Pattern evolution and interactions in subaqueous dune fields: North loup river, Nebraska, U.S.A.

A time series of aerial images of a dune field on a migrating free bar in the North Loup River, Nebraska, is used to generate a quantified dataset that allows analyses of crestline deformation, dune interaction type and spatial density, and impact of spurs.Measurement of dune parameters show that the dune field maintained a dynamic steadystate pattern, despite high rates of deformation, common interactions, and sediment bypassing.Mapped crestlines had a mean migration rate of 8 cm/min. The mean deformation rate, quantified using a partial Procrustes analysis, was 2 cm/min, indicating that along individual crestlines, local migration varied ± 25% from the bedform mean. Dune interactions caused the break-apart and recombination of crestline segments, thus limiting pattern variability caused by deformation. Although most of the 50 documented interactions are comparable to those observed in aeolian dune fields, defect-driven interactions are less common and interactions caused by migration of the convex-downstream nose of the dunes were observed, which has not been reported in aeolian dunes. The spatial density of interactions is consistent with that derived for aeolian crescentic-dune fields, in spite of differences in ambient fluid and dune size. Although spurs were ubiquitous, their presence did not have a quantifiable impact on deformation and interactions as compared to areas where spurs were absent, suggesting that these short-lived features affect instantaneous flux rates only.