Publication Date

8-2016

Date of Final Oral Examination (Defense)

5-5-2016

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Geosciences

Department

Geosciences

Supervisory Committee Chair

Jennifer Pierce, Ph.D.

Supervisory Committee Member

Shawn Benner, Ph.D.

Supervisory Committee Member

James P. McNamara, Ph.D.

Supervisory Committee Member

Alejandro N. Flores, Ph.D.

Supervisory Committee Member

Erkan Istanbulluoglu, Ph.D.

Abstract

Aspect has long been recognized as a significant source of landscape variability, which is induced by the orientation of land surfaces relative to solar incidence. Insolation differences on opposing aspects (e.g., north and south-facing slopes) act as localized climatic perturbations, altering surface energy balances and temperatures. Over shorter timescales, aspect-induced changes to the energy balance alter snow pack dynamics, soil water input rates and seasonality, and plant available water and water stress. Over longer timescales, aspect-induced insolation variability affects bedrock weathering rates and depths, soil and regolith development, vegetation type and density, erosion rates and processes, and ultimately hillslope and drainage forms. In turn, differences in landscape evolution on the opposite sides of valleys lead to valley asymmetry development.

The primary goals of this work are to 1) summarize available aspect-related hydrologic, ecologic and pedologic data for the Dry Creek Experimental Watershed (DCEW), fill knowledge gaps by investigating aspect-related differences in geomorphic characteristics and processes, and develop an integrative conceptual framework for how landscapes respond to aspect-induced insolation variability and how valley asymmetry develops. In particular, we assess 1) how aspect drives differences in fire and erosion rates, 2) how critical zone characteristics vary with aspect and how aspect-sensitivity changes with elevation, and 3) how aspect-related differences in critical zone response alter runoff production, drainage incision, and catchment competition, which drive valley asymmetry development.

Surprisingly, erosion rates for north and south-facing catchments do not appear to have varied significantly during the Holocene, despite dramatic differences in landscape characteristics (e.g., vegetation cover, soils, hydrologic processes, and landforms). This suggests that the valley asymmetry has not been actively developing, and is a relict feature of either initial landscape response to aspect, or specific climate intervals (e.g., glacial periods). Erosion rates for the margin of the Idaho batholith are lower than those in the interior batholith, reflecting lower rates of incision.

Elevation appears to modify the sensitivity of landscapes to aspect-induced climate perturbation. Critical zone properties appear to be most sensitive to aspect-induced climate perturbation at lower elevations (~1,100 m), and aspect-sensitivity diminishes towards higher elevations (~2,100 m). Changes in precipitation and temperature with increasing elevation appear to alleviate moisture stress, causing aspect-induced insolation and temperature variability to have less of an effect. Reduced landscape sensitivity to aspect at higher elevations explains why previously mapped slope asymmetry diminishes towards these elevations throughout the region.

Drainage incision and expansion are more pronounced on south-facing valley sides. South-facing catchments have shallower, coarser soils that yield more runoff per unit drainage area. Changes to the water balance at pedon-scales appear to influence how fluvial process scale with drainage area, which impacts catchment-scale erosive efficiency. Enhanced drainage incision in south-facing catchments, in conjunction with more effective diffusive erosion, appears to have promoted divide migration and land surface elongation.

Importantly, land surface degradation and elongation reduce geomorphic gradients, which serve as negative feedbacks by reducing denudation differences between north and south-facing valley sides, and effectively drive valley asymmetry development towards dynamic equilibrium. This suggests that although valley asymmetry clearly reflects differences in past erosion, it may actually develop as a landscape response to counteract aspect-induced differences in erosion. Where valley asymmetry is most pronounced, we suspect there may be little difference in rates of erosion. Valley asymmetry may be a remotely measurable characteristic of landscapes that reflect their proximity to stable states.

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