Uplift stages are typically marked by successively elevated terraces. Each terrace was once a wave-cut bench below a sea cliff, indicating a brief pause for a steadily rising coastline. The coastal terrain is riddled with landslides, an outcome accentuated by occasional earthquake shaking and by the infrequent cloudbursts. The problem is apparent, but identification of the most vulnerable slopes requires the careful scrutiny of engineering geologists.
The present site of the southeastern seashore of the United States stood well above sea level during the most recent continental glaciations. Rivers entering the sea did so farther to the east, through valleys incised into the coastal plain. As the ice sheets melted, the sea level rose and flooded the valleys, forming estuaries such as the Chesapeake Bay. The Patapsco, Potomac, and James rivers were all once tributaries of a Susquehanna River that flowed out past the present bay and formed a delta on what is now the continental shelf. As drastic as such changes might seem, they are relatively recent, having occurred over the past 10,000 years, and have cycled back and forth as ice sheets waxed and waned repeatedly over the past few hundreds of thousands of years.
Within the past 10,000 years of high sea level stand, currents and wave action along the shores have piled up barrier beaches such as those near Cape Hatteras, protecting shallow lagoons on the landward side. These are ephemeral and fragile creations, even without human dredging, building, and destruction of the vegetative cover. The lagoons and barrier islands record a complex history of deposition, erosion, and redeposition. A 1-m variation in sea level will change the sea-land interface substantially. In areas such as the Texas Gulf coast, where a thick section of sediments is gradually consolidating, scientists anticipate that shoreline features will be severely affected, probably with major losses of valuable property (Figure 5.8). Proposals to recover energy from geopressured fluids from southeast Texas aquifers would almost surely result in further subsidence of the surface and encroachment of the gulf on present-day shores.
As rivers erode the land, deltaic and estuarine environments become the first sites of major sediment deposition—except for constructed reservoirs and rare natural lakes. These brackish tidal waters are nursery grounds for many marine organisms as well as invitations for the establishment of human populations because of access to those nursery grounds, to transportation routes, and to freshwater sources just up river. Many, if not most, of the major estuaries of the country have become contaminated by natural or human-introduced pollutants. Boston Harbor is, unfortunately, an example of a contaminated estuary. The fish populations have declined to the point of disappearance, and the beaches are nearly deserted throughout the summer because of pollution. Chesapeake Bay pollution, first detected by the Public Health Service before World War I, has contributed to a decline of striped bass. Pollution of estuaries has an extremely deleterious effect on fish spawning, and encroachment often leads the way in destroying the estuarine habitat, the eventual result being diminution of potential food supplies.
Deltas, barrier islands, lagoons, tidal flats, and estuaries support a number of dynamic physical, chemical, and biological processes that operate in complex association. Although scientific understanding encourages informed management, only a few of the world's populated coastal environments have been studied in the required detail. The United States has engaged in a major effort in coastal-zone planning, but there has been no systematic effort at data collection to determine either baseline information or trends in essentials such as estuarine quality. Study of estuaries and other coastal environments should prove most productive when approached by interdisciplinary teams that can appreciate the complexities of the physical, chemical, and biological processes. The sensitivity and significance of coastal processes warrant a systematic integration of biological, sedimentological, hydrological, and geochemical data collection and analysis. Such an effort should result in a product that will inform decision makers in the near future.
Hazards such as landslides, subsidence, and floods are often exacerbated by human activities. But they are also often triggered by violent tectonic upheavals—earthquakes or volcanoes, or by the former's menacing effects, tsunamis. There is nothing that society can do at present to prevent tectonic upheavals, but there are methods to monitor, predict, mitigate, and avoid potential tectonic disasters (Figure 5.9 ). In the United States, progress in these strategies has involved many geologists over the past 20 years because of the need to protect population centers from threats such as the 1980 Mount St. Helen's eruption and the 1989 Loma Prieta earthquake. Scientists and policy makers know that