Vegetation‐precipitation controls on Central Andean topography

Climatic controls on fluvial landscapes are commonly characterized in terms of mean annual precipitation. However, physical erosion processes are driven by extreme events and are therefore more directly related to the intensity, duration, and frequency of individual rainfall events. Climate also influences erosional processes indirectly by controlling vegetation. In this study, we explore how interdependent climate and vegetation properties affect landscape morphology at the scale of the Andean orogen. The mean intensity, duration, and frequency of precipitation events are derived from the TRMM 3B42v7 product. Relationships between mean hillslope gradients and precipitation event metrics, mean annual precipitation, vegetation, and bedrock lithology in the central Andes are examined by correlation analyses and multiple linear regression. Our results indicate that mean hillslope gradient correlates most strongly with percent vegetation cover ( r  = 0.56). Where vegetation cover is less than 95%, mean hillslope gradients increase with mean annual precipitation ( r  = 0.60) and vegetation cover ( r  = 0.69). Where vegetation cover is dense (>95%), mean hillslope gradients increase with increasing elevation ( r  = 0.74), decreasing inter‐storm duration ( r  = −0.69), and decreasing precipitation intensity by ~0.5°/(mm d −1 ) ( r  = −0.56). Thus, we conclude that at the orogen scale, climate influences on topography are mediated by vegetation, which itself is dependent on mean annual precipitation ( r  = 0.77). Observations from the central Andes are consistent with landscape evolution models in which hillslope gradients are a balance between rock uplift, climatic erosional efficiency and erosional resistance of the landscape determined by bedrock lithology and vegetation. Key Points Hillslope gradients in central Andes increase with increasing vegetation cover Precipitation intensity affects topography most in densely vegetated areas Mean annual precipitation affects erosional efficiency through vegetation coverPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108109/1/jgrf20258.pd

Quantifying the role of paleoclimate and Andean Plateau uplift on river incision

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99035/1/jgrf20055-sup-0002-2012JF002533fs02.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99035/2/jgrf20055.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99035/3/jgrf20055-sup-0001-2012JF002533fs01.pd

Identifying spatial variations in glacial catchment erosion with detrital thermochronology

Understanding the spatial distribution of glacial catchment erosion during glaciation has previously proven difficult due to limited access to the glacier bed. Recent advances in detrital thermochronology provide a new technique to quantify the source elevation of sediment. This approach utilizes the tendency of thermochronometer cooling ages to increase with elevation and provides a sediment tracer for the elevation of erosion. We apply this technique to the Tiedeman Glacier in the heavily glaciated Mount Waddington region, British Columbia. A total of 106 detrital apatite (U‐Th)/He (AHe) and 100 apatite fission track (AFT) single‐grain ages was presented from the modern outwash of the Tiedemann Glacier with catchment elevations between 530 and 3960 m. These data are combined with nine AHe and nine AFT bedrock ages collected from a ~2400 m vertical transect to test the hypotheses that erosion is uniformly or nonuniformly distributed in the catchment. A Monte Carlo sampling model and Kuiper statistical test are used to quantify the elevation range where outwash sediment is sourced. Model results from the AHe data suggest nearly uniform erosion in the catchment, with a preference for sediment being sourced from ~2900 to 2700 m elevation. Ages indicated that the largest source of sediment is near the present‐day ELA. These results demonstrate the utility of AHe detrital thermochronology (and to a lesser degree AFT data) to quantify the distribution of erosion by individual geomorphic processes, as well as some of the limitations of the technique.Key PointsDetrital thermochronometers record spatial pattern of erosionNearly uniform erosion under the present‐day glacierThe largest observed source area of erosion is near the ELAPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112265/1/jgrf20399-sup-0001-supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112265/2/jgrf20399.pd

Lithologic Effects on Landscape Response to Base Level Changes: A Modeling Study in the Context of the Eastern Jura Mountains, Switzerland

Landscape evolution is a product of the forces that drive geomorphic processes (e.g., tectonics and climate) and the resistance to those processes. The underlying lithology and structural setting in many landscapes set the resistance to erosion. This study uses a modified version of the Channel‐Hillslope Integrated Landscape Development (CHILD) landscape evolution model to determine the effect of a spatially and temporally changing erodibility in a terrain with a complex base level history. Specifically, our focus is to quantify how the effects of variable lithology influence transient base level signals. We set up a series of numerical landscape evolution models with increasing levels of complexity based on the lithologic variability and base level history of the Jura Mountains of northern Switzerland. The models are consistent with lithology (and therewith erodibility) playing an important role in the transient evolution of the landscape. The results show that the erosion rate history at a location depends on the rock uplift and base level history, the range of erodibilities of the different lithologies, and the history of the surface geology downstream from the analyzed location. Near the model boundary, the history of erosion is dominated by the base level history. The transient wave of incision, however, is quite variable in the different model runs and depends on the geometric structure of lithology used. It is thus important to constrain the spatiotemporal erodibility patterns downstream of any given point of interest to understand the evolution of a landscape subject to variable base level in a quantitative framework.Key PointsA landscape evolution model is used to show how topographic history is influenced by regional geologyExhumation of different lithologies modulates the transient response to base level changes over millions of yearsSignificantly different erosion and topographic histories result depending on the stratigraphic architecture, even over a small range in erodibilityPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141336/1/jgrf20766_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141336/2/jgrf20766.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141336/3/jgrf20766-sup-0001-Data_S1.pd

High magnitude and rapid incision from river capture: Rhine River, Switzerland

Landscape evolution is controlled by the development and organization of drainage basins. As a landscape evolves, drainage reorganization events can occur via river capture or piracy, whereby one river basin grows at the expense of another. The river downstream of a capture location will generate a transient topographic response as the added water discharge increases sediment transport and erosion efficiency. This erosional response will propagate upstream through both the captured and original river basins. Here we focus on quantifying the impact of drainage reorganization along the Rhine/Aare River system (~45,000 km 2 ) during the late Pliocene/early Pleistocene, where gravel remnants indicate total incision of ~650 m during the last ~4.2 Myr in the region of the recent Aare‐Rhine confluence. We develop a numerical model of drainage capture to quantify the range of possible magnitudes of erosion and the transient river response resulting from the reorganization of the Rhine River. The model accounts for both fluvial incision and sediment transport. Our model estimates 400–800 m of river elevation change (lowering profiles) during the last ~4 Myr due to river capture events, providing an important component to the recent exhumation budget of the Swiss Alpine Foreland. The model indicates a rapid response to capture events (re‐equilibration timescale of ~1 Myr). The predicted incision magnitudes are consistent with incision measured from the elevation of Pliocene and early Pleistocene river gravels, suggesting that across northern Switzerland, a significant amount of incision can be explained by drainage reorganization. Key Points Drainage capture has caused significant erosion along the Rhine River The transient erosional wave propagates quickly through the landscape The incision is a significant fraction of Plio‐Pleistocene erosion in the regionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99064/1/jgrf20056.pd

The influence of sediment cover variability on long‐term river incision rates: An example from the Peikang River, central Taiwan

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94660/1/jgrf824.pd

The influence of sediment cover variability on long-term river incision rates: An example from the Peikang River, central Taiwan

[1] This study explores the hypothesis that the relative frequency of rock exposure in the bed of an incising channel can have a first-order impact on the long-term average erosion rate. The 1999 Chi-Chi earthquake in central Taiwan generated thousands of landslides along the middle reach of the Peikang River. Sediment from these landslides produced widespread aggradation, such that much of the river's bed remains shielded from active bedrock incision. We present data that constrain the spatial and temporal variability of sediment cover for the Peikang River. Because the river is undergoing spatially variable Holocene bedrock incision (1-10 mm/yr), it offers a unique natural experiment to test the influence of intermittent sedimentation on long-term incision rates. Published electrical resistivity surveys at seven locations along the river reveal median sediment depth values ranging from 1.9 to 11.5 m. The sediment depth correlates inversely with long-term incision rate and sediment transport capacity. We interpret this as an indication that the frequency of bedrock exposure exerts a major influence on incision along the Peikang River

Controls and limits on bedrock channel geometry

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94837/1/jgrf728.pd

The Dynamics of Channel Slope, Width, and Sedimentin Actively Eroding Bedrock River Systems

The evolution of rivers in eroding landscapes plays a key role in determining landscape relief and modulating climate‐tectonic interactions. A common approach to quantifying river system evolution uses a one‐dimensional, detachment‐limited stream power equation. One potential drawback of this model is that it does not incorporate the effects of changes in channel width or the role of sediment transport dynamics. Here I present a new method for modeling the influence of channel width on river dynamics to explore how variable width and sediment transport impact river profile evolution. With this approach, vertical river erosion can operate based on any number of river erosion models, such as a simple shear stress model (e.g., detachment limited), sediment cover‐shear stress hybrid models, or mechanistic saltation‐abrasion models. I explore the sensitivity of these three models to increases in rock‐uplift rate (i.e., 2, 3, 5, 10, and 20× increase). Generally, the results show that incorporating channel width adjustment or sediment transport dynamics lowers the sensitivity of a river profile to rock‐uplift rate. For the sediment transport‐dependent models, the degree of sensitivity depends on whether the system is limited by bedrock exposure or erosion potential (i.e., detachment potential). The approach produces transient responses that reveal distinct patterns of width and slope, which may provide valuable insight into the limiting physical mechanisms of bedrock erosion in a region. The implications of the work are broad and include the potential to distinguish underlying erosion controls from field observations of width and slope as well as understanding climate‐tectonic interactions