RECENT EVOLUTION OF THE HUDSON ESTUARY WITHIN THE TAPPAN ZEE

An extensive suite of geophysical and sampling data have recently been collected within over 100 km of the Hudson River Estuary. The geophysical data include sidescan sonar and high resolution Chirp subbottom data collected on a dense grid of perpendicular lines spaced ~100-200 m apart. Core data provide age constraints on subbottom horizons via radiocarbon dating of shells and permit reconstruction of the sedimentary history of the estuary within the past several thousand years.

Here, we focus on a 20 km stretch of the river spanning the Tappan Zee Bridge crossing. In this area, wide (~ 3 km) shallow mudflats are found within the western portion of the estuary. These are occupied by oyster beds which thrived in this region within two distinct time periods: from the recent past to 2000-3000 years BP and prior to ~ 5-6000 years (uncorrected 14C dates). Radiocarbon data indicate that both of these time periods were characterized by low sedimentation rates (1-2 mm/yr) with higher rates of sedimentation during the intervening time period. The transition to the most recent low sedimentation period coincides with a sub-bottom horizon that can be mapped beneath extensive portions of the western mudflat sediments.

Sediments of the main channel stratigraphically overlie the west flat sediments indicating modern channel sedimentation followed the era of oyster bed formation. In places, the mudflats have been eroded by the modern channel, with remnants of mudflat sediments found as isolated peaks within the channel. At present this stretch of the estuary appears to be in morphological equilibrium with current sediment load, with little recent sedimentation occurring except in regions of anthropogenic activity (dredging) or obstructions (bridges and piers).