The large 1993 freshet that sent the largest amount of fresh water down the Susquehanna since Tropical Storm Agnes in 1972 failed to "scour" the huge amount of sediments behind the Conowingo Dam.
In fact, figures from the U.S. Geological Survey and the Susquehanna River Basin Commission indicate that - despite the large flows - slightly more material was accumulated in the reservoir behind the dam than was washed out during 1993.
"All the material that was there is still there, and probably a little bit more," said Lloyd Reed, a hydrologist with the U.S. Geological Survey.
After reviewing surveys of bottom sediments, Reed concluded that while sediments at the bottom of the reservoir shifted around "like drifting snow" during the heavy flows, no significant amount was washed out.
The observation is backed by efforts to estimate sediment movement above and below the dam. The Susquehanna River Basin Commission, which monitors water at Marietta, Pa., north of Conowingo, estimated that about 5 million tons of sediment washed down the river in the 1993 "water year" which ran from Oct. 1, 1992 through Sept. 30, 1993.
Estimates taken by USGS at the Conowingo dam indicate that about 3 million tons of sediment passed through the dam. That means about 2 million tons were trapped.
The finding indicates that higher flows than previously thought may be required to scour sediments which have been accumulating for decades behind the large dam.
Before, it was thought that river flows of about 400,000 cubic feet per second was enough to begin eroding the accumulated sediments. But flows during last year's freshet peaked at 500,000 cfs, and surveys of bottom sediments indicate little change from before and after the freshet, Reed said.
"They didn't hit the level they needed for a significant scour," he said. "Apparently it's going to take a little bit more flow than that to get it to scour."
The issue is significant because the reservoir behind the 100-foot-high dam is thought to be nearing the point of "equilibrium" where as much sediment would be washed out of the reservoir as flows into it. When that happens, large amounts of nutrient and sediments washed down the river that are now trapped behind the dam will begin flowing straight to the Bay, adversely impacting water quality.
In an average year, about 9.1 million pounds of phosphorus wash down the Susquehanna, but 4 million pounds are trapped in the Conowingo reservoir. The dam also traps most of the 3.7 million tons of sediment that wash down the river, allowing an average of only 890,000 tons to reach the Bay. Only a small amount of nitrogen is trapped because, unlike phosphorus which tends to bind with sediment and become trapped, nitrogen is more water soluble.
The implication is that the actual amount of phosphorus reaching the Bay could eventually increase despite Pennsylvania's nutrient reduction effort. To meet the Bay Program goal of reducing nutrients that reach the Bay by 40 percent by the year 2000, Pennsylvania is trying to reduce the amount of phosphorus flowing down the river by 2.2 million pounds a year. That fi gure, though, is less than what is now being trapped by the dam.
Pennsylvania's draft nutrient reduction strategy for the Susquehanna suggests that options for managing sediments trapped behind the dams be explored, but warned that any action would be complex and expensive.
Phosphorus spurs the growth of algae in areas of the Bay with fresh or low-salinity water. That algae blocks sunlight to important Bay grasses and can reduce the amount of oxygen in the water.
Sediment has already reached equilibrium in the reservoirs behind two upstream dams, Holtwood and Safe Harbor. The Conowingo reservoir has a sediment capacity of about 200 million tons and is about three-quarters full, according to USGS estimates Reed said it was difficult to say when the Conowingo reservoir would reach equilibrium, but that it would likely be in the next two decades, barring any Agnes-like storms. "It's obviously still trapping sediment," he said.
Flows during Tropical Storm Agnes in 1972 exceeded 1 million cfs, scouring about 22.6 million tons of sediment from behind the Conowingo Dam, Reed said. Scouring of sediment from behind the dam likely begins with flows of 500,000 cfs, and increases as the flows become greater, he said.
Last year's freshet sent 6.1 trillion gallons of water down the Bay - more than during Agnes - but last year's high flows were spread over a 41-day period compared to 16 days for Agnes, so peak flows were not as high as those in the 1972 storm.