequent growth of NE-trending ranges. Examination of crustal-scale deformation via an established numerical model, which portrays the Otago Schist basement as rheologically weak compared to adjacent greywacke-dominated structural blocks, suggests that uplift geometry was controlled by the inherited Cretaceous boundary between these crustal blocks. Pre-existing faults had relatively minor effects on uplift geometry. A low-relief corridor between Canterbury and Southland permitted genetic connections of freshwater fish populations through to <1 Ma, to the west of the developing Clutha catchment.

Squeezing river catchments through tectonics: Shortening and erosion across the Indus Valley, NW Himalaya

Tectonic displacement of drainage divides and the consequent deformation of river networks during crustal shortening have been proposed for a number of mountain ranges, but never tested. In order to preserve crustal strain in surface topography, surface displacements across thrust faults must be retained without being recovered by consequent erosion. Quantification of these competing processes and the implications for catchment topography have not previously been demonstrated. Here, we use structural mapping combined with dating of terrace sediments to measure Quaternary shortening across the Indus River valley in Ladakh, NW Himalaya. We demonstrate ∼0.21 m k.y.–1 of horizontal displacement since ca. 45 ka on the Stok thrust in Ladakh, which defines the southwestern margin of the Indus Valley catchment and is the major back thrust to the Tethyan Himalaya in this region. We use normalized river channel gradients of the tributaries that drain into the Indus River to show that the lateral continuation of the Stok thrust was active for at least 70 km along strike. Shortening rates combined with fault geometries yield vertical displacement rates that are compared to time-equivalent erosion rates in the hanging wall derived from published detrital 10Be analyses. The results demonstrate that vertical displacement rates across the Stok thrust were approximately twice that of the time-equivalent erosion rates, implying a net horizontal displacement of the surface topography, and hence narrowing of the Indus Valley at ∼0.1 m k.y.–1. A fill terrace records debris-flow emplacement linked to thrust activity, resulting in damming of the valley and extensive lake development. Conglomerates beneath some of the modern alluvial fans indicate a northeastward shift of the Indus River channel since ca. 45 ka to its present course against the opposite side of the valley from the Stok thrust. The integration of structural, topographic, erosional, and sedimentological data provides the first demonstration of the tectonic convergence of drainage divides in a mountain range and yields a model of the surface processes involved.

New approaches to analysing landscape development: Some contributions from Edinburgh

Abstract The Institute (formerly Department) of Geography of the University of Edinburgh has been active in the rejuvenation of the sub-discipline of geomorphology over the past two decades with its emphasis on addressing macroscale questions of long-term landscape development. In addition to interdisciplinary collaborations in the fields of low-temperature thermochronology and numerical landscape modelling, researchers there have played a key role both nationally and, more recently, internationally in applying cosmogenic isotope analysis to landform dating and the estimation of denudation rates. Some examples of research are discussed from the passive margin setting of southern Africa, the active tectonic setting of southern California, and the arid environments of Chile and Namibia.