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A recent breach in the western (right-hand, looking downcurrent) levee of the main valley on the Rhone Fan resulted in a rejunevated thalweg within the valley-floor sediment fill and formation of an incipient, sandy, depositional lobe high on the fan. The new thalweg exhibits an unusual cuspate or scalloped form in contrast with the abandoned thalweg downstream of the avulsion that exhibits the more typical meandering or sinusoidal form. The cusps have a radius of curvature that is usually about 0.25 km. Long-range (GLORIA) and high-resolution, deep-tow (SeaMARC I) side-looking sonar images provide details concerning the surface texture (through acoustic-backscatter patterns) and morphology of the relatively young entrenched channel and its surroundings. These data suggest that mass wasting, seen as small gullies on channel walls and deposits on the thalweg floor, accompanied the downcutting of the thalweg and played a major role in forming the cusp-shape morphology. The side-looking sonar images also confirm the widespread occurrence of sediment failure on the main levee areas, as suggested by previous work based on seismic-reflection profiles.

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