Important Unsolved Problems

Available observations provide us with information about all the major processes occurring in the estuary. The Hudson, however, is both spatially diverse and temporally variable so without adequate spatial and temporal observations, the integrated behavior of sediment in the estuary remains elusive. As a result, managers often find answers to their questions unsatisfying and inadequate. For example, a sediment budget for the estuary requires deposition rates in the various substrates on the estuary floor. These facies have not been mapped in detail, although a current effort by the New York State Department of Environmental Conservation is moving in that direction (see chapter by Bell et al., this volume). Even when they are, deposition rates have been determined in only a few locations, so large uncertainties will remain in the amount of sediment deposited.

' the interval used for contouring: 10 units (i.<

Figure 4.6. Axial distribution of salt (a) and suspended sediment (b) under low discharge (September, 1995). The salinity contour interval is one part per thousand; the suspended sediment concentration contour interval is 10 mg L-1. The Battery is at kilometer 31 on this scale.

' the interval used for contouring: 10 units (i.<

Figure 4.6. Axial distribution of salt (a) and suspended sediment (b) under low discharge (September, 1995). The salinity contour interval is one part per thousand; the suspended sediment concentration contour interval is 10 mg L-1. The Battery is at kilometer 31 on this scale.

As we have seen, even straightforward questions, like how much sediment is brought into the estuary by rivers, are answered only within fairly broad ranges. Monitoring has not been continuous. Storm events are hostile to any measurement program and easy to miss. Most of the tributaries are not monitored, forcing reliance on indirect calculations.

The amount of resuspension poses similar difficulties. Because of the wide variety of variables that influence resuspension rates, the only way to know them are to measure them directly (Bokuniewicz, McTiernan, and Davis, 1991). Very few measurements have been made and no attempt has been made to measure spatial or temporal variability. As a result, resuspension can only be quantified in the most general terms. An oceanic source also seems likely, but what is its magnitude? How does it vary with changes in discharge? Could it be that the oceanic source buffers the system? Even without increases in the fluvial sources, increased dredging may result in an increased deposition of sediments of marine origin.

Likewise, the broad outlines for the estuarine turbidity structure in the Hudson are known but many questions remain concerning the mechanisms and dynamics. Predictive models of the suspended sediment transport will remain elusive. An important question on the road to these answers are the degree and mechanism of lateral transport in the estuary. Some workhas been done on this issue but its significance remains to be integrated into a more comprehensive conceptual model of the estuarine sedimentary system.

In the future of estuarine research, in general, there needs to be attention given to comparisons between estuaries. Thirty years ago, Emery and Uchupi (1972) pointed out that far more effort has gone into making detailed studies of sediments of individual estuaries than into either comparing results from a variety of estuaries with similar physical or geological characteristics, or into critical evaluation of processes. Their observation still holds true today. Schemes for classifying the hydraulic regimes of estuaries have been developed and widely used but little effort has been devoted to comparing estuarine sedimentary systems. Holistic comparative studies are needed to better understand those sedimentary processes that characterize estuaries. The Hudson-Raritan estuarine system invites comparisons with a wide variety of other local systems such as Narrangansett Bay, Long Island Sound, Peconic Bay, and the Connecticut River estuary. Of these, the Hudson system is probably the most heavily impacted by human activities. Anthropogenic effects add an additional complicating factor that is not present in less urbanized estuaries.

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