Muddy Waters: Sediments Pose Threat
to the Bay
Satellite photo from September 2011. Source: National Aeronautics and Space Administration.
Source: NASA

THE MUDDY-BROWN WATER in the satellite photo of the Chesapeake Bay, taken from 400 miles up, tells a story about the health of the Bay in 2011 and, perhaps, future years.

The photo was taken earlier this year a few days after a pair of troublesome tropical storms, Irene and Lee, walloped the mid-Atlantic within a two-week period in late August and early September. In the photo, the long smear of brown extends in a thin pencil down the Susquehanna River in Pennsylvania. The stain fills the upper Bay and continues south some 70 miles before the brown gives way to blue.

The two storms brought record amounts of rainfall to parts of the Chesapeake's watershed, and the Susquehanna reached its third-highest flow on record. That deluge in turn brought millions of tons of suspended sediment, turning the water the color of cafe au lait and threatening the Bay's plants and animals.

This fall, scientists and managers traveled out on the water to take measurements and try to gauge the storms' effects. Although the full impact may not be known until summer 2012 or later, in this issue of Chesapeake Quarterly in an article titled, "Big Year for Storms, Bad Year for Bay Sediment?" we present a number of their preliminary findings, some of which are surprising and cause for cautious optimism.

People cannot control the path and timing of storms (although climate change is predicted to increase the intensity of Atlantic storms and hurricanes in coming years). But people can — and already have — influenced the quantities of sediments and other water-quality contaminants flowing down the Susquehanna River into the Bay every day during normal weather conditions. In considering the effect of this year's storms, we were struck by a threat to the Bay's ecology that has grown for years but until recently has been largely ignored: the massive buildup of sediments behind three hydroelectric dams on the lower Susquehanna River. "Countdown for the Conowingo" looks at what this might mean for the Chesapeake.

The southernmost dam, the Conowingo, is the only one of the three with capacity remaining, and that space is forecast to run out within as few as 15 years. After that, the amount of sediment flowing down the river could more than double — placing a big burden on the Bay's ecology and jeopardizing plans to reduce sediments under the Bay's new pollution "diet." A new study of the dam begun this year may lead to solutions, but they could prove difficult and expensive.

Although the Conowingo Dam is especially important in the flow of sediment to the Bay, other sources have also attracted attention. The third article in this issue, "Those Dammed Old Rivers," explores the effects of removing dams on sediment flows along rivers and creeks smaller than the Susquehanna. Fishing groups and environmentalists have been pushing for dam removals to free up fish passage and restore the spawning runs of native species. Those smaller dams, however, may have collectively played a big role in holding back sediments from reaching the Bay. When many of those dams are removed, what happens to all the sediment they held back?

Considered from a very long perspective — the passage of time in geologic periods — humans' ability to control sediment is limited. In fact, over a period of millions of years, the Chesapeake will inexorably fill up with silt. This is a result of natural processes that would occur even if human activities, like construction and agriculture, were not adding sediment to the Bay. The Susquehanna carved out the current extent of the Chesapeake over millennia; when the most-recent ice age ended, glaciers retreated, sea level rose, and the Bay filled with water. Over time, sedimentation will fill in the Bay, creating a narrow river valley of the Susquehanna.

But a long time scale offers us small comfort today — nor excuses. The continuing challenge for us during our constrained lifetimes is to reduce sedimentation and other pollutants by enough that we improve the health of the Bay's ecology.

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