Southern Appalachian hillslope erosion rates measured by soil and detrital radiocarbon in hollows

Understanding the dynamics of sediment generation and transport on hillslopes provides important constraints on the rate of sediment output from orogenic systems. Hillslope sediment fluxes are recorded by organic material found in the deposits infilling unchanneled convergent topographic features called hollows. This study describes the first hollow infilling rates measured in the southern Appalachian Mountains. Infilling rates (and bedrock erosion rates) were calculated from the vertical distribution of radiocarbon ages at two sites in the Coweeta drainage basin, western North Carolina. At each site we dated paired charcoal and silt soil organic matter samples from five different horizons. Paired radiocarbon samples were used to bracket the age of the soil material in order to capture the range of complex soil forming processes and deposition within the hollows. These dates constrain hillslope erosion rates of between 0.051 and 0.111 mm yr^− 1. These rates are up to 4 times higher than spatially-averaged rates for the Southern Appalachian Mountains making creep processes one of the most efficient erosional mechanisms in this mountain range. Our hillslope erosion rates are consistent with those of forested mountain ranges in the western United States, suggesting that the mechanisms (dominantly tree throw) driving creep erosion in both the western United States and the Southern Appalachian Mountains are equally effective.     

Geomorphic characteristics of hillslope and channelized debris flows: A case study in the Shitou area of central Taiwan

The data on the hillslope and channelized debris flows in the Shitou area of central Taiwan occurred during Typhoons Toraji and Nali in 2001 were applied in this paper. The geomorphic parameters, including the flow length, gully gradient, drainage area and form factor of the debris flows were determined by spatial analysis using a Geographic Information System （GIS） based on the data derived from field investigation, aerial photographs, and topographical maps. According to such determined geomorphic parameters, the threshold conditions and empirical equations, such as the relationship between the gully gradient and drainage area and that between gully length and drainage area and topographic parameter, are presented and used to distinguish the geomorphic characteristics between the channelized and hillslope debris flows.     

Flow variability and hillslope hydrology

Examination of spatial variability of streamflow in headwater areas can provide important insight about factors that influence hillslope hydrology. Detailed observations of variations in stream channel input, based on a tracer experiment, indicate that topography alone cannot explain flow variability. However, determination of changes in channel input on a small spatial scale can provide valuable clues to factors, such as structural geology that control subsurface flows.     

A linear erosion/elevation measuring instrument (LEMI)

The Linear Erosion/Elevation Measuring Instrument (LEMI) offers an alternative to erosion pins for assessing changes in the ground surface that need not disturb the soil at the point of measurement. The device, fabricated from two carpenter's levels, is mounted on each of a series of support rods implanted along a hillslope profile. The vertical distance between the LEMI and the surface may be recorded to the nearest one-half millimetre. Testing indicates that the use of the LEMI sacrifices about one millimetre of measurement replicability while obtaining undisturbed site data.     

Heathland vegetation,soil moisture and dynamic contributing area

Gurnell (1978) demonstrated the potential of the composition of wet heathland vegetation as an indicator of hydrological processes in a small heathland catchment. The present paper investigates this potential and demonstrates that on a hillslope in the study catchment simple prediction equations for pressure head based entirely on position on the slope can be substantially improved by the inclusion of vegetation related variables. It is suggested that in this context the vegetation composition is largely reflecting soil permeability and that the remaining scatter around the prediction equations results from other factors controlling micro-scale variations in the vegetation pattern.     

Conceptual model of sediment processes in the upper Yuba River watershed, Sierra Nevada, CA

This study examines the development of a conceptual model of sediment processes in the upper Yuba River watershed; and we hypothesize how components of the conceptual model may be spatially distributed using a geographical information system (GIS). The conceptual model illustrates key processes controlling sediment dynamics in the upper Yuba River watershed and was tested and revised using field measurements, aerial photography, and low elevation videography. Field reconnaissance included mass wasting and channel storage inventories, assessment of annual channel change in upland tributaries, and evaluation of the relative importance of sediment sources and transport processes. Hillslope erosion rates throughout the study area are relatively low when compared to more rapidly eroding landscapes such as the Pacific Northwest and notable hillslope sediment sources include highly erodible andesitic mudflows, serpentinized ultramafics, and unvegetated hydraulic mine pits. Mass wasting dominates surface erosion on the hillslopes; however, erosion of stored channel sediment is the primary contributor to annual sediment yield. We used GIS to spatially distribute the components of the conceptual model and created hillslope erosion potential and channel storage models. The GIS models exemplify the conceptual model in that landscapes with low potential evapotranspiration, sparse vegetation, steep slopes, erodible geology and soils, and high road densities display the greatest hillslope erosion potential and channel storage increases with increasing stream order. In-channel storage in upland tributaries impacted by hydraulic mining is an exception. Reworking of stored hydraulic mining sediment in low-order tributaries continues to elevate upper Yuba River sediment yields. Finally, we propose that spatially distributing the components of a conceptual model in a GIS framework provides a guide for developing more detailed sediment budgets or numerical models making it an inexpensive way to develop a roadmap for understanding sediment dynamics at a watershed scale.     

Forms of bluffs degraded for different lengths of time in emmet county,Michigan, U.S.A.

Distinct differences in morphology are found among bluffs cut and abandoned by Glacial Lake Algonquin and by the Nipissing Great Lakes in Emmet County, Michigan and modern, active bluffs presently being eroded by Lake Michigan in Mason and Manistee Counties, Michigan. The Algonquin, Nipissing, and modern bluffs are all composed of a sandy, morainic material. Because the composition and mode of formation of the three sets of bluffs are similar, it is assumed that the abandoned bluffs had the same initial configuration as the modern bluff. The morphologic differences are likely a function of the different periods of time each bluff has degraded. With time, the angle of the bluff midsection decreased, the extent of the convex crest and the concave base increased, and the angle of the crest increased. This course of hillslope degradation is believed similar to that occurring elsewhere on heavily vegetated hillslopes underlain by cohesionless material and situated in temperate, humid climates. The morphologic changes may be understood in terms of an analytical model which assumes that the rate of change in elevation at a point on a hillslope profile is proportional to the profile curvature at that point.     

Soil saturation patterns in steep,convergent hill-slopes under forest and pasture vegetation

The relationships between hillslope form and the development of zones of hillslope saturation are central to studies of process hydrology. These relationships are examined in two small, steep (30°) first-order basins under forest and pasture vegetation. Hillslope saturation patterns are described using a probability analysis of observations of peak water table elevations made at individual sites in each basin. In both first-order basins, the effects of topographic convergence on the spatial variability of hillslope saturation zones are most pronounced on shallow slopes and in steeper areas where hillslope form is strongly concave. The implications of vegetation type on the formation and variation of hillslope saturation are also examined. The differences in hillslope saturation patterns in each vegetation type are attributed primarily to differences in net rainfall and not soil physical properties or other features of topography.     

Anthropogenic hillslope terraces and swidden agriculture in Jiuzhaigou National Park, northern Sichuan, China

Small, irregular terraces on hillslopes, or terracettes, are common landscape features throughout west central China. Despite their prevalence, there is limited understanding of the nature of these topographic features, the processes that form them, and the role humans played in their formation. We used an interdisciplinary approach to investigate the geology, ecology, and cultural history of terracette development within Jiuzhaigou National Park, Sichuan Province, China. Terracettes occur on south facing, 20° slopes at 2500 m elevation, which appears to coincide with places people historically preferred to build villages. Ethnographic interviews suggest that traditional swidden agricultural cycles removed tree roots, causing the loess sediments to lose cohesion, slump, and the terrace risers to retreat uphill over time. This evidence is supported by landslide debris at terracette faces. Archaeological analysis of terracette sites reveal remains of rammed spread soil structures, bones, stone tools, and ceramics dating from at least 2200 years before present within a distinct paleosol layer. Radiocarbon and optically stimulated luminescence dating of terracette sediments ranged in age from between 1500 and 2000 ¹⁴C yr BP and between 16 and 0.30 ka, respectively. These multiple lines of evidence indicate a long history of human habitation within Jiuzhaigou National Park and, taken together, suggest strong links between terracette formation and human-landuse interactions.