The flood of January 1996 bought time for the Bay states to defuse a time bomb that threatens much of the Chesapeake restoration effort, according to a new U.S. Geological Survey report.

The “bomb” is the huge buildup of sediment and phosphorus in reservoirs behind a series of large dams on the lower Susquehanna River.

Those dams trap two-thirds of the sediment and two-fifths of the phosphorus that flow down the river in an average year.

But once the dams filled, all that material will reach the Bay. As a result, according to the USGS, the amount of phosphorus reaching the Bay from its largest tributary could increase by 70 percent, while sediment discharge would jump by 250 percent a year.

That “bomb” would hit the Bay sometime between 2010 and 2015, according to a recent USGS report that examined the issue.

The report doesn’t suggest what to do about the problem. “We were just trying to raise the issue of how fast the reservoirs might fill, so solutions can be formulated,” said Scott Phillips, who coordinates USGS Chesapeake Bay activities.

According to the report, is the new deadline for finding a solution is about six years later than what was earlier thought. This is because strong river flows accompanying the January 1996 flood “scoured” about six years of sediment buildup from behind the dams, according to the USGS.

Officials will very likely need all the time they can get. Right now, there are no plans to defuse the problem.

“No one has taken it seriously, and the Bay Program is going to have to,” said Bill Matuszeski, director of the EPA’s Chesapeake Bay Program Office. “It’s that simple.”

If nothing is done, one of two things will happen. The reservoirs could fill to the point where they no longer trap sediment and phosphorus, unleashing millions of pounds of additional dirt and nutrients into the Bay year.

Or, a dramatic event like Hurricane Agnes could excavate much of the reservoirs’ sediment in one dramatic event that smothers thousands of acres of downstream clam, oyster and crab habitat and grass beds with one giant surge of sediment, as happened in 1972.

“Those are two pretty unacceptable options,” Matuszeski said.

There are three major dams between Harrisburg and the Bay. But the reservoirs behind the two upper dams, Safe Harbor and Holtwood, are already filled, according to the USGS. Only the largest of the dams, Conowingo — located just south of the state line in Maryland — has any significant sediment trapping capacity left.

A 1995 report by the Susquehanna River Basin Commission that studied the issue found no easy options for dealing with the sediment buildup.

The SRBC estimated that about 2.3 million cubic yards of sediment were trapped behind the Conowingo Dam each year. Just to keep up with that rate of deposition, the report said, would require dredging enough sediment to fill about a hundred, 100-ton railroad cars every work day of the year.

“However one looks at the quantities involved, the magnitude of the task if formidable,” the report said.

Such dredging would cost about $28 million a year, the report said. That’s nearly three times what the Bay Program annually gives all three Bay States combined to control runoff.

And that would only to keep up with what is coming in every year. Excavating what has been built up since the Conowingo Dam was completed in 1928, the SRBC report said, “requires resources well beyond present technological capabilities.”

The report went on to say that any dredging could also pose a variety of adverse environmental impacts, such as the potential of releasing toxics trapped in the sediment. In addition, removing just the annual amount of deposition — which amounts to about half of what is dredged to maintain Baltimore Harbor shipping lanes each year — would require a huge disposal site for the sediment.

While the issue is sorted out, the SRBC report recommended that efforts to control sediment runoff from the land be stepped up — something that could at least slow the rate at which the reservoir fills.

Pennsylvania’s 1996 nutrient reduction strategy agreed that was the best short-term action. It called for the expanded use of structural devices and management practices that help stem sediment runoff, primarily from agricultural areas.

Noting that “solutions to this problem of the Susquehanna River dams may be complex and expensive,” the strategy added that a variety of interests would have to participate in any long-term strategy, such as the Federal Energy Regulatory Commission, which regulates the utilities; the Army Corps of Engineers, which handles dredging of waterways; and the state of Maryland, where the Conowingo Dam is located.

“This issue demonstrates the importance of installing the most effective best management practices to control sediment .runoff.” said Pat Buckley, of the Pennsylvania Department of Environmental Protection. “Pennsylvania is encouraging the Bay Program to re-examine existing and alternative practices to identify the most cost-effective.”

Buckley said the state may examine the issue in more detail shortly after the turn of the century, when the Bay Program is expected to revise nutrient reduction goals for major tributaries.

“It’ll have to be one of the issues that we look at when we develop our new strategy in 2001 when our new goals come out,” she said.

The USGS estimates that the reservoir behind the Conowingo Dam can hold about 450 billion pounds of sediment. Sediment would not actually reach the surface — flowing water keeps sediments from settling in many areas — but the reservoirs would reach a point of “equilibrium” where as much sediment would move beyond the dam as what arrives from upstream.

About 4.6 billion pounds of sediment is deposited behind the dam annually, but that amount varies largely depending on river flow. Wet years with lots of rain wash more sediment from the land into the river, while dry years bring less.

Very high flows, which exceed 400,000 cubic feet per second, erode — or “scour” — more sediment from the dam than is left behind.

That happened most recently in January and early February of 1996, when heavy rains, coupled with warm temperatures melted much of the 3- to-4-foot snowpack in the watershed, resulting in floods. The situation was worsened by an ice jam downstream of Harrisburg that resulted in the river rising 8 feet in one hour. When the jam broke on Jan. 20, a huge wave was unleashed. More than 900,000 cubic feet of water per second surged past the Conowingo Dam — more than twice what was needed to scour sediments from the reservoir. Flows remained high enough to scour through Feb. 4.

During that time, about 11.8 million tons of sediment was removed from behind the dams — mostly from Conowingo. Coupled with another 3.2 million tons of sediment being carried from upstream, about 15 million tons were dumped into the Bay — about 16 times the normal sediment load from the river, along with huge amounts of nutrients.

Because the flood took place in January, when there is little biological activity, the Bay did not suffer severe consequences. Had the timing been different, the results may have been devastating. The flooding that accompanied Hurricane Agnes in 1972 was one of the most devastating events ever recorded in the Bay, smothering huge areas of grass beds, and causing severe setbacks to crab, clam, shad and other populations in the Chesapeake. Some things, such as grass beds, have still not returned to pre-Agnes levels.

After the 1996 flood, the USGS re-examined the storage capacity behind the dams and concluded that the Conowingo reservoir could hold another 86 billion pounds before it is filled — something less than 20 years away at the current rate of sedimentation.

The problem compounds two growing concerns for the Bay: Phosphorus and sediment. In recent years, much of the nutrient reduction effort has focused on nitrogen, which is generally considered more difficult to control than phosphorus. But recent work has shown that phosphorus has been building up in the soils in many parts of the watershed, and maintaining phosphorus reduction goals may be more difficult than earlier thought — even without taking the reservoir problem into account.

Unlike nutrients, sediment has not been targeted for specific reductions. But sediment is increasingly seen as a major Bay water quality problem, and monitoring has shown that the Bay has become increasingly clouded with sediment in recent years. Sediment can prevent light from reaching important underwater grass beds, smother habitats for bottom-dwelling organisms such as oysters and clams, and clog the gills of fish.

Because of increasing trends, some believe more emphasis will be placed on controlling sediment runoff in the future, possibly with reduction goals being set for rivers as the Bay Program has done for nitrogen and phosphorus.

The USGS estimated that in a typical year, about 8.7 million pounds of phosphorus flows from the Susquehanna basin, of which 3.48 millions pounds is trapped and 5.22 million pounds reaches the Bay.

Once the Conowingo reservoir is filled, essentially everything that comes down the river will reach the Bay, more than offsetting the Susquehanna phosphorus reduction goal of 1.7 million pounds a year. Instead of decreasing, the amount of phosphorus reaching the Bay would increase from 5.22 million pounds a year to about 7 million pounds a year once the reservoir is filled and the nutrient reduction goal attained.

The story for sediment is even gloomier. Right now, about 6.2 billion pounds of sediment flows down the river each year, of which 4.4 billion pounds is trapped. When the reservoir is filled, the full 6.2 billion pounds would hit the Bay — a 250 percent increase.

Nitrogen would increase only slightly — by 3 million pounds, or about 2 percent — because nitrogen is more water soluble than phosphorus, and little is trapped by the dams.

Not only would nutrients and sediments increase during “normal” flows, the USGS cautioned that the lost storage capacity would result in even greater increases in the amount of sediment and nutrients flushed into the Bay during future scour events.

The report, “Changes in Bottom-Surface Elevations in Three Reservoirs on the Lower Susquehanna River, Pennsylvania and Maryland, Following the January 1996 Flood — Implications for Nutrient and Sediment Loads to the Chesapeake Bay,” by Michael Langland and Robert Hainly, is available from the USGS. The report number is 97-4138.

The USGS also has a fact sheet that summarizes the issue, “Changes in Sediment and Nutrient Storage in Three Reservoirs in the Lower Susquehanna River Basin and Implications for the Chesapeake Bay.” For a copy of the report or the fact sheet, contact the USGS at 410-238-4202.

The Susquehanna River Basin Commission report, “Managing Sediments and Nutrients in the Susquehanna River Basin,” is available from the SRBC, 717-238-0423.