R. Craig Kochel

Geomorphological controls on coastal vegetation at the Virginia Coast Reserve

Abstract The establishment and succession of vegetation on migrating, low-profile barrier islands is greatly affected by the physical hydrogeomorphological processes that regulate island topography, saline and fresh groundwater table surfaces. Apart from the physical destruction of plants by overwash processes, fluctuations in water table elevations and variations in groundwater salinity, both spatially and temporally, also appear to have significant impact on the nature and distribution of vegetation on these islands. Species composition, community structure and biodiversity on the Virginia barrier islands are controlled by the same processes that give rise to landforms and maintain their form. These processes include marine water inundations, groundwater salinity variations and changes in depth to the fresh-water table. Land surface elevation, landform morphology and position on the barrier island determine exposure to high tides, storm surges, sand burial, and the extent of the fresh-water reserves. In this article, the underpinnings of a Long-term Ecological Research Program in which 25 geologists, geomorphologists, climatologists, and ecologists have a common research plan is presented and several examples of the product of this research partnership dealing with geomorphological and hydrologic controls on vegetation dynamics are detailed. Among the aspects of ecological dynamics examined in terms of geomorphological processes are vegetation zonation, succession, disturbance, and ecosystem state change.

Role of debris flows in long-term landscape denudation in the central Appalachians of Virginia

Four major storms that triggered debris flows in the Virginia‐West Virginia Appalachians provide new insights into the role of high-magnitude, low-frequency floods in longterm denudation and landscape evolution in mountainous terrain. Storm denudation in the Blue Ridge Mountain drainage basins is approximately an order of magnitude greater compared to basins located in the mountains of the Valley and Ridge province. This difference is probably the result of higher storm rainfall from the Blue Ridge storms. Radiocarbon dating of debris-flow deposits in the Blue Ridge indicates a debris-flow return interval of not more than 2‐4 k.y. in mountainous river basins. This finding, combined with measurements of basin denudation, suggests that approximately half of the long-term denudation from mechanical load occurs episodically by debris-flow processes. Although floods of moderate magnitude are largely responsible for mobilizing sediment in lowgradient streams, our data suggest that high-magnitude, low-frequency events are the most significant component in delivering coarse-grained regolith from mountainous hollows and channels to the lowland floodplains.

Fluvial responses to land-use changes and climatic variations within the Drury Creek watershed, southern Illinois

Abstract Fluvial responses to climatic variation and Anglo-American settlement were documented for the Drury Creek watershed, southern Illinois by examining stratigraphic, geomorphic, climatic, and historical data. Regional analyses of long-term precipitation records document a period of decreasing mean annual precipitation from 1904 to about 1945, and an increasing trend in annual precipitation from 1952 to the present. The period between 1945 and 1951 experienced a large number of intense storms that resulted in high annual precipitation totals. Statistical relationships illustrate that changes in precipitation totals are transferred to the hydrologic system as fluctuations in stream discharge. Historical records of southern Illinois show that a maximum period of settlement and deforestation occurred between the 1860s and 1920s. This era ended in the 1940s when large tracts of land were revegetated in an attempt to curtail erosion which had caused extensive upland degradation. In response to hillslope erosion at least two meters of fine-grained sediments were deposited on valley floors. Average sedimentation rates, determined using decdrochronologic techniques, are estimated to be 2.11 cm/yr for the period between 1890 and 1988; rates that are 1 to 2 orders of magnitude greater than pre-settlement values calculated for other areas of the midwest. However, botanical data suggest that aggradation was episodic, possibly occurring during three periods characterized by greater annual precipitation. Since the 1940s, sedimentation rates have declined. Reduced rates of sedimentation are related to an episode of channel entrenchment that reduced overbank flooding. Entrenchment coincided with a period of: (1) reduced sediment yields associated with watershed revegetation and the introduction of soil conservation practices, and (2) intense storm activity that resulted in long periods of high discharge. As a result of channel incision and hillslope erosion, newly exposed bedrock in upstream areas currently provides a source of gravel load to the channels. The distribution of coarse bedload material along tributary streams combined with downstream decreases in width:depth ratios and tractive force estimates suggest that channels in the Drury Creek watershed are slowly adjusting their configuration to transport coarse bedload material. The fluvial response to the increased influx of coarse sediment began more than 45 years ago and continues today.

Quaternary deposits and landscape evolution of the central Blue Ridge of Virginia

Abstract A catastrophic storm that struck the central Virginia Blue Ridge Mountains in June 1995 delivered over 775 mm (30.5 in) of rain in 16 h. The deluge triggered more than 1000 slope failures; and stream channels and debris fans were deeply incised, exposing the stratigraphy of earlier mass movement and fluvial deposits. The synthesis of data obtained from detailed pollen studies and 39 radiometrically dated surficial deposits in the Rapidan basin gives new insights into Quaternary climatic change and landscape evolution of the central Blue Ridge Mountains. The oldest depositional landforms in the study area are fluvial terraces. Their deposits have weathering characteristics similar to both early Pleistocene and late Tertiary terrace surfaces located near the Fall Zone of Virginia. Terraces of similar ages are also present in nearby basins and suggest regional incision of streams in the area since early Pleistocene–late Tertiary time. The oldest debris-flow deposits in the study area are much older than Wisconsinan glaciation as indicated by 2.5YR colors, thick argillic horizons, and fully disintegrated granitic cobbles. Radiocarbon dating indicates that debris flow activity since 25,000 YBP has recurred, on average, at least every 2500 years. The presence of stratified slope deposits, emplaced from 27,410 through 15,800 YBP, indicates hillslope stripping and reduced vegetation cover on upland slopes during the Wisconsinan glacial maximum. Regolith generated from mechanical weathering during the Pleistocene collected in low-order stream channels and was episodically delivered to the valley floor by debris flows. Debris fans prograded onto flood plains during the late Pleistocene but have been incised by Holocene stream entrenchment. The fan incision allows Holocene debris flows to largely bypass many of the higher elevation debris fan surfaces and deposit onto the topographically lower surfaces. These episodic, high-magnitude storm events are responsible for transporting approximately half of the sediment from high gradient, low-order drainage basins to debris fans and flood plains.

Role of tree dams in the construction of pseudo-terraces and variable geomorphic response to floods in Little River valley, Virginia

Geomorphic response to a 1985 flood in Little River valley, northern Virginia, was different in magnitude and style from the largest historic flood in the same valley in 1949. The primary geomorphic activity during the 1985 flood was severe bank erosion and channel-gravel deposition rather than the debris flow and avalanching of the 1949 event. An unusual and widespread phenomenon of the recent flood was that large trees eroded and transported by the floodwater were aligned parallel to the river banks and, at isolated sites, were braced and stacked against trees still standing on the floodplain. Lateral barriers or dams created from these displaced trees allowed the channel to be locally aggraded above the level of the flood-plain. In these reaches, little, if any, river gravel was deposited on the floodplain, even though the adjacent channel floor was raised well above that surface. The river has now shifted around the filled segments, leaving flat, isolated surfaces, underlain by channel gravel, standing above the level of the modern floodplain. These features may be mistaken for terraces alter they become vegetated and the trees bracing the gravels decay. Interpreting these surfaces to be terrace remnants would lead to a faulty reconstruction of geomorphic history in the Little River valley and other valleys where floodplain morphology is controlled by infrequent flood events.

The disruption of Grassy Creek: implications concerning catastrophic events and thresholds

Two catastrophic events, occurring simultaneously in the valley of Little Grassy Creek, IL, allow for an examination of the threshold concept in geomorphology. Movement of debris associated with failure and sliding of valley-side material, caused damming and avulsion of Little Grassy Creek. Slope and river disruptions, both severe in character, were linked because the effect of one event (slope failure) was the cause of the second event (fluvial avulsion). The slope failure represents a true threshold-crossing event because the results are irreversible on a graded-time scale. In contrast, the fluvial disruption was not a threshold crossing, although the event was catastrophic and short-term instability occurred. In the fluvial case, a new channel developed, and the re-establishment of equilibrium, as estimated by channel characteristics, occurred within 10 years. The river system functions as it did before the slope failure/avulsion, though the channel reach is now in a different location. Criteria needed to employ thresholds to explain geomorphic events are suggested, and a definition of thresholds as time-dependent phenomena is presented as a means of reducing confusion over the use of the threshold concept.

Unexpected Geomorphic Effects of the Hurricane Agnes Storm and Flood, Conestoga Drainage Basin, Southeastern Pennsylvania

The Hurricane Agnes flood, the greatest ever recorded in the drainage basin of the Conestoga River, raised two significant questions: why was such massive flooding caused by such a relatively small total rainfall? and why did this catastrophic event cause only minor geomorphic changes in channels and flood plains? Total precipitation averaged only 8.5 inches in 54 hours. However, other factors-including high rainfall intensity in one 10-hour period, even distribution of rainfall throughout the basin, pre-Agnes saturation of the ground, the storms arrival in June before crops were well established, and Mans shortsightedness-combined to produce exceptionally high runoff. The insignificant geomorphic effects of the storm also resulted from a combination of factors. The low stream velocity of flood waters associated with low gradients, a 15-20 times increase in stream widths and water depths rising 7-20 feet above floodplain surfaces minimized the waters erosive power. Also, the silt and clay loads of the flooding streams throughout much of the basin failed to provide effective abrasive tools for channel widening. Deposition on channel bottoms and floodplains was also minor because by the time the flood was ebbing, the suspended silt-clay load had diminished markedly and there was little sediment to deposit except at scattered localities possessing special characteristics. The effect of this flood with a several 100-year recurrence interval was to transport large quantities of fine sediment out of the basin but not to alter significantly the shape of channels or the contour of floodplains.

The Role of Overwash on a Mid-Atlantic Coast Barrier Island

The importance of storms and their overwash in barrier island transgression is assessed quantitatively by determining net sediment budgets of four representative Assateague Island washover sites for the 1982-83 season. The results show that most of the annual accretion occurred during one large storm which had a return interval of 10 to 15 years. The combined effects of eight subsequent storms with return intervals of less than 2.5 years accounted for less sedimentation. Change due to eolian redistribution was insignificant.

Beach Cusp Destruction, Formation, and Evolution during and Subsequent to an Extratropical Storm, Duck, North Carolina

Many studies have debated whether beach cusps are erosional or depositional features. The April 12-14, 1988, extratropical storm provided an opportunity to view the direct effects of one of the largest storms of the past decade upon beach sedimentology and morphology on barrier islands near Duck, North Carolina. Prior to the storm, the beach at Duck was characterized by a well-defined pattern of beach cusps with horn-to-horn spacings averaging 35 m. Storm-induced alterations were dominated by an initial period of beach erosion that remobilized the upper 30 to 50 cm of beach sediment, followed by aggradation. Net aggradation was most prominent along the middle beachface and within the pre-storm cusp bays. These morphologic adjustments resulted in the destruction of cusps, which were replaced with a post-storm planar beachface composed of horizontally bedded fine- to coarse-grained sediments. Within 24 hrs of storm subsidence, new beach cusps formed sequentially along the coast in the direction of longshore transport. Initial cusp formation resulted from beach erosion and the creation of bays in the planar storm-beach surface at positions of preferential post-storm runup. The initial cusp horns were composed of truncated horizontal beds of the planar beach accreted during the storm. After their formation, the cusps sequentially migrated downdrift. Migrating horns were composed of a coarse-grained sediment wedge that thickened toward horn crests, suggesting formation by deposition. It is concluded from these observations that beach cusps are both erosional and depositional in nature.

Geomorphic response to minor cyclic climate changes, San Diego County, California

Abstract Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region. Using aerial photographs and field investigations we found little response to the 1978–1983 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.