Response to Channel Deepening of the Salinity Intrusion, Estuarine Circulation, and Stratification in an Urbanized Estuary

TLDR

Navigation modifications since the late 1800s have increased channel depth in the lower Hudson River estuary by 10–30%, more than doubling at the mouth. Observations and models show that channel deepening has raised salinity intrusion by ~30 % and stratification over the past century, yet the estuarine circulation remains largely unchanged, the salinity increase threatens drinking water supplies more than 100 km inland, and the overall parameter shift is modest compared to tidal and discharge variability.

Abstract

Abstract Modifications for navigation since the late 1800s have increased channel depth ( H ) in the lower Hudson River estuary by 10–30%, and at the mouth the depth has more than doubled. Observations along the lower estuary show that both salinity and stratification have increased over the past century. Model results comparing predredging bathymetry from the 1860s with modern conditions indicate an increase in the salinity intrusion of about 30%, which is roughly consistent with the H 5/3 scaling expected from theory for salt flux dominated by steady exchange. While modifications including a recent deepening project have been concentrated near the mouth, the changes increase salinity and threaten drinking water supplies more than 100 km landward. The deepening has not changed the responses to river discharge ( Q r ) of the salinity intrusion (~ Q r −1/3 ) or mean stratification ( Q r 2/3 ). Surprisingly, the increase in salinity intrusion with channel deepening results in almost no change in the estuarine circulation. This contrasts sharply with local scaling based on local dynamics of an H 2 dependence, but it is consistent with a steady state salt balance that allows scaling of the estuarine circulation based on external forcing factors and is independent of depth. In contrast, the observed and modeled increases in stratification are opposite of expectations from the steady state balance, which could be due to reduction in mixing with loss of shallow subtidal regions. Overall, the mean shift in estuarine parameter space due to channel deepening has been modest compared with the monthly‐to‐seasonal variability due to tides and river discharge.