Project Outcomes Report

Estuaries are environments where natural processes and human activities are strongly interconnected.  The natural processes of estuaries have positive and negative impacts on human welfare, and human alterations can ameliorate or exacerbate the interactions between nature and society.  This study examined the natural processes of estuaries in context with alterations by human activities, most notably the changes in estuarine morphology by dredging, with the goal of determining how these changes affect the environmental services and hazards.  Two case studies were examined?Delaware Bay, where rapid shoreline erosion is threatening the sustainability of the coastal environment, and the New York Harbor/Hudson River estuarine network, where centuries of human modifications have altered the estuarine environment in ways that are only beginning to be recognized.  The field methodologies involved a combination of ship-based surveys, instrumented mooring deployments, and land-based marsh surveys.  These were complemented by realistic numerical modeling studies of the two estuarine systems, in order to examine the influence of historical changes in the bathymetry that have altered the physical processes and the ecosystem functions of these environments. 

Not surprisingly, we found that dredging has significantly affected the natural function of these estuaries, but some of the results were unexpected.  In the unexpected category, we did not find that dredging makes these estuaries more vulnerable to flooding.  On the contrary, we found that while the tides have gotten larger due to the deepening of channels in the upper Hudson tidal river (extending to Albany, NY), it is significantly less prone to flooding associated with river discharge events.  In the Delaware, we found that wave-induced shoreline erosion, which is the main agent of shoreline retreat, is not particularly sensitive to the presence of dredged channels. 

It is possible that shoreline loss along Delaware Bay is largely a natural adjustment to sea-level rise. This adjustment can lead to changes in other important ecosystem services, however, including carbon sequestration in salt marshes.  The tidal wetlands of the Delaware are currently being lost at a rate of about one square kilometer each year, mostly to open water and woody wetlands, but also to encroaching human development.  Depending upon assumptions about the rate of remineralization of carbon, each lost square kilometer constitutes a conservatively estimated ecosystem service loss of $5 million, and the future accumulation of economic losses at this rate would approach $0.5 billion for scenarios of rapid sea-level rise.

Another important influence of dredging is its influence on salinity.  All estuaries have salt water that slides under the outgoing fresh water from river flow, and its position continually shifts with the river flow and tides. These interactions between salt and fresh water are particularly sensitive to the variations in estuarine depth?where the estuary is deep the salt water can readily extend up the estuary, but where it is shallow the mixing between fresh and salt water retards its advance.  We expect that as estuaries become deeper, the salt water should extend farther into estuaries, and that is exactly what we found in both estuaries, by examining historical data on salinity and with numerical modeling of the changes in bathymetry.  In the Delaware, this extension is likely to raise the costs of water treatment in electric generating facilities, especially those utilizing evaporative cooling towers, due to increased salinity in water withdrawn from the Delaware because of a deepened channel and sea-level rise.  The increased costs of water treatment for the Hope Creek nuclear power plant alone would reduce the estimated net benefits of the Delaware channel deepening project by as much as seven percent.

In the Hudson, we found that during low river flow conditions, the salt intrusion now extends more than 30 km farther up the Hudson than it did in the pre-dredging conditions of the mid-1800?s.  This has serious consequences for the city of Poughkeepsie, which gets its water from the Hudson at a location that used to be fresh even during the most extreme droughts, but now becomes saline during moderate to severe droughts.  This change is not due to climate change?it is due to dredging.  The effect of dredging the lower estuary is undeniably far reaching.  Existing policies to respond to elevated salinity at Poughkeepsie?s water intake on the Hudson would require the drawdown of a reservoir in New York?s Adirondack State Park, releasing water to lower the salinity downstream, and leading to significant losses in the generation of renewable energy by hydropower at the reservoir upstream.

Our research has provided tangible results that illustrate the influence of human activities on natural processes of particular importance to society.  The value of this research comes from our increased ability to understand the causes of these environmental changes and to develop more effective methods of predicting them and mitigating their negative consequences.

Last Modified: 01/09/2020
Modified by: George R Parsons