Date of Final Oral Examination (Defense)

12-2011

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Major Advisor

Jennifer L. Pierce

Abstract

Deciphering the role and importance of climate vs. tectonism in controlling rates and processes of erosion in mountainous regions remains an area of active research in geomorphology. Through dating of alluvial fan aggradation in the Lost River Range of Idaho, USA, we investigate the influence of Quaternary climate on hillslope sediment supply and transport capacity of mountain streams in this region. Fans along the normally-faulted western side of the Lost River Range extend ~2-6 km from the range front. Sediment is predominantly limestone-derived pebble to cobble gravels with a sandy matrix and is often sorted into coarse-fine couplets that suggest sheetflooding has been the dominant process transporting and depositing sediment. We selected five alluvial fans for correlation and mapping of similar-age surfaces using geomorphic positions of surfaces, optically stimulated luminescence (OSL) dating, soil development, and thickness of pedogenic CaCO3 coats on clasts within soil profiles. Late Pleistocene glacial extent within alluvial fan contributing basins varied from 0-80% of catchment area, allowing investigation of the influence of glaciation versus regional climate on fan aggradation. OSL ages range from 4-115 ka and suggest synchronous fan aggradation regardless of the extent of past glaciation in catchments. Geomorphic maps of the fans suggests that approximately 10% of total fan area was constructed during the Holocene, while ~80% was constructed during the late Pleistocene (undated/older fan surfaces account for the remaining ~10% of fan area). The more significant fan growth of the late Pleistocene may be divided into distinct periods at 10-20 ka, 20-35 ka, and 35-60 ka. Differences in the extent, facies, grain-size distributions, and average surface slopes between Holocene and late Pleistocene deposits suggest greater stream transport capacity and sediment delivery to fans during the late Pleistocene. These results suggest that the warmer and drier conditions of the Holocene (OIS 1) have promoted limited deposition and largely inactive fans. In contrast, the cold climate conditions throughout OIS 2-4 promoted aggradation and active fans in this region. While glaciers may have played a role in fan growth, deposition on fans with and without extensive glaciation in contributing basins suggests that glaciation was not required and that regional climate conditions drive other processes that result in fan aggradation. From these results, we infer that cold climate enhanced both stream transport capacity and sediment delivery to fans, resulting in aggradation. Greater stream transport capacity may have resulted from increased effective moisture as well as greater peak flows and total discharge during spring snowmelt. Greater sediment delivery to fans may have resulted from the direct effects of glaciers where present, but also through reduced vegetation density in contributing basins and more rapid weathering rates driven by greater effective moisture and temperature regimes that promoted more effective frost weathering processes. These findings suggest that in the Lost River Range and perhaps greater region, conditions during intervals of cold climate have most effectively driven denudation and subsequent alluvial fan growth.

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