NEAR-STREAM DEPOSITIONAL SEDIMENT TRAITS, INTERPRETATION, AND HYDROLOGICAL IMPLICATIONS IN HEADWATERS OF THE SOUTHERN BLUE RIDGE MOUNTAINS

The headwater valleys of the Southern Blue Ridge Mountains contain saprolite, bedrock, and a mix of colluvial and alluvial deposits. Differences in origin and characteristics of the surficial deposits cause these areas to vary in hydrological traits. This study aims to characterize valley bottom sediments, interpret their origin, and determine their hydrological traits in different geomorphic settings near streams located within two small-scale watersheds in the Southern Blue Ridge Mountains.

The study watersheds are Boyd Branch (284 ha) and Gribble Gap (44 ha); both are underlain mostly by gneiss and have ~200-300 m of relief. Anthropogenic disturbances over the past two centuries in the form of timber harvest, clearing land for pastures, and small-scale mining operations have occurred in both areas. Small alluvial-colluvial fans are common at tributary junctions. Sediment samples were taken near well sites and analyzed for grain size distribution and source indicators. Shallow groundwater wells in depositional zones provide groundwater table levels, and hydraulic conductivity was measured in the field using a permeameter and well tests.

Sediment grain size, mineralogy, and carbon content provides insight to material sources. Preliminary analysis of near-stream sediments show a wide range of sediment types, including organic rich silt-clay loam, sand, matrix-supported bouldery loam, and paleosols. Previous research at Gribble Gap has shown truncated top soils on valley slopes with deposition at the toe slope and downstream fans. A lack of soil development in most sediment sections points to recent depositional events. However, some bouldery loams show texture traits and weathering typical of Pleistocene debris flow deposits, which have been described in the region. Preliminary field observations indicate different hydrological properties in deposits along streams in the catchments, with evidence of perched streams, losing streams, and wetlands. Perched streams indicate low permeability layers beneath streams, which decreases the exchange of groundwater and stream water. The history and geomorphic setting of the study catchments is not unique; future work will examine how pervasive the sediment-hydrologic traits described are throughout the region.