Geology topics

Alan D. Howard

Publications and source records attributed to Alan D. Howard.

3 recordsLinked to original sources

USGS2011

Quantifying home range habitat requirements for bobcats (Lynx rufus) in Vermont, USA

We demonstrate how home range and habitat use analysis can inform landscape-scale conservation planning for the bobcat, Lynx rufus , in Vermont USA. From 2005 to 2008, we outfitted fourteen bobcats with GPS collars that collected spatially explicit locations from individuals every 4 h for 3–4 months. Kernel home range techniques were used to estimate home range size and boundaries, and to quantify the utilization distribution (UD), which is a spatially explicit, topographic mapping of how different areas within the home range are used. We then used GIS methods to quantify both biotic (e.g. habitat types, stream density) and abiotic (e.g. slope) resources within each bobcat’s home range. Across bobcats, upper 20th UD percentiles (core areas) had 18% less agriculture, 42% less development, 26% more bobcat habitat (shrub, deciduous, coniferous forest, and wetland cover types), and 33% lower road density than lower UD percentiles (UD valleys). For each bobcat, we used Akaike’s Information Criterion (AIC) to evaluate and compare 24 alternative Resource Utilization Functions (hypotheses) that could explain the topology of the individual’s UD. A model-averaged population-level Resource Utilization Function suggested positive responses to shrub, deciduous, coniferous forest, and wetland cover types within 1 km of a location, and negative responses to roads and mixed forest cover types within 1 km of a location. Applying this model-averaged function to each pixel in the study area revealed habitat suitability for bobcats across the entire study area, with suitability scores ranging between −1.69 and 1.44, where higher values were assumed to represent higher quality habitat. The southern Champlain Valley, which contained ample wetland and shrub habitat, was a concentrated area of highly suitable habitat, while areas at higher elevation areas were less suitable. Female bobcat home ranges, on average, had an average habitat suitability score of near 0, indicating that home ranges consisted of both beneficial and detrimental habitat types. We discuss the application of habitat suitability mapping and home range requirements for bobcat conservation and landscape scale management.

Vermont↗

USGS2003

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

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.

Virginia↗

USGS1993

Landslides triggered by the storm of November 3-5, 1985, Wills Mountain Anticline, West Virginia and Virginia

More than 3,000 landslides were triggered by heavy rainfall in the central Appalachian Mountains of West Virginia and Virginia, November 3-5, 1985. These landslides provided the opportunity to study spatial controls on landslides, magnitude and frequency of triggering events, and the effects of landslides on flood-induced geomorphic change. The study area consists of parts of the Wills Mountain anticline, a major NE-trending structure in the central Appalachians, and a portion of the adjacent Appalachian Plateau. Across the anticline and adjacent plateau, bedrock lithologies vary markedly and include pure marine limestone, marine shale, deltaic mudstone/sandstone sequences, and orthoquartzites. Because of the geologic structure, bedrock lithology varies little along strike. The spatial distribution of landslides triggered by the storm was controlled primarily by rainfall, bedrock lithology, surficial lithology, land cover, and slope morphology. The triggering rainfall was of moderate intensity and long duration. Two-day storm totals varied from 170 mm to more than 240 mm in the study area. Most landslides occurred at the northeast end of the study area, where 48-h rainfall totals were in excess of 200 mm. Different rainfall thresholds are apparent for triggering landslides on different bedrock lithologies. The highest density of landslides occurred in shallow colluvium and residuum of the Reedsville Shale (Ordovician), followed by regolith of the Greenbriar and Mauch Chunk Groups (Mississippian). Most of the landslides in these fine-grained regoliths were shallow slides and slumps, many of which transformed to mudflows and delivered sediment directly to streams; a smaller number of debris avalanches were triggered high on quartzite ridges. Instability of colluvium and residuum derived from the Reedsville Shale, compared with regolith from four other fine-grained bedrock lithologies, is attributable to its low strength combined with moderate infiltration rates that allowed soil moisture to accumulate under the moderate intensities of the rainfall. Slopes covered by coarse, cobbly debris flow and alluvial deposits, mostly of Pleistocene age, were very stable due to their low slope angles and high frictional strength. For a particular bedrock lithology, the spatial distribution of landslides appears controlled by interdependent influences of slope morphology and land cover. On the Reedsville Shale, most landslides occurred on north- to northeast-facing slopes, which might have had higher antecedent levels of soil moisture; these slopes have also been preferentially cleared because they produce better pasture forage for livestock. A secondary concentration of landslides on south- to southwest-facing slopes cannot be explained by conventional soil-moisture models. Landslide density was 100--200 percent higher on cleared land than on forested land. On pastured land, most landslides occurred on laterally planar slopes, but on forested land, most landslides occurred in slope positions that were laterally concave (hillslope hollows). Compared with other documented Appalachian storms that have triggered landslides, the November 1985 storm had lower rainfall intensities over longer durations. Comparison with these other storms suggests that the anomalously high degree of slope instability in 1985 is due to the long duration of low-intensity rainfall on fine-grained regolith derived from shale; the triggering rainfall can be approximated by the 48-h storm total. Landslide density in Reedsville Shale regolith is linearly related to the varying 48-h rainfall along the anticline. These data define a probabilistic model that estimates return intervals of 43 to 300 yr for landslide densities ranging from 1 to 70 landslides/km 2 . Analysis of flood-induced geomorphic changes in 79 small drainage basins that received 210-240 mm of rainfall showed a clear local association between landslides and channel erosion or deposition adjacent to where the landslides delivered sediment to the stream. When channel change was quantified using an index evaluated at each basin mouth, most of the channel change was attributable to the influence of basin morphology on flood discharge. Landslide density in the basins was of secondary, although measurable, importance in explaining flood-induced channel changes at the basin scale.

Virginia↗