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Nonbuoyant front formation at the confluence of Nanjemoy Creek and the main Potomac River (MD) channel is examined. Terra satellite ASTER imagery reveals a sediment color front emerging from Nanjemoy Creek when the Potomac is near maximum ebb. Nearly contemporaneous ASTER and Landsat ETM1 imagery are used to extract surface velocities, which suggest a velocity shear front is collocated with the color front. In situ velocities (measured by RiverRay traverses near the Nanjemoy Creek mouth) confirm the shear front's presence. A finite-element simulation (using ADCIRC) replicates the observed velocity-shear front and is applied to decipher its physics. Three results emerge: (1) the velocity-shear front forms, confined to a shoal downstream of the creek-river confluence for most of the tidal cycle, (2) a simulation with a flat bottom in Nanjemoy Creek and Potomac River (i.e., no bathymetry variation) indicates the velocity-shear front never forms, hence the front cannot exist without the bathymetry, and (3) an additional simulation with a blocked-off Creek entrance demonstrates that while the magnitude of the velocity shear is largely unchanged without the creek, shear front formation is delayed in time. Without the Creek, there is no advection of the M 6 tidal constituent (generated by nonlinear interaction of the flow with bottom friction) onto the shoals, only a locally generated contribution. A tidal phase difference between Nanjemoy and Potomac causes the ebbing Nanjemoy Creek waters to intrude into the Potomac as far south as its deep channel, and draw from a similar location in the Potomac during Nanjemoy flood.

A fundamental challenge in river analysis and modelling is the lack of readily available and reliable information on river bank geometry. Traditional survey methods are expensive and time consuming and often difficult to execute in many river systems because of hazardous terrain or lack of access. However, as high quality aerial and satellite imagery becomes available for more of the globe, it is increasingly possible to extract these bank locations directly from imagery. The most direct method of doing this involves manually designating edges based on visual criterion. This, however, is often prohibitively time consuming and labour intensive, and the quality is dependent on the individual doing the task. This paper describes a quick and fully automated method for locating water surface and river banks in high resolution aerial imagery without recourse to any multispectral information, by segmenting based on the local entropy of the image. This method is demonstrated on imagery of several rivers and its advantages and limitations are discussed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

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[1] An experimental study of vertical mixing and along-channel dispersion parameterized in terms of horizontal mixing near the mouth of the Duplin River (a tidal creek bordered by extensive intertidal salt marshes on Sapelo Island, Georgia) was carried out over several spring/neap cycles in the fall of 2005. Vertical mixing is modulated on both M4 and fortnightly frequencies with maximum turbulent stresses being generated near the bed on periods of maximum flood and ebb and propagating into the water column showing a linear dependence with depth. Values are significantly greater on spring tide than on neap. Horizontal mixing evaluated by salt fluxes is driven and dominated by tidal dispersion, which is also modulated by the fortnightly spring/neap cycle. Net export of salt from the lower Duplin is shown to be due to residual advection modified by upstream tidal pumping. The tidal dispersion coefficient exhibits a pulsating character with greater values on spring tide followed by smaller values on neap tide.