Date of Graduation

1999

Document Type

Dissertation/Thesis

Abstract

Bedrock lithology affects watershed morphometry, regolith composition, and caliber of sediment transported by drainage basins. This study involves a comparative geomorphic analysis of three watersheds similarly underlain by the Acadian clastic wedge: (1) Fernow Experimental Forest (19 km 2), Tucker County, West Virginia; (2) North Fork basin (49 km 2), Pocahontas County, West Virginia; and (3) Little River basin (41 km2), Augusta County, Virginia. Surficial mapping (1:9,600), GIS, morphometry, lithofacies analysis, and channel-gravel studies are utilized to make inferences regarding controls on sediment-transport efficiency. Mapping of small-scale debris fans and morphometric analysis elucidate local controls on fan accommodation at tributary junctions. Optimum conditions for fan preservation include: (1) high drainage density, (2) high tributary-junction frequency, (3) steep low-order channels, (4) high valley-width expansion rates, and (5) steep, debris-flow-prone hillslopes. Sandstone lithofacies play an important role in shaping watershed morphometry. High sandstone content produces comparatively steeper hillslopes at Little River. Bouldery colluvium accumulates in hollows until threshold thicknesses are attained, and a triggering meteorological event releases it via debris flow. Conversely, lithofacies relations are such that gentler hillslopes are maintained at Fernow and North Fork. Regolith in low-order tributaries is incrementally transported by normal stream flow. The model implies that spatial distribution of debris flow depends on sandstone lithofacies, hillslope morphology, and occurrence of high-intensity precipitation events. Valley-bottom storage volumes are examined in tandem with channel morphology and gravel analyses to make inferences regarding sediment-transport efficiency. Debris-flow-prone hillslopes at Little River deliver high volumes of sediment at magnitudes greater than channel transport capacity. Aggraded alluvial fill insulates valley-floor bedrock from vertical erosion, while lateral incision dominates. Valley-width expansion creates a positive response via increased storage capacity and lower unit stream power. Conversely, Fernow and North Fork hillslopes are characterized by diffusive mass wasting with incremental bedload transport. Hillslope delivery rates are in balance with channel export. Low valley-width expansion promotes high unit stream power and vertical erosion. The model implies that Fernow and North Fork have been more effective at sediment transport during the Late Quaternary. Given similar climatic and tectonic settings, bedrock lithofacies variation is the modulating factor.

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