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Sierra Nevada

Tim Wuenscher, defended his Master of Science research in August 2018

Regolith mobility and its effects on hillslope grain size distributions, Mono Basin, CA

Abstract

 Earth’s landforms constantly change as a result of erosion at the surface. Surface regolith is composed of a variety of different grains sizes, each with a different rate of
transport. The varying rates of transport between grain sizes cause the distribution of grain sizes to change along hillslopes. The ability to predict grain size distributions as a
function of time would be useful for determining slope stability and determining ages of landforms. However, current knowledge is limited for the varying rates of transport of
different grain sizes. To better understand grain transport rates, a set of experiments was carried out in Mono Basin, CA, where glacial moraines provide ideal hillslopes to
measure grain transport rates and grain size distributions. This data was used within a hillslope diffusion model to simulate transport of grains and generate grain size
distributions along the slope over time. 


Results from the field experiments showed that transport rate does not necessarily increase with decreasing grain size, and that there may be a certain grain size (1-4 mm)
that has the highest velocity on hillslope surfaces. This may be due to cohesiveness of finer grains, a buoyancy of coarser grains in the mobile surface layer, or a combination of
both.  


The measured grain size transport rates are consistent with the transport rates predicted by the best fit of the grain size distribution model. The grain size distribution
model can also somewhat accurately predict grain size distributions at the footslope and crest of the moraines over time.



Research Updates (2009-2016): Climate Driven Hillslope Degradation and Time-dependent Topographic Diffusivity

Risa's dissertation (2015):   Climate driven hillslope degradation of Mono Basin moraine, Sierra Nevada, CA, USA, used a space-for-time substitution of present-day topographic diffusivities from climates similar to those over the past 85 ka in the Mono Basin region of Sierra Nevada, CA,  as assessed from paleo-climate records, to test the effects of climate fluctuations on hillslope erosion rates on a glacial/interglacial time scale. Topographic diffusivities that varied in accord with the glacial chronology of the region were used to model  hillslope diffusion of the Mono Basin moraine with the basic hillslope diffusion equation. Using this approach, three scenarios of  the diffusivity parameter were applied to compare the resulting hillslope degradation models: a long term average, a time-dependent parameter, and a diffusivity based on current measures on the moraine. On a glacial/interglacial time scale, the current diffusivity was found to be significantly lower than either the long term average or those of that occurred over the past 85 ka. It was also found that in glacially active environments, diffusivities can vary significantly, depending on the region. Results are published in Geomorphology:

Madoff, R.D. and Putkonen, J., 2016, Climate and hillslope degradation vary in concert; 85 ka to present, eastern Sierra Nevada, CA,  USA, Geomorphology, v. 266, p.33-40.   doi:10.1016/j.geomorph.2016.05.010 

For updates on Risa's research, follow her website here.

Summary of Earlier Group Research: Landscape Evolution, Glacial Geology, and Surface Processes

Glaciers on land leave behind characteristic landforms and bedrock topography. The spatial extent of this signature and dating of the erosional and depositional features has led to good understanding of temporal and spatial shifts in the past glaciers and climate. At best this record contains considerable information on past glacial processes in the forms of moraines, and other depositional landforms. However, evidence suggests that all unconsolidated landforms degrade, manifested in the decreasing slope angles through time. To determine the amount and pattern of degradation on glacial moraines a well documented moraine degradation model was used (Putkonen et al., 2008). The model produced testable predictions of: 1) moraine cross profile, 2) spatial pattern of surface boulders, and 3) relative frequency of surface boulders. Comparable field data was collected by manually counting boulders, running GPS profiles, hand leveling, and low altitude airphoto analysis of alpine, tropical and Antarctic moraines. The correspondence between model and field data was remarkably good and strongly supported our current understanding of the evolution of glacial landscape. These results highlighted the evolving nature of the glacial landforms, complete removal of original moraine surfaces, and postglacial emergence of spatial pattern in boulder frequency.


(Left) Eastern Sierra Nevada Mountains, Bloody Canyon, and  Walker Lake area just south of Mono Lake, California.          

(Right) A lateral moraine near the town of Bishop, CA, eastern Sierra Nevada.

 

 

 


 

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