Amanda Keen-Zebert

Channel Responses to Urbanization: Scull and Mud Creeks in Fayetteville, Arkansas

Urbanization has several effects on the controls on stream morphology. In order to provide a basis for future study, and to determine whether changes in urban stream-channel morphology can be detected in a short time period, two urban streams in Fayetteville, Arkansas were chosen for repeat cross-section measurement and suspended sediment sampling over 18 months. Standard U.S. Geological Survey measurement and sampling techniques were used. An overall decrease in mean depth was observed over the study period. Significant channel variation occurred where there was ongoing development and channel obstruction. Channel stability was observed in locations with diverse riparian vegetation. Channel shape varied with land use in the study area. Sites with more disturbances in the near-channel environment were variable over the course of the study, exhibiting a complex response to several types of disturbances.

Cosmogenic 3He Measurements Provide Insight into Lithologic Controls on Bedrock Channel Incision: Examples from the South African Interior

Resistant bedrock outcrops can exert control on river long-profile adjustment, upstream transmission of base level fall, and valley development, particularly in postorogenic settings. To examine how variation in lithologic resistance impacts landscape development in the postorogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and the Schoonspruit are widely variable, with channel bed denudation rates ranging from 14 to 255 m/m.yr. and valley side and top denudation rates ranging from 11 to 50 m/m.yr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (∼20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 k.yr.). These results suggest that in the postorogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics.

Evidence of an early projectile point technology in North America at the Gault Site, Texas, USA

Human presence in North America before ~16 thousand years ago is confirmed by age measurements from Area 15 of the Gault Site. American archeology has long been polarized over the issue of a human presence in the Western Hemisphere earlier than Clovis. As evidence of early sites across North and South America continues to emerge, stone tool assemblages appear more geographically and temporally diverse than traditionally assumed. Within this new framework, the prevailing models of Clovis origins and the peopling of the Americas are being reevaluated. This paper presents age estimates from a series of alluvial sedimentary samples from the earliest cultural assemblage at the Gault Site, Central Texas. The optically stimulated luminescence age estimates (~16 to 20 thousand years ago) indicate an early human occupation in North America before at least ~16 thousand years ago. Significantly, this assemblage exhibits a previously unknown, early projectile point technology unrelated to Clovis. Within a wider context, this evidence suggests that Clovis technology spread across an already regionalized, indigenous population.

The effect of lithology on valley width, terrace distribution, and bedload provenance in a tectonically stable catchment with flat-lying stratigraphy: Effects of lithology on valley width, terraces, & bedload provenance

How rock resistance or erodibility affects fluvial landforms and processes is an outstanding question in geomorphology that has recently garnered attention owing to the recognition that the erosion rates of bedrock channels largely set the pace of landscape evolution. In this work, we evaluate valley width, terrace distribution, and bedload provenance in terms of reach scale variation in lithology in the study reach and discuss the implications for landscape evolution in a catchment with relatively flatlying stratigraphy and very little uplift. A reach of the Buffalo National River in Arkansas was partitioned into lithologic reaches and the mechanical and chemical resistance of the main lithologies making up the catchment was measured. Valley width and the spatial distribution of terraces were compared among the different lithologic reaches. The surface grain size and provenance of coarse (2–90mm) sediment of both modern gravel bars and older terrace deposits that make up the former bedload were measured and defined. The results demonstrate a strong impact of lithology upon valley width, terrace distribution, and bedload provenance and therefore, upon landscape evolution processes. Channel down-cutting through different lithologies creates variable patterns of resistance across catchments and continents. Particularly in post-tectonic and non-tectonic landscapes, the variation in resistance that arises from the exhumation of different rocks in channel longitudinal profiles can impact local base levels, initiating responses that can be propagated through channel networks. The rate at which that response is transmitted through channels is potentially amplified and/or mitigated by differences between the resistance of channel beds and bedload sediment loads. In the study reach, variation in lithologic resistance influences the prevalence of lateral and vertical processes, thus producing a spatial pattern of terraces that reflects rock type rather than climate, regional base level change, or hydrologic variability. Copyright

The rarefied (non-continuum) conditions of tracer particle transport in soils, with implications for assessing the intensity and depth dependence of mixing from geochronology

We formulate tracer particle transport and mixing in soils due to disturbance-driven particle motions in terms of the Fokker–Planck equation. The probabilistic basis of the formulation is suitable for rarefied particle conditions, and for parsing the mixing behavior of extensive and intensive properties belonging to the particles rather than to the bulk soil. The significance of the formulation is illustrated with the examples of vertical profiles of expected beryllium-10 (10Be) concentrations and optically stimulated luminescence (OSL) particle ages for the benchmark situation involving a one-dimensional mean upward soil motion with nominally steady surface erosion in the presence of either uniform or depth-dependent particle mixing, and varying mixing intensity. The analysis, together with Eulerian–Lagrangian numerical simulations of tracer particle motions, highlights the significance of calculating ensemble-expected values of extensive and intensive particle properties, including higher moments of particle OSL ages, rather than assuming de facto a continuum-like mixing behavior. The analysis and results offer guidance for field sampling and for describing the mixing behavior of other particle and soil properties. Profiles of expected 10Be concentrations and OSL ages systematically vary with mixing intensity as measured by a Péclet number involving the speed at which particles enter the soil, the soil thickness, and the particle diffusivity. Profiles associated with uniform mixing versus a linear decrease in mixing with depth are distinct for moderate mixing, but they become similar with either weak mixing or strong mixing; uniform profiles do not necessarily imply uniform mixing.