Evolution and deposits of a gravelly braid bar, Sagavanirktok River, Alaska

摘要

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).