Floodwaters from the remnants of Hurricane Ivan sent tree branches, road barricades and tires—and a potentially large load of sediment—down the Susquehanna River and into the upper Chesapeake, creating hazards for boaters and raising concerns about environmental impacts.

According to the U.S. Geological Survey, river flows on Sept. 19 and 20 were strong enough to scour sediments stored behind the Conowingo Dam and flush them into the Bay—the first time that has happened since 1996. USGS scientists said river flow reached a peak of 606,000 cubic feet per second and remained above 400,000 cfs from about two full days.

Normally, the 100-foot-high Conowingo Dam traps abut 4.4 billion pounds of sediment a year—more than two-thirds of what flows down the river. The dam also traps about two-fifths of the phosphorus flowing toward the Bay.

But when river flows rise above 400,000 cfs, it scours the sediment built up behind the dam and flushes it down the river and into the Bay, where it can smother underwater grass beds, oyster bars and other bottom habitats.

It was too early for scientists to assess the impact, but because the sediment hit the area at the end of the growing season, the hope was that the impact would be less severe than if it had occurred in the spring or summer when biological activity is at its peak.

“It’s never good to have something like this happen, but it’s likely to have less of an effect now,” said Rich Batiuk, associate director for science with the EPA’s Bay Program Office.

Of particular concern was whether the sediment would impact extensive grass beds in the Susquehanna flats at the mouth of the river, which had been rebounding this year. One scientist measuring water quality in the area after the discharge described the amount of sediment in the water as “astounding.”

The last time sediment was scoured from behind the dam was in early 1996, when heavy rains coupled with rapid snowmelt led to scouring for 16 consecutive days, with flows exceeding 900,000 cfs. That event flushed about 11.8 million tons of sediment from behind the dam.

In September, as remnants of Hurricane Ivan passed over Pennsylvania and New York, it caused flooding along the Susquehanna and flushed huge amounts of debris into the Bay.

Marine authorities cautioned that uprooted trees and debris that raced through the floodgates at Conowingo Dam would likely linger for weeks in the Chesapeake Bay. The Coast Guard is broadcasting warnings three times a day, cautioning boaters to be on the lookout.

“There’s all this stuff in the water,” said Mark Huppert, a fisherman who had to navigate his way around tree limbs and floating barrels near Pooles Island. “You definitely could do some serious damage to your boat if you didn't watch out.”

The Associated Press contributed to this report.