Estuaries: Where the River Meets the Sea

By Boicourt, William C. | Oceanus, Summer 1993 | Go to article overview

Estuaries: Where the River Meets the Sea


Boicourt, William C., Oceanus


As the world's population has grown and moved toward the sea, the oceanic effects of this progression have been felt first and most acutely in the estuary. Perhaps this should come as no surprise, for the estuary is where the river, with all its waterborne materials draining off the altered landscape, meets the sea. But there is more than proximity at work here. The estuary is nearly a world unto itself, buffered from a strong marine influence by a controlled communication with the ocean, and protected by enclosing coastal boundaries. Within this domain, the estuary's unique water motion retains and recycles nutrients essential to living organisms, inducing the richest productivity per square kilometer on the earth's surface. Labeled the "protein factory" by writer H.L. Mencken, an estuary such as Chesapeake Bay is made bountiful through the workings of physical, chemical, and biological engines in complex and sometimes mysterious balance. We are learning that these balances can be very delicate, and that humans can tilt and dramatically shift the outcome, usually to their own detriment. We are also learning that the estuary forms only a fragile line of defense for the coastal ocean.

The gradual rise in sea level that created estuaries from ancient river valleys, glaciated grooves in the landscape, or faults in the earth's crust also produced natural harbors. These were fortuitously situated where maritime commerce could connect to the hinterlands via river highways. Cities grew, creating "urban estuaries" threatened not only by unnatural amounts of nutrients, sediments, and toxic materials drained from the watershed, but also by the concentration of human activity on the edge of major fish and waterfowl habitats. These estuarine habitats support both native and transient populations. Because their shallow marshes and seagrass beds serve as spawning grounds and nurseries for mobile fish as well as havens for migrating birds, their influence extends far beyond the confines of the local estuary.

A quick tour of the coastal US reveals how many of the larger estuaries have become urban. Starting in the northeast, the large estuaries of the Gulf of Maine, such as Penobscot Bay, are relatively rural. Boston Harbor represents both an urban water body and an environmental cause celebre. It is, however, just barely an estuary. Narragansett Bay, not far to the south, is a genuine estuary flowing in a series of glacial grooves, loosely separated by islands, with the city of Providence located at its head. The rivers that drive Long Island Sound's estuarine circulation discharge along its broad northern flank. At its western end, Long Island Sound is connected to the most urban of estuaries, the Hudson River, via the East River. The Hudson River and most of the larger estuaries to the south are river valleys drowned by the rise in sea level that followed the last ice age.

From Delaware Bay south, the river valleys were cut from low-lying, coastal-plain sediments. Delaware Bay seems placidly nonurban in its lower reaches, where it opens like the bell of a trumpet toward the sea. Extensive salt marshes border the broadening estuary. Immediately upstream, the Delaware River connects the Bay to Wilmington, Philadelphia, and Trenton. In Chesapeake Bay, the largest estuary in the coastal US, there are locales where the view in all directions is nearly as unspoiled as in the days when Captain John Smith first set eyes on these waters. But the bay and its tributaries reach to Baltimore, Washington, Norfolk, and nearly to Richmond. The bay is so large that it is not just one estuary, but a 300-kilometer-long backbone connecting a series of tributaries such as the Potomac, Rappahannock, York, and James rivers, each robust estuaries in their own right. South of Chesapeake Bay, the broad North Carolina sounds and the estuaries and tidal marshes of the South Atlantic Bight are comparatively nonurban. The estuaries of Wilmington (North Carolina), Charleston (South Carolina), Savannah (Georgia), and Jacksonville (Florida) are modest exceptions. …

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