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The evolution, migration and deposits of a gravelly braid bar in the Sagavanirktok River, northern Alaska, are described in unprecedented detail using annual aerial photographs, ground-penetrating radar (GPR) profiles, trenches and cores. Compound braid bars in the Sagavanirktok River form by chute cut-off of point bars and by growth of mid-channel unit bars. Subsequent growth is primarily by accretion of unit bars onto their lateral and downstream margins. The upstream ends of braid bars may be sites of erosion or unit bar deposition. Compound braid bar deposits vary in thickness laterally and are thickest in medial sections and near cut banks. Compound bar deposits are typically composed of three to seven sets of simple large-scale inclined strata, each simple set formed by a unit bar. The simple large-scale strata contain medium-scale cross-strata (from dune migration) and planar strata (from migration of bedload sheets). The upstream and medial parts of compound braid bar deposits show very little vertical variation in grain size, but downstream and lateral margins tend to fine upwards. The deposits are mostly poorly sorted sands and gravels, although sands tend to be deposited at the top of the braid bar, and open-framework gravels preferentially occur near the top and base of the braid bar. The patterns of braid bar growth and migration, and the nature of the deposits, described from the Sagavanirktok River are generally similar to other sandy and gravelly braided rivers, and consistent with the theoretical braid bar model of Bridge (1993). [ABSTRACT FROM AUTHOR]

Instrumentation and methods for measuring snow properties are compared in an investigation of millimeter- to meter-scale stratigraphy in a snowpack not influenced by topography, vegetation, or a warm and variable ground surface. Field measurements were conducted within a 20×20×2 m plot at Pika Glacier, Alaska. The snow was characterized by more than 600 point measurements of density, stratigraphic mapping in 19 snow-pits, and by pulse-radar imaging along 20 cross-plot profiles. Density was measured manually and was calculated from electric permittivity, which was determined with a hand-held probe and by radar velocity analysis. Stratigraphic mapping in snow-pit walls with manual measurements of density identified comparatively few layers, suggesting a relatively homogeneous snowpack. Both the permittivity probe and the radar imaging, however, identified a larger number of layers based on vertical density contrasts. Image analysis of a back-illuminated column of snow revealed the highest level of stratigraphic complexity, identifying layers mm in thickness that extended up to 10 cm laterally. Despite minor variations in snow properties at the mm scale, major features in the vertical density profiles were laterally continuous over tens of meters. These results provide evidence for spatial homogeneity of densification processes leading to decimeter scale layering in a situation where the snowpack is not influenced by local terrain factors. In addition, these observations demonstrate that the complexity of snow stratigraphy is highly dependent upon choice of scale and measurement tool. [Copyright &y& Elsevier]