How much Susquehanna River debris could have passed through Conowingo Dam floodgates?

In the days after record July rainfall filled Chesapeake Bay waterways with debris, Comptroller Peter Franchot said Maryland was "literally drowning in Pennsylvania’s trash."

How much of the detritus actually came down the Susquehanna River and through the Conowingo Dam’s opened floodgates?

It's impossible to quantify, but one scientist who has studied the effects of major rainstorms on the bay said it could have been significant. The Susquehanna drains an area that makes up 43 percent of the Chesapeake watershed.

As for any debris already built up behind the dam that might have been released when the gates opened, officials at Exelon Corp. say they had recently cleared the filters that protect the Conowingo’s 11 hydroelectric turbines.

And photos Exelon officials shot show a significant load of branches, tires and plastic barrels that the dam caught, and remained behind it when the flooding subsided.

Larry Sanford, a professor at the University of Maryland Center for Environmental Science, said the Susquehanna is a dominant influence on the Maryland portion of the Chesapeake. The river contributes about 90 percent of the upper bay’s fresh water — far more the half commonly thought, Sanford said.

The Susquehanna watershed stretches 27,500 square miles across parts of New York, Pennsylvania and Maryland — by far the largest fragment of the Chesapeake’s 64,000-square-mile drainage area. So it easily could have been carrying mountains of debris as dam owner Exelon was required to open the Conowingo’s gates to prevent flooding.

“It’s flowing really fast because it’s got a huge watershed and there’s a big elevation drop,” Sanford said.

Some of the Susquehanna debris could have made it into bay tributaries such as the Severn River and even the Annapolis harbor on the mouth of Spa Creek, he said. When a surge of fresh water enters the bay from the Susquehanna, the flow of such smaller, brackish tidal rivers actually reverses.

The record July rainfall, more than 16 inches in July at Baltimore-Washington International Thurgood Marshall Airport, resulted in one of the biggest surges of stormwater into the bay in years, Sanford added.

The more time in between such large rain events, the larger the amount of built-up debris that can be washed into waterways, he said.

Exelon officials said they have removed 600 tons of debris from the dam so far this year, much of it built up along its western half, the Harford County side of the dam. And debris in that area “generally does not pass through [the dam] in flood conditions,” said Kristen Otterness, a spokeswoman for Exelon Generation.

A photo of that portion of the dam taken July 24, which Exelon provided to The Baltimore Sun, shows a relatively small buildup of branches, foam and plastic. That same day, 4 inches of rain fell at BWI, forcing the Conowingo gates to open over the following days.

A photo of the same area taken two weeks later shows the same field of debris — only multiplied, stretching across the river.

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