Shoal formation in the Piscataqua River, New Hampshire

The formation of a shoal was investigated in the Piscataqua River, New Hampshire, which is a well-mixed channel with low freshwater flow and tidal currents up to 2.3 m s^-1. Observations of sediment characteristics, bathymetry, and bottom current were made, and theory was used to predict bedload transport. Sediment sampling showed the bottom material to be coarse sand and gravel, and sidescan sonar revealed large sand waves directed upriver at the shoal. Bottom current measurements were made along transects upriver and downriver of the shoal and downriver of an adjacent deepwater area that was also studied for comparison. Bedload flux inferred from current measurements using the Brown-Einstein theory indicated that transport is generally directed upriver. Sediment budget calculations showed the shoal area to be depositional before, immediately after, and subsequent to a dredging operation at rates of 0.36 m yr^-1, 1.06 m yr^-1, and 0.35 m yr^{- 1}, respectively. Predredge and subsequent rates were consistent with the historical record of removal by dredging at the shoal.