Date of Award

Summer 7-9-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Gregory E. Tucker

Second Advisor

Robert S. Anderson

Third Advisor

Eric Tilton

Abstract

The power of water shapes landscapes, redistributes sediment on the earths surface, and creates natural hazards that have an enormous impact on the natural and built environment. In quasi-equilibrium landscapes, slope angles balance water erosion and sedimentation processes. However, after major disturbances such as wildfire or drought rapid landscape evolution can result, causing massive landscape reorganization. How do these disturbances propagate through a landscape? What is the imbalance in sediment mass conservation that creates catastrophic erosion or deposition? These questions are dependent on how landscapes respond to transient perturbations, and they are important for understanding both long-term landscape evolution and short-term hazards. In this research I focus on two transient deviations that interrupt landscape steady-state equilibrium: gully and post-wildfire erosion.

To understand gully erosion I relied on a natural laboratory in a semi-arid grassland. I instrumented a watershed with rain gages, soil-moisture sensors, a flume to measure water discharge, and time-lapse cameras. This instrumentation allowed me to track the hydrologic drivers of erosion. I measured the short-term erosion response in gullies with repeat terrestrial LiDAR surveys on a sub- annual basis. Combining LiDAR data with field hydrology data, I found that most erosion occurs during times of high soil moisture, which are present during summer thunderstorms and winter snowmelt. To put this erosion into a long-term context, I used optically stimulated luminescence dating to identify past episodes of gully erosion and deposition.

Additionally, I studied the landscape response following a wildfire by using terrestrial LiDAR to map evolving erosion patterns over a period of two years following a wildfire. To date, it has been difficult to assess which morphologic units on a landscape are most susceptible to erosion after wildfire, but this approach made it possible to document where erosion occurred in a catchment after a wildfire. I found that although deep erosion within channels is visible to any observer, the majority of eroded sediment comes from shallow hillslope erosion. Additionally, I showed that the first summer thunderstorms after a wildfire created the most geomorphic change, because of an abundance of available sediment. Over time, subsequent storms of a similar magnitude generated less erosion because available sediment on the landscape decreased. Therefore, this work identified patterns that would be difficult to assess without high-precision equipment.

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