Rising water, increasing sediment

ON THE BAY

March 27, 1993|By TOM HORTON

Basic to the nature of estuaries like Chesapeake Bay is their variability.

From its geologic history to its life forms, currents and sediments, the bay displays rich overlays of comings and goings that help make the place so fascinating.

For example, perhaps a dozen different Chesapeakes have existed during the last several million years. As ice ages wax and wane, the oceans are alternately bound up and released from the polar ice; sea level rises and falls by several hundred feet.

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Only in the warmest parts of the short periods -- tens of thousands of years -- between glaciers do the seas gorge the coastal river valleys like the one the Susquehanna River has cut through our region.

During those brief geological summers, estuaries swell and blossom within the nooks and crannies of the continental fringes like rare flowers, only to wither with the onset of the next glaciation. Through most of time, bays like the Chesapeake aren't even here.

Other comings and goings that we can see range from the twice-daily tidal cycles to wet and dry years that can make the bay fresher or saltier and dramatically affect oxygen levels and the distribution of fish.

One thing that until recently seemed predictable, however, was our current bay's eventual fate: It would, during the next several thousand years, fill in with sediment washing down its rivers from the land, and become a marsh, then dry land. That has been the route for all previous Chesapeakes.

But is it possible humans have changed the equation? Large areas of marshes in places like Dorchester County are clearly going underwater. Comparisons of aerial photos taken in the 1930s and 1980s confirm that thousands of acres at Blackwater National Wildlife Refuge and along the Nanticoke River are submerging. Residents in areas like southern Dorchester county have begun to bulldoze humps in their driveways on which to park during high tides.

And, University of Maryland researchers think there has been a significant leap in the last century in the rate at which many of the bay's islands have been eroding away. And in areas like Norfolk and Solomons, Md., geological surveys have found "hot spots of depression," where deepening is occurring at the rate of nearly three feet per century.

Possible causes for all this run the gamut. Global warming, caused by human pollution, may be making sea levels rise faster. Or, the land around the bay and the bay bottom in places may be subsiding. Why? Some scientists wonder whether pumping too much ground water for industrial and residential use is a cause; others say it is possible that the sheer weight of sediment thathas accumulated on the bay's bottom is depressing the floor of the estuary, and the edges of land around it.

So we have clear evidence that the water is rising, the bay deepening; yet abundant evidence also that the Chesapeake is filling in. Colonial seaports like Port Tobacco in Charles County and Joppatowne in Harford County now are no longer even on navigable waters. Records show that water depth at the Hanover Street Bridge in South Baltimore went from 17 feet in 1845 to six inches by 1923. (It is now kept deep by dredging.)

Most of the marshes that seem such timeless landscapes along the bay's western shore tributaries were not even there when John Smith explored the bay in 1608 -- their growth was, in effect, "fertilized" by all the sediments washing down from the clearing of forests during succeeding centuries.

What's the bay's bottom line? "No one has ever done a careful mapping of where things are eroding and where they are filling," says Court Stevenson, a University of Maryland scientist who has studied the impacts on marshes of sea-level rise.

Even the movement of sediment, into and within the estuary, is not well understood. Many scientists, for example, are convinced that the bayactually sucks lots of sediment in from the ocean, meaning that a great, sandy delta will appear someday in the southern Chesapeake.

At the bay's other end, the Susquehanna River, source of half the Chesapeake's fresh water, carries more than 3 million tons of sediment downstream in a year of average rainfall. In the 18th and 19th centuries, this helped create extensive rice marshes that made the upper bay's Susquehanna Flats famous for ducks. But large hydro dams erected in the 20th century began to trap about two-thirds of all the riverborne sediment. The dams now are reaching the point at which they can't trap any more sediment in their reservoirs, and will resume passing it all downstream, presumably filling in the Flats again. Unless, of course, we get another mammoth storm such as Agnes in 1972, which in a week blasted nearly 50 million tons of trapped sediment from behind the dams.

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