Matching Items (5)
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- All Subjects: Erosion
- Creators: Whipple, Kelin
- Creators: Heimsath, Arjun M
Description
Climate and its influence on hydrology and weathering is a key driver of surface processes on Earth. Despite its clear importance to hazard generation, fluvial sediment transport and erosion, the drawdown of atmospheric CO2 via the rock cycle, and feedbacks between climate and tectonics, quantifying climatic controls on long-term erosion rates has proven to be one of the grand problems in geomorphology. In fact, recent attempts addressing this problem using cosmogenic radionuclide (CRN) derived erosion rates suggest very weak climatic controls on millennial-scale erosion rates contrary to expectations. In this work, two challenges are addressed that may be impeding progress on this problem.
The first challenge is choosing appropriate climate metrics that are closely tied to erosional processes. For example, in fluvial landscapes, most runoff events do little to no geomorphic work due to erosion thresholds, and event-scale variability dictates how frequently these thresholds are exceeded. By analyzing dense hydroclimatic datasets in the contiguous U.S. and Puerto Rico, we show that event-scale runoff variability is only loosely related to event-scale rainfall variability. Instead, aridity and fractional evapotranspiration (ET) losses are much better predictors of runoff variability. Importantly, simple hillslope-scale soil water balance models capture major aspects of the observed relation between runoff variability and fractional ET losses. Together, these results point to the role of vegetation water use as a potential key to relating mean hydrologic partitioning with runoff variability.
The second challenge is that long-term erosion rates are expected to balance rock uplift rates as landscapes approach topographic steady state, regardless of hydroclimatic setting. This is illustrated with new data along the Main Gulf Escarpment, Baja, Mexico. Under this conceptual framework, climate is not expected to set the erosion rate, but rather the erosional efficiency of the system, or the steady-state relief required for erosion to keep up with tectonically driven uplift rates. To assess differences in erosional efficiency across landscapes experiencing different climatic regimes, we contrast new CRN data from tectonically active landscapes in Baja, Mexico and southern California (arid) with northern Honduras (very humid) alongside other published global data from similar hydroclimatic settings. This analysis shows how climate does, in fact, set functional relationships between topographic metrics like channel steepness and long-term erosion rates. However, we also show that relatively small differences in rock erodibility and incision thresholds can easily overprint hydroclimatic controls on erosional efficiency motivating the need for more field based constraints on these important variables.
The first challenge is choosing appropriate climate metrics that are closely tied to erosional processes. For example, in fluvial landscapes, most runoff events do little to no geomorphic work due to erosion thresholds, and event-scale variability dictates how frequently these thresholds are exceeded. By analyzing dense hydroclimatic datasets in the contiguous U.S. and Puerto Rico, we show that event-scale runoff variability is only loosely related to event-scale rainfall variability. Instead, aridity and fractional evapotranspiration (ET) losses are much better predictors of runoff variability. Importantly, simple hillslope-scale soil water balance models capture major aspects of the observed relation between runoff variability and fractional ET losses. Together, these results point to the role of vegetation water use as a potential key to relating mean hydrologic partitioning with runoff variability.
The second challenge is that long-term erosion rates are expected to balance rock uplift rates as landscapes approach topographic steady state, regardless of hydroclimatic setting. This is illustrated with new data along the Main Gulf Escarpment, Baja, Mexico. Under this conceptual framework, climate is not expected to set the erosion rate, but rather the erosional efficiency of the system, or the steady-state relief required for erosion to keep up with tectonically driven uplift rates. To assess differences in erosional efficiency across landscapes experiencing different climatic regimes, we contrast new CRN data from tectonically active landscapes in Baja, Mexico and southern California (arid) with northern Honduras (very humid) alongside other published global data from similar hydroclimatic settings. This analysis shows how climate does, in fact, set functional relationships between topographic metrics like channel steepness and long-term erosion rates. However, we also show that relatively small differences in rock erodibility and incision thresholds can easily overprint hydroclimatic controls on erosional efficiency motivating the need for more field based constraints on these important variables.
ContributorsRossi, Matthew (Author) / Whipple, Kelin X (Thesis advisor) / DeVecchio, Duane E (Committee member) / Vivoni, Enrique R (Committee member) / Arrowsmith, J Ramon (Committee member) / Heimsath, Arjun M (Committee member) / Arizona State University (Publisher)
Created2014
Description
Drylands (arid and semi-arid grassland ecosystems) cover about 40% of the Earth's surface and support over 40% of the human population, most of which is in emerging economies. Human development of drylands leads to topsoil loss, and over the last 160 years, woody plants have encroached on drylands, both of which have implications for maintaining soil viability. Understanding the spatial variability in erosion and soil organic carbon and total nitrogen under varying geomorphic and biotic forcing in drylands is therefore of paramount importance. This study focuses on how two plants, palo verde (Parkinsonia microphylla, nitrogen-fixing) and jojoba (Simmondsia chinensis, non-nitrogen fixing), affect sediment transport and soil organic carbon and total nitrogen pools in a dryland environment north of Phoenix, Arizona. Bulk samples were systematically collected from the top 10 cm of soil in twelve catenae to control for the existence and type of plants, location to canopy (sub- or intercanopy, up- or downslope), aspect, and distance from the divide. Samples were measured for soil organic carbon and total nitrogen and an unmanned aerial system-derived digital elevation map of the field site was created for spatial analysis. A subset of the samples was measured for the short-lived isotopes 137Cs and 210Pbex, which serve as proxy erosion rates. Erosional soils were found to have less organic carbon and total nitrogen than depositional soils. There were clear differences in the data between the two plant types: jojoba catenae had higher short-lived isotope activity, lower carbon and nitrogen, and smaller canopies than those of palo verde, suggesting lower erosion rates and nutrient contributions from jojoba plants. This research quantifies the importance of biota on influencing hillslope and soil dynamics in a semi-arid field site in central AZ and finishes with a discussion on the global implications for soil sustainability.
ContributorsAlter, Samuel (Author) / Heimsath, Arjun M (Thesis advisor) / Throop, Heather L (Committee member) / Walker, Ian J (Committee member) / Arizona State University (Publisher)
Created2018
Description
The morphology of mountainous areas is strongly influenced by stream bed incision rates, but most studies of landscape evolution consider erosion at basin scales or larger. The research here attempts to understand the smaller-scale mechanics of erosion on exposed bedrock channels in the conceptual framework of an established saltation-abrasion model by Sklar and Dietrich [2004]. The recirculating flume used in this experiment allows independent control of bed slope, water discharge rate, sediment flux, and sediment grain size – all factors often bundled together in simple models of river incision and typically cross-correlated in natural settings. This study investigates the mechanics of erosion on exposed bedrock channels caused by abrasion of transported particles. Of particular interest are saltating particles, as well as sediment near the threshold between saltation and suspension - sediment vigorously transported but with significant interaction with the bed. The size of these erosive tools are varied over an order of magnitude in mean grain diameter, including a sand of D¬50 = 0.56 mm, and three gravel sizes of 3.39, 4.63, and 5.88 mm. Special consideration was taken to prevent any flow conditions that created a persistent alluvial cover. The erodible concrete substrate is fully exposed at all times during experiments reported here. Rates of erosion into the concrete substrate (a bedrock proxy) were measured by comparing topographic data before and after each experimental run, made possible by a precision laser mounted on a high speed computer-controlled cart. The experimental flume was able to produce flow discharge as high as 75 liters per second, sediment fluxes (of many varieties) up to 215 grams per second, and bed slopes up to 10%. I find a general positive correlation is found between erosion rate and bed slope, shear stress, grain size, and sediment flux.
ContributorsAdams, Mark (Author) / Whipple, Kelin (Thesis advisor) / Heimsath, Arjun (Committee member) / Schmeeckle, Mark (Committee member) / Arizona State University (Publisher)
Created2016
Description
For this dissertation, three separate papers explore the study areas of the western Grand Canyon, the Grand Staircase (as related to Grand Canyon) and Desolation Canyon on the Green River in Utah.
In western Grand Canyon, I use comparative geomorphology between the Grand Canyon and the Grand Wash Cliffs (GWC). We propose the onset of erosion of the GWC is caused by slip on the Grand Wash Fault that formed between 18 and 12 million years ago. Hillslope angle and channel steepness are higher in Grand Canyon than along the Grand Wash Cliffs despite similar rock types, climate and base level fall magnitude. These experimental controls allow inference that the Grand Canyon is younger and eroding at a faster rate than the Grand Wash Cliffs.
The Grand Staircase is the headwaters of some of the streams that flow into Grand Canyon. A space-for-time substitution of erosion rates, supported by landscape simulations, implies that the Grand Canyon is the result of an increase in base level fall rate, with the older, slower base level fall rate preserved in the Grand Staircase. Our data and analyses also support a younger, ~6-million-year estimate of the age of Grand Canyon that is likely related to the integration of the Colorado River from the Colorado Plateau to the Basin and Range. Complicated cliff-band erosion and its effect on cosmogenic erosion rates are also explored, guiding interpretation of isotopic data in landscapes with stratigraphic variation in quartz and rock strength.
Several hypotheses for the erosion of Desolation Canyon are tested and refuted, leaving one plausible conclusion. I infer that the Uinta Basin north of Desolation Canyon is eroding slowly and that its form represents a slow, stable base level fall rate. Downstream of Desolation Canyon, the Colorado River is inferred to have established itself in the exhumed region of Canyonlands and to have incised to near modern depths prior to the integration of the Green River and the production of relief in Desolation Canyon. Analysis of incision and erosion rates in the region suggests integration is relatively recent.
In western Grand Canyon, I use comparative geomorphology between the Grand Canyon and the Grand Wash Cliffs (GWC). We propose the onset of erosion of the GWC is caused by slip on the Grand Wash Fault that formed between 18 and 12 million years ago. Hillslope angle and channel steepness are higher in Grand Canyon than along the Grand Wash Cliffs despite similar rock types, climate and base level fall magnitude. These experimental controls allow inference that the Grand Canyon is younger and eroding at a faster rate than the Grand Wash Cliffs.
The Grand Staircase is the headwaters of some of the streams that flow into Grand Canyon. A space-for-time substitution of erosion rates, supported by landscape simulations, implies that the Grand Canyon is the result of an increase in base level fall rate, with the older, slower base level fall rate preserved in the Grand Staircase. Our data and analyses also support a younger, ~6-million-year estimate of the age of Grand Canyon that is likely related to the integration of the Colorado River from the Colorado Plateau to the Basin and Range. Complicated cliff-band erosion and its effect on cosmogenic erosion rates are also explored, guiding interpretation of isotopic data in landscapes with stratigraphic variation in quartz and rock strength.
Several hypotheses for the erosion of Desolation Canyon are tested and refuted, leaving one plausible conclusion. I infer that the Uinta Basin north of Desolation Canyon is eroding slowly and that its form represents a slow, stable base level fall rate. Downstream of Desolation Canyon, the Colorado River is inferred to have established itself in the exhumed region of Canyonlands and to have incised to near modern depths prior to the integration of the Green River and the production of relief in Desolation Canyon. Analysis of incision and erosion rates in the region suggests integration is relatively recent.
ContributorsDarling, Andrew Lee (Author) / Whipple, Kelin (Thesis advisor) / Semken, Steven (Committee member) / Arrowsmith, Ramon (Committee member) / DeVecchio, Duane (Committee member) / Heimsath, Arjun (Committee member) / Arizona State University (Publisher)
Created2016
Description
Mountain landscapes reflect competition between tectonic processes acting to build topography and erosive processes acting to wear it down. In temperate mountain landscapes, bedrock rivers are the primary erosional agent, setting both the pace of landscape evolution and form of the surrounding topography. Theory predicts that river steepness is sensitive to climatic, tectonic, and lithologic factors, which dictate the rates and mechanics of erosional processes. Thus, encoded into topography is an archive of information about forces driving landscape evolution. Decoding this archive, however, is fraught and climate presents a particularly challenging conundrum: despite decades of research describing theoretically how climate should affect topography, unambiguous natural examples from tectonically active landscapes where variations in climate demonstrably influence topography are elusive. In this dissertation, I first present a theoretical framework describing how the spatially varied nature of orographic rainfall patterns, which are ubiquitous features of mountain climates, complicate expectations about how climate should influence river steepness and erosion. I then apply some of these ideas to the northern-central Andes. By analyzing river profiles spanning more than 1500 km across Peru and Bolivia, I show that the regional orographic rainfall pattern this landscape experiences systematically influences fluvial erosional efficiency and thus topography. I also show how common simplifying assumptions built into conventional topographic analysis techniques may introduce biases that undermine detection of climatic signatures in landscapes where climate, tectonics, and lithology all covary – a common condition in mountain landscapes where these techniques are often used. I continue by coupling this analysis with published erosion rates and a new dataset of 25 cosmogenic 10Be catchment average erosion rates. Once the influence of climate is accounted for, functional relationships emerge among channel steepness, erosion rate, and lithology. I then use these functional relationships to produce a calibrated erosion rate map that spans over 300 km of the southern Peruvian Andes. These results demonstrate that accounting for the effects of climate significantly enhances the ability to decode channel steepness patterns. Along with this comes the potential to better understand rates and patterns of tectonic processes, and identify seismic hazards associated with tectonic activity using topography.
ContributorsLeonard, Joel Scott (Author) / Whipple, Kelin (Thesis advisor) / Arrowsmith, Ramon (Committee member) / Christensen, Philip (Committee member) / Forte, Adam (Committee member) / Heimsath, Arjun (Committee member) / Hodges, Kip (Committee member) / Arizona State University (Publisher)
Created2022