HUMAN DISTURBANCE AND SEDIMENTARY RESPONSE OF AN ESTUARINE BASIN

The Delaware River estuary provides an informative case study of changes in sedimentation conditions related to waterway engineering practices during historical times. Construction of the federal shipping channel in the early 1900’s had well-known impacts on estuarine circulation, including increasing the tidal prism and causing the salt limit to intrude farther landward. The present study was undertaken to document sedimentary and morphologic consequences of these hydraulic changes, focusing on the subtidal zone of the basin. Analysis of historical bathymetry indicates that the subtidal basin shifted from net accretionary to net erosional in the 1930s when the axial shipping channel was deepened to a uniform depth, and when a program of annual maintenance dredging was implemented. Sediment deposition from suspension (and the need for dredging) is greatest at the head of the estuary on account of convergent circulation and sediment transport. Dredge spoils are disposed of shoreside, not reintroduced to the system, greatly reducing the down-estuary flux of suspended sediment. On average the mass of sediment dredged exceeds the annual suspended load derived from river tributaries, thus a large amount of fine sediment is sourced from within the estuarine basin. Historical changes in basin hypsometry manifest widespread bottom erosion; the mean depth of the estuary floor has increased from a depth of 6 m in 1848 to its current depth of 8 m. Although erosion rates have decreased in recent decades, the areal extent of erosion continues to outweigh that of accretion by more than a factor of three.

We contend that maintenance dredging at the head of the estuary has deprived the system of fine sediment, causing previously accreting bottom areas to erode. Specifically, the diminished mobile sediment pool appears to have reduced tidal depositional fluxes to below the critical limit for net accretion. Another factor is vessel-generated waves, which expose the bed to unnaturally high shear stresses and give rise to resuspension and erosion on the long term. The net erosional state of the estuary has major implications its stability and the ecosystem it supports.