Debris flows and landscape evolution in the upper Rapidan basin, Blue Ridge Mountains, central Virginia

This dissertation examines the role of catastrophic storms and debris flow processes in long-term landscape evolution. Of the major storms that triggered debris flows in the Virginia-West Virginia Appalachians this century, four have been studied extensively. These studies have provided new insights into the role of high magnitude, low frequency storms in long term denudation and landscape evolution of the Appalachians. The most recent of these storms occurred in the upper Rapidan River basin, located in the central Blue Ridge province of Virginia, where up to 775 mm (30.5 in) of rain fell in 16 hours on June 27, 1995. The deluge triggered more than 1000 slope failures and left stream channels and debris fans deeply incised, exposing the stratigraphy of earlier fluvial and mass movement events. Radiocarbon dating of organic-rich units indicates that debris flow activity since 25,000 yr. B.P. has on average recurred once every 2,500 years. Measurements of basin denudation combined with debris flow frequency data suggest that approximately half of the long term denudation from mechanical load occurs episodically by debris flow processes.; These observations suggest that episodic storm events are a major factor in delivering coarse-grained regolith from mountainous hollows and channels to the lowland flood plains. Regolith, much of which is presumably generated from the higher rates of mechanical weathering during the late Pleistocene, has collected in low-order stream channels by mass wasting processes. This material is episodically delivered to the valley floor in the form of debris flows. The sediment is conveyed through the stream channels and comes to rest onto debris fans, which prograde over the flood plains of the larger fluvial systems. The end of the Pleistocene presumably reduced the sediment supply to the fluvial system, and initiated stream incision through the Pleistocene-age debris fans. Field evidence indicates the entrenched system of stream channels allowed the Holocene debris flows to bypass many of the higher elevation debris fans and deposit onto the lower Holocene-age fans. Pedogenesis of pre-Holocene debris fans show a long history of progradation and incision that may date to the early Pleistocene or even to late Tertiary.