Scott A. Lecce

Use of mining-contaminated sediment tracers to investigate the timing and rates of historical flood plain sedimentation

Abstract Changes in land use practices following European settlement in the 1830s produced accelerated sedimentation on virtually all valley floors in the Blue River Watershed, Wisconsin. The contamination of sediments by Pb and Zn mining allowed us to calculate cross-valley rates of flood plain sedimentation for three time periods: the pre-mining period (1830–1900), the mining period (1900–1920), and the post-mining period (1920–1997). Most of the eight valley floors examined contained multiple presettlement surfaces. Significantly higher rates of sedimentation occurred on the lower flood plain surfaces, while the terraces were high enough to prevent sedimentation from most floods. Higher rates of sedimentation on the lower surfaces eventually reduced valley floor relief and, consequently, lateral differences in sedimentation rates. Tributaries and larger valleys downstream exhibited differences in the timing and rates of historical flood plain sedimentation. While rates of sedimentation were high during the pre-mining period in tributary valleys, the lower valleys were receiving little or no pre-mining alluvium. Little pre-mining alluvium was found in mid-basin reaches, suggesting that most of the pre-mining sedimentation was limited to headwater locations. During the mining period, lateral channel migration and the development of meander belts increased the conveyance capacity of tributary and mid-basin channels, which decreased overbank flooding and produced lower rates of sedimentation during the post-mining period. The meander belt channels also had an effect on the lower portion of the watershed by increasing flood magnitudes and the transport of sediment downstream, thereby shifting the locus of sedimentation from the tributaries to the larger, lower valleys. Sedimentation rates in the largest, most downstream site were an order of magnitude higher during the post-mining period than any of the sites upstream.

Stream Channel Enlargement Response to Urban Land Cover in Small Coastal Plain Watersheds, North Carolina

Stream channel response to urban land use has not been well documented for southeastern Coastal Plain streams. In this study, urban channel response was evaluated in small Inner Coastal Plain watersheds (<5 km2) in eastern North Carolina. Reaches were selected across a range of watershed total impervious area (0-67% TIA). Channel dimensions and sediment grain size data were collected along 20 urban (>10% TIA) and 20 rural reaches (<10% TIA), and at 10 stormwater outfall sites (180 cross-sections). Urban cross-sectional area, channel incision ratio, and channel grain size (gravel%, D50, and D84) were greater, relative to rural channels. Bankfull cross-sectional areas were approximately 1.78 times greater for urban watersheds than for rural watersheds. Channels in urban watersheds were incised and had median full-channel capacities approximately 3.4 times greater than channels draining rural watersheds. Watershed TIA explained 65-72% of channel capacity enlargement. Urban expansion in the region began in the 1960s, with major urbanization occurring over the last 25 years. Channels draining urban watersheds are still responding to this land use change by downcutting and widening. Urban channel incision has frequently cut off streams from their floodplains, reducing floodplain sediment retention and water quality functions.

Floodplain Sedimentation During an Extreme Flood: the 1999 Flood on the Tar River, Eastern North Carolina

This study examines floodplain sedimentation following the largest flood in the 98-yr. record on the Tar River, North Carolina. Hurricane Floyd made landfall just 10 days after Hurricane Dennis in September 1999, bringing unprecedented rainfall (30-46 cm) and flooding to eastern North Carolina. A field survey of the lower 350 km of the river showed that this >500 yr. flood deposited very little overbank sediment (<1 mm) on most of the floodplain. We used suspended sediment concentrations measured on the Tar River from 1958-1967 to suggest that the seasonal timing and sequencing of flood events in 1999 are the most probable explanations for the minimal geomorphic impact of this extreme flood. The early autumn timing of the flood coincided with crops that were mature but not yet harvested, and when natural vegetation was very dense and effective at stabilizing channel banks, hillslopes, and floodplain soils. Hurricane Dennis may have exhausted the available sediment supply and transported this sediment to the Pamlico Sound before reaching flood stage, thereby reducing the sediment available to be transported and deposited by the flood that followed Hurricane Floyd.

Seasonality of Flooding in North Carolina

This paper examines spatial patterns in the timing of floods in North Carolina. Regions with distinctly different seasonal flood regimes were identified using cluster analysis of monthly flood frequencies for the annual and partial duration series. The partial duration series produced results similar to those using the annual flood series, which was used to delineate three regions: the Appalachian Region, the Piedmont Region, and the Coastal Plain Region. In the Appalachian Region, about three-fourths of all floods occur during winter and early spring (December-April), while less than one fourth occur from May to November. The Appalachian Mountains are an effective barrier to warm, moist air masses that influence the rest of the state during the summer and fall. The frequency of floods in summer and fall increases to the east in the Piedmont and Coastal Plain regions, where maximum frequencies (~25-30%) occur during August-October. Although flooding in summer and fall is greater than in the Appalachian Region, floods still occur most frequently from January to April in the Piedmont (44%) and Coastal Plain (55%) regions. The smallest watersheds tend to experience a larger proportion of floods in summer and fall.

EOLIAN DUST EROSION FROM AN AGRICULTURAL FIELD ON THE NORTH CAROLINA COASTAL PLAIN

Eolian erosion typically has not been considered a significant process on the humid southeastern coastal plain of the United States. A preliminary study of eolian erosion from an agricultural field was undertaken during the late winter of 2002 and early spring of 1999. During those times local agricultural practices leave fields bare while frontal systems produce frequent high wind events. Dust emissions were measured with two samplers; modified Wilson and Cooke passive dust traps and high-volume air samplers. Results of the study indicate that wind erosion is a significant process on agricultural fields of the North Carolina Coastal plain. Dust flux off of the field during the largest of five measured events was estimated as high as 126 kg/m with total losses of 3070 kg/ha. Atmospheric concentrations of suspended material were measured at 58,815 μgm-3. Sediment erosion was not evenly distributed across the field. Erosion was focused over soils that are better drained. Low levels of soil moisture did not eliminate erosion but instead produced pulses of sediment emission as sustained wind continually dried then activated sequential layers of the field surface. Soil moisture and topography appear to be the primary controls on spatial erosion differences and soil characteristics likely play a secondary role.

Metal Contamination from Gold Mining in the Cid District, North Carolina

The purpose of this paper was to assess contamination from 19th century gold (Au) mining in the Cid district, North Carolina. Sediment samples collected from active channel sediments and floodplain cores were analyzed for mercury (Hg), copper (Cu), lead (Pb), and zinc (Zn). Analysis of trace metal concentrations shows that although Hg contamination exists at relatively low levels (i.e., no samples exceeded the probable effect concentration for Hg), the active channel sediments and historical floodplain deposits are contaminated by Hg downstream from all mines in the district. We also found significant contamination by Cu, Pb, and Zn. The use of Hg and other metals as tracers associated with mining activities suggests that long-term rates of floodplain sedimentation in the Cid district (0.3-0.9 cm/yr) were less than half as high as those in the nearby Gold Hill district. This suggests that the intensity of land disturbance in the Cid district was less than in the more intensively mined Gold Hill district.

Suggestions for Low-Cost Equipment for Physical Geography II: Field Equipment

Abstract Fieldwork and laboratory experiences have always been important components of physical geography education, at universities as well as secondary schools. However, the rising cost of necessary equipment and dwindling education budgets of most universities and secondary schools have placed such experiences in crisis. This is the second of two papers that present lab- and field-based items we have designed and built for student research. The equipment is easy to construct and made from low-cost materials like PVC plumbing pipe. Photographs, construction notes, and costs have been included for each of the pieces of equipment, as well as measured schematics for the more complex items.

Legacy Hg-Cu Contamination of Active Stream Sediments in the Gold Hill Mining District, North Carolina

The purpose of this study is to evaluate the longitudinal trends of mercury (Hg) and copper (Cu) in active channel sediments downstream from the Gold Hill mining district in the Piedmont of North Carolina. Mining for gold (Au) and Cu from 1844 to 1915 released both Hg (associated with Au processing) and Cu in a 254 km2 watershed. Multiple linear regression is used to quantify spatial and geochemical trends in 93 active channel samples collected from contaminated main stem and background tributary sites. Simple two-parameter regression models combining the effects of both watershed-scale dispersal processes (distance downstream) and reach-scale sediment transport (percent sand) explain 85 percent of the variance in Hg and 90 percent of the variance in Cu in active channel sediments. Contamination trends in two different sediment media, low bar and higher elevation bench deposits, were effectively similar when local grain size influence was accounted for in the two-parameter models. Background geochemistry models explain 84 percent of the variance of Hg and Cu in uncontaminated tributary samples using parameters related to grain-size, secondary geochemical substrates, and mineral weathering sources. More than 45 percent of the variance of Hg and 20 percent of Cu in contaminated sediment can be explained by background parameters. Geochemical signatures differ between Hg and Cu in active channel sediments due to variations in mining inputs, background geochemistry, and present-day pollution sources.

Drainage Ditches as Sediment Sinks on the Coastal Plain of North Carolina

This paper examines the role that slope-channel linkages and seasonal variations in vegetation play in explaining spatial and temporal variations in sediment flux through agricultural drainage ditches in eastern North Carolina. We used biannual cross-sectional surveys of drainage ditches to assess erosion/deposition during a five-year period in the headwaters of a small agricultural watershed. Although net accumulations of sediment were observed in three-fourths of the cross sections surveyed, the rate of sedimentation varied considerably from ditch to ditch and cross section to cross section. The ditches were sediment sinks during the growing season in summer and autumn when they became choked with dense vegetation growth, and more hydraulically efficient after removal of vegetation in December during annual maintenance operations. The ditches experienced erosion or modest deposition while the vegetation was dormant during the late winter/early spring. Sediment was delivered to the ditches from isolated gullies that linked the primary source of sediment, soil eroded on agricultural fields, to the channels. Except for these isolated linkages, ditches and fields are largely decoupled.

Stream power, channel change, and channel geometry in the Blue River, Wisconsin

Discharge and bank resistance have long been considered two of the more important factors controlling downstream changes in stream channel geometry. Although stream power has been shown to have an important impact on many components of the fluvial system, it has received little use as an explanatory variable in the consideration of questions related to channel geometry. This study examines spatial and temporal changes in channel geometry and stream power in a moderate-sized watershed in the Driftless Area of Wisconsin, USA. Pre-settlement (~1830) channel geometry and stream power estimates derived from General Land Office survey notes were compared with conditions obtained from modern field surveys. Comparison of pre-settlement and modern channels shows that channel width and cross-sectional area have increased considerably during the post-settlement period, producing on average a threefold increase in cross-sectional stream power. When compared to discharge, slope, and bank sediment variables, cross-sectional stream power is more strongly associated with channel shape, explaining a maximum of 47% of the variance in one of the four study reaches. These results are compared with a reanalysis of previously published data from the midwestern USA, in which cross-sectional power also explains more of the variance in channel shape than discharge or bank sediment.