Multibeam sonar data provide new high-resolution characterization of morphological and sedimentological features on the upper slope of the Roberts Bank section of the Fraser River delta. Subaqueous dunes are more abundant and broadly distributed over the slope than previously documented and include large asymmetric two-dimensional dunes (sinuous crested and straight crested), large asymmetric three-dimensional dunes (high relief, low relief) and irregularly-spaced dunes. The two-dimensional and three-dimensional dunes fall within the typical range of subaqueous dune heights and wavelengths and show dimensions that do not universally scale to water depth but are probably scaled to the dynamic conditions. The irregularly-spaced dunes fall outside the typical scaling, having excessively low height to wavelength ratios. Current data indicate that these irregularly-spaced dunes form under conditions where the ratio of suspended to bedload transport is too high to maintain fully-scaled bedforms. The orientation of the dunes indicates predominant alongshore transport to the northwest, in the direction of the flood tidal current, although a small patch of ebb-oriented dunes is present near Point Roberts, where the ebb-flow is accelerated.
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Vol. 2009 • No. 252