FINE GRAIN SEDIMENT TRANSPORT ON THE MONO BASIN MORAINE, SIERRA NEVADA, CALIFORNIA

Hillslopes are ubiquitous features in landscapes, yet little detail is known about the erosional processes and transport rates of fine sediments that establish the hillslope soil mantle. The fine-grained sediments (<2mm) are the primary constituent grain size by weight of many hillslopes, excluding boulders.

Better understanding of the mechanics and transport rates of the fine grain sediments mantling the hillslope will allow for improved understanding of landscape evolution, slope stability, forecasting of soil erosion and illuminate the physics governing the transportation of regolith.

To better understand the regolith transport on bare soil hillslopes we conducted a 5-month long experiment on the Mono Basin moraine in east-central California in which 0.5-1.0mm and 2.0mm glass particles were used as surface tracers to monitor downslope movement of granular materials under natural conditions. Preliminary results indicate that 2.0mm grains have a faster transport rate than 0.5-1.0mm grains. Additionally, some grains of both experimental sizes travelled upslope at some of the field sites.

These results are incompatible with the simplest expectation that particle transport rate is a function of size, with smaller particles moving progressively faster. Everything else being the same, larger particles experience transport inducing threshold events less frequently than smaller particles; therefore smaller particles should travel a greater distance over a given period of time. Also, movement of grains in the upslope direction past the limit of rain-splash indicates some other actor besides water and gravity is at play.

It is suspected that bioturbation is the cause of the upslope grain movement indicating that further investigation into animal induced sediment transport is needed. Additionally, transport rates with respect to grain geometry will be investigated further in this project, as the surface characteristics of the experimental grains were not identical between the two size groups.