Dan Royall

A Fifty-Year Record of Historical Sedimentation at Deer Lake, North Carolina

Abstract Although many once-deforested areas of the eastern United States are now revegetated, impacts of this disturbance on watershed processes may persist. In this study, lake sediment stratigraphy and magnetism were used to assess the recovery of a small watershed in the southern Blue Ridge Mountains following abrupt reduction of human impacts. Average sediment yields were found to be higher than those of less disturbed basins nearby, and lower than those reported from the early twentieth-century Piedmont Province. Temporal trends in sediment yield appear to reflect both meteorological and land-use histories. Although most of the lake sediment is magnetically similar to bottomland sources, two instances of local upland sediment input, possibly related to human activities, are evident in the record. Interpreting relationships between sediment yield and changing environmental influences is impeded by poor temporal control in the methodology as well as by the intrinsic dynamics of the fluvial system. *I thank Henry McNabb and the U.S. Forest Service at Bent Creek Experimental Forest for resources, useful discussion, historical information, and other means of facilitating this research. I am also indebted to I. L. Larsen, Oak Ridge National Laboratory, Environmental Sciences Division, for crucial assistance with Cs-137 counting.

In-Channel Benches in Small Watersheds: Examples from the Southern Piedmont

A knowledge of the process dynamics and forms of river sediment storage is important for understanding the impacts of climate and land use change on sediment yield, longitudinal and cross-sectional stream channel morphology, and aquatic and riparian habitat. Among the important forms of sediment storage are in-channel benches: level, step-like fluvial deposits occurring at different heights above the channel bed but below the main floodplain surface. In this study, the contemporary occurrence of in-channel benches in Piedmont streams was examined, and hydrologic characteristics of study sites investigated as potential drivers of bench formation. Nine stream channel sites, originally studied in 1964, were resurveyed to determine changes in the number of in-channel bench levels and current bench morphological characteristics. One to three bench levels not documented in 1964 were observed at several sites that have drainage areas less than 600 km2. Low and mid-level benches were generally found to be highly discontinuous, dominated almost exclusively by herbaceous vegetation, and frequently well-stratified. Although specific instances of bench occurrence may be locally controlled, correspondence between elevations of in-channel benches and drought period flow stages with recurrence intervals similar to those of the higher and larger active floodplain under average hydroclimatic conditions suggest a link between bench morphogenesis and drought. The recent growth of benches over the last decade of Piedmont drought is demonstrable at one site, using dateable cultural debris. However, the unequivocal assignment of specific ages to entire bench deposits remains problematic.

A LiDAR-Based Analysis of Stream Channel Cross Section Change Across an Urban–Rural Land-Use Boundary

Little is known about the downstream propagation of stream channel alterations initiated by urban flow regimes. In this study, freely available Light Detection and Ranging (LiDAR) data were combined with ground survey data to derive information on changes in stream cross sections in a North Carolina watershed, moving downstream across an urban-to-rural land-use boundary. Regression analyses relating LiDAR model and ground survey data exhibit poor relationships for all channel morphometric measurements except reach-average channel capacity. Application of the channel capacity regression reveals a negative power function pattern of downstream decline in channel enlargement. The largest declines occur before streams enter the rural landscape, with more gradual declines afterward. The largest declines in channel size might be attributable to reservoir effects along reaches with wide floodplains. Model results indicate no discernible enlargement once rural land covers and percentage impervious areas of about 60 percent and 16 percent of watershed area, respectively, are reached.

Spatial Patterns of Surface Soil Magnetism and Soil Redistribution Across a Fallow Field, Northern Alabama

Distributed assessments of total soil loss from agricultural fields are important for understanding both on-site and off-site consequences of historical and current erosion. In this study, the potential for using cheap and rapid measurements of soil magnetism to identify relative degrees of total soil loss is tested in a fallow field in northern Alabama. This is accomplished by defining and mapping erosion categories based on two different magnetic measurements, and an existing soil magnetism model describing soil loss in a nearby field. Surface soil magnetism patterns generally accord with Revised Universal Soil Loss Equation soil loss estimates, but may contain more useful information regarding historical soil redistribution (mass balance at a point) for the fallow field tested. Due to current limitations of the technique and the intrinsic complexity of soilscapes, the best use of magnetic surveying at this time is likely to be in rapid erosion reconnaissance.

Land Suitability Evaluation for Organic Agriculture of Wheat Using GIS and Multicriteria Analysis

ABSTRACT Despite the growing interest in organic farming, its practice remains limited because of its lower productivity relative to conventional farming. Land suitability evaluation for organic crops can potentially improve productivity, and thus the economic viability of organic farming. The best analytical procedures for such evaluations have not yet been fully explored. This article addresses the evaluation of land suitability of present agricultural lands for organic agriculture of rain-fed winter wheat using Duplin County, North Carolina, a location economically dependent on agriculture, as a case study. A novel land suitability evaluation procedure is developed combined with seventeen suitability criteria from five principal categories including climatic parameters, soil characteristics and qualities, soil chemistry, soil organic matter and fertility, and flood and erosion hazards by using geographic information systems (GIS), multicriteria analysis, and the square root method. Our analysis demonstrates that although 18.6 percent of agricultural lands in Duplin County are highly suitable for organic winter wheat production, a large proportion (76.8 percent) of agricultural lands are also moderately suitable. The method of suitability analysis used in this research, which allows specific consideration of soil organic matter and fertility as particularly critical factors for organic farming, can be easily exported to other locations, for similar applications.

Dendrohydrological Evidence of Flooding Along a Small Appalachian Stream

Tree-ring records from flood-damaged trees have been most frequently used for reconstructing flood histories along large streams. Rarely has this approach been taken to gain hydrological information about small headwater channels. In this study, I analyze dendrohydrological evidence of flooding along two headwater streams in Virginia to determine its usefulness for hydrologic and geomorphic investigations of small, unmonitored watersheds. Easily obtained tree-ring dates for corrasion scars, adventitious sprouts and thin-ring sequences comprise the fundamental data set. The most noteworthy aspects of the data set are the youthfulness of most samples and a large corrasion scar peak in 1989. Elevations of corrasion scars above the channel bed indicate higher peak 1989 discharge than that expected based on field observations. These observations, coupled with hydrological monitoring data from closest stations and HEC-1 modeling results, emphasize the importance of wet antecedent conditions and repeated moderate events in scar creation. Earlier studies of lake sediment do not resolve sediment yield from year-1989 flooding, and average sediment yield was generally low at this time. Geomorphic work accomplished and/or sediment delivery may have been lower during 1989 than those associated with the Hurricane Agnes event in 1972. Although botanical flood evidence from small streams, and in particular corrasion scarring, are demonstrated to be useful for some hydrologic analyses, their use in reconstructing flood chronology is limited to recent decades by rapid tree healing.

Evaluating the effects of urbanization age on the morphology of low-order urban streams in the U.S. southern Piedmont

ABSTRACT We used space-for-time substitution, correlation, and regression analyses to study the effects of urbanization age on stream channel variables from 19 low-order urban watersheds in the southern Piedmont of the United States. The results are used to address questions regarding which variables are most responsive to urbanization age, the duration of response relative to system size, and changes in adjustment rate over time. Few morphological variables were found to be well correlated with metrics of urbanization age. The maximum-width/maximum-depth ratio (Wmax/Dmax) exhibited the best correlations to multiple metrics of urbanization age, with all correlations positive. A dearth of young urbanization ages in our dataset makes mathematically derived response times and end states for Wmax/Dmax highly uncertain. Inferences from the age range that is better represented suggest that the period of major response has occurred within the first three decades following urbanization. Subsequently, gradual increase in Wmax/Dmax appears to continue over 40 or more years. Response of channel form to urbanization and time elapsed in these low-order Piedmont streams is constrained by the presence of resistant bedrock below streambeds, a circumstance addressed by recent channel evolution models.