Spatial patterns of sand-wave size and migration rate were carefully monitored using side-scan sonar and narrow-beam echo sounding on Southwest Middle Ground Shoal in Vineyard Sound, Massachusetts, from March 22 to November 16, 1978. Navigation was provided by an acoustic-transponder system with a positioning accuracy of 3 m.

Sand waves migrate obliquely upshoal from depths greater than 19 m to depths less than 11 m. Erosion at depths less than 16 m is caused by a downcurrent increase in the sediment-transport rate, represented by a twofold increase in bed-form height that more than offsets a small decrease in migration rate. Deposition farther downcurrent, above 16 m, is caused by a downcurrent decrease in sediment-transport rate, manifested by downcurrent decreases in both bed-form height and migration rate. These bed-form-size trends are consistent with a kinematic control on bed-form size that causes bed forms to grow or shrink while they acquire or lose sediment owing to erosion or deposition.

The observed pattern of erosion on the shoal's lower flanks, and of deposition on the upper flanks and crest (a constructional phase of shoal dynamics), may be seasonal because the period of our study did not include the more energetic winter months. Alternatively, this constructional pattern may occur during all seasons, and only be balanced by such major storm events as hurricanes, which would transfer sediment from the shoal crest to its lower flanks.

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