By Karl Blankenship, Chesapeake Bay Journal

Since the early 1990s, scientists have warned that the Conowingo Dam loomed as an ominous threat to the Chesapeake. When the reservoir behind the massive 100-foot dam filled, more sediment and nutrients would begin pouring down the Susquehanna River.

For nearly as long, dealing with the issue has been largely put off; the reservoir issue has always been considered a problem for the future.

But the future may be here, according to new research.

“It’s not a decade out,” said Bob Hirsch, a research hydrologist with the U.S. Geological Survey. “It’s now.”

To be sure, the giant dam, located near the Maryland-Pennsylvania border 10 miles upstream from the river’s mouth, is still trapping much of what washes down the Bay’s largest tributary. But it appears to be trapping less than it used to, particularly during high flows, according to Hirsch.

Hirsch’s analyses, presented at a recent Chesapeake Bay Program scientific meeting, suggests that more sediment and phosphorus have been reaching the Chesapeake over the last decade as the reservoir behind the dam has neared its storage capacity.

If correct, that may means that portions of Pennsylvania, New York and Maryland upstream from the dam may need to take additional steps to control sediment and phosphorus runoff to achieve the Chesapeake Bay Total Maximum Daily Load, or “pollution diet.” The TMDL established the maximum amount of nutrients and sediment that can enter the Bay from each major tributary.

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6/4/2012