The sediment buildup behind Susquehanna River hydroelectric dams has been likened to a time-bomb — when their reservoirs are filled, huge amounts of dirt and nutrients will spill over, fouling Bay water quality.
But a recent symposium about the problem revealed some good news: Scientists believe the fuse on the time bomb may be longer than previously thought. Instead of being filled in less than 20 years, it may take an extra decade before the reservoirs reach capacity.
The bad news is that no one knows how to defuse the sediment bomb.
The two-day symposium looked at options ranging from dredging millions of tons of mud from behind the dams to stream corridor restoration to better land use practices that reduce erosion.
But no “silver bullet” solution emerged as participants raised questions as to whether any single solution — all of which have huge price tags — could solve the problem. In fact, a task force headed by the Susquehanna River Basin Commission and the Chesapeake Bay Commission, which studied the issue for 18 months, found more questions than answers and called for more research.
“This is a complex issue,” said Task Force Chair Tom Beauduy, Pennsylvania director of the Chesapeake Bay Commission, a panel that represents state legislatures. “It is one where we lack sufficient information to make comprehensive management decisions for the long term.”
Remaining questions are huge. No one knows exactly where all the sediment is coming from, how long it takes to move downstream or who would pay to keep the dirt out of the water and the Bay.
One thing is clear. Once the reservoirs reach their silt-trapping capacity, the amount of sediment pouring into the Upper Chesapeake will more than double while phosphorus inputs would rise more than 50 percent, severely setting back Bay cleanup efforts.
The issue is so serious that the Chesapeake 2000 Agreement called for the Bay Program to work with the Susquehanna River Basin Commission, a multistate agency that regulates water use in the watershed, to develop and begin implementing strategies to deal with the problem by 2003.
Although the problem of the dams is unique to the Susquehanna, other Chesapeake tributaries will be facing their own sediment issues in coming years. When the Bay Program sets new cleanup goals at the end of 2001, it will establish reductions for sediments as well as nutrients. At that point, all watersheds will be struggling with questions of where sediment is coming from — and how to stop it from reaching the Bay.
The issue emerged in the Susquehanna because recent studies by the U.S. Geological Survey showed that the 250-million-ton sediment trapping capacity at four dams built between 1904 and 1931 was being reached. In fact, reservoirs at three of the dams are already filled; the only trapping capacity that remains is at Conowingo Dam, the largest and southernmost of the structures.
A 1997 study calculated that its reservoir could be filled by about 2015. Revised calculations from the USGS presented at the symposium indicated there may be an additional decade before the capacity is reached.
But even before that time, scientists caution, increased amounts of sediment will begin passing Conowingo as its reservoir nears its silt-holding capacity.
Right now, about 1.2 million tons of sediment a year flow into the Bay from the Susquehanna — or about 95 pounds a second. The four large hydroelectric dams trap on average between 1.4 and 2 million tons of dirt a year. Also trapped is about 3.5 million pounds of phosphorus annually, while about 5.2 million pounds reaches the Bay.
(Little nitrogen is trapped because it tends to be water soluble, while phosphorus tends to attach to particles.)
That added sediment could bury the Upper Bay in silt, smothering grass beds and oyster bars, while the added phosphorus could spur more algae blooms in the upper and mid sections of the Bay.
The sediment flowing down the river stems from a variety of watershed activities over time. Scientists estimate that the pre-settlement forests probably yielded only about one-tenth of a ton of sediment per acre per year.
But sediment greatly increased as the forests were cleared and farms developed. According to Lloyd Reed, of the USGS, erosion rates probably peaked in the late 1800s and early 1900s, when many steep slopes were farmed and — instead of practicing contour plowing along the sides of slopes — furrows led straight down the hillside, speeding water runoff and erosion.
Also adding sediment to the streams was the mining of anthracite — or hard — coal. Water was used to drive chunks of coal through a sorting process, and the pieces too small to market — about 15 percent of all coal — went back into the streams with the water.
So much coal was in the river, Reed said, that by the late 1800s, sand and gravel operations were recovering “river coal” as a byproduct of their operations. Whole bars of coal were observed moving down the river at rates of about 3 miles a year, until they would be obliterated during floods.
When the watershed was totally forested, about a million tons of sediment a year flowed down the Susquehanna — just a bit less than gets past the dams today, according to Reed’s estimates. By around 1900, about 6 million tons of soil a year were going down the river, along with another 3 million tons of coal.
Sediment has decreased over time, with only about 3 million tons a year reaching the dams today. Still, much of the sediment from past activities remain in rivers and streams throughout the basin where it has become part of the sediment “bedload” which can take decades to work its way downstream.
“Even if you put best management practices to control runoff on every acre in a watershed, it could be decades before you see the effect of that because of bedload,” Beauduy said.
Runoff from developing areas and agriculture still generates high loads compared with the natural forested landscape. Modern farming practices reduce runoff, but conservation plans still consider soil losses of about 3 to 4 tons per acre per year to be normal.
But today’s sediment problems don’t just stem from dirt washing off the land. When silt-laden water blasts into an unstable stream bank, barren of vegetation, it causes the bank to collapse into the waterway, adding still more sediment.
“Streambank erosion is analogous to gasoline being thrown on a fire,” said Bill Weihbrecht, an environmental specialist with the consulting firm Skelly and Loy. In one 68-square-mile watershed in southern Pennsylvania, Weihbrecht estimated that between 37,000 and 78,000 tons of sediment came not from the land, but streambank erosion during a 16-month period.
The low estimate, Weihbrecht said, is equivalent to covering a football field 20 feet deep in sediment; the high estimate would bury the same field 36 feet deep. And all of it is now slowly working its way toward the Bay.
That poses a particularly difficult question, Weihbrecht said. While farmers or other landowners might be asked to control sediment running off their land, who is responsible for sediment eroded by the stream itself?
It’s only one of many sediment questions without answers. How long will it take the material to reach the dams? Different types of sediment move at different speeds.
Nor does anyone know how much of the sediment in the streams is from “new” runoff, how much is bedload and how much is from streambank erosion.
Without knowing when and where the sediment originated, officials say, it’s hard to come up with effective management strategies. “We really need to know the sources,” said Robert Edwards, of the Susquehanna River Basin Commission.
If streambank erosion is a major source of sediment, that means simply trying to keep sediment on the land isn’t enough — major efforts are needed to restore streams as well.
“The long-term solution is going to be the restoration of the watershed,” Edwards said. “A short-term answer is dredging. But that is more of a reactive response than a proactive response. It’s all so complex, it may be a combination of those things.”
Many, in fact, see dredging old sediment from behind the dams as the way to maintain their sediment trapping capacity, at least until the benefits of other stream restoration and runoff control actions are “felt” at the dams years from now. That’s because enough silt is probably already in the streams to fill the reservoirs.
But dredging is no panacea. In Western Pennsylvania — outside the Bay watershed — the U.S. Army Corps of Engineers sought to dredge a mere 1 percent of the 14.8 million cubic yards of sediment behind a flood control dam. The project required securing a number of permits and, despite the fact the Corps was able to dispose of the dredged material on-site rather than ship it elsewhere, the operation still carried a price tag of $2.3 million.
The project was halted before it was finished when a single storm washed 50,000 cubic yards back in. The lesson, said the Corps’ Werner Loehlein, who oversaw the project, is that dredging is a “short-term, expensive and complex solution.”
By comparison, dredging behind the Susquehanna dams would be a huge undertaking — just finding somewhere to put the material could be a problem, officials say.
Dredging isn’t unprecedented. For about 20 years, until 1973, some of the sediment was dredged annually from behind the Safe Harbor Dam, the second largest on the river, to recover coal. About 10 million tons of coal were excavated from the reservoir in all.
But at present, no one knows how much dredging would cost, how it would be done, or where the material would go. Many argue that a feasibility study that lays out the dredging option in detail is needed now.
Paul Swartz, executive director of the Susquehanna River Commission, said the Corps of Engineers estimates a feasibility study for dredging would cost about $8 million, and would require the states, or other interests, to cover 25 percent of the cost.
It’s not clear where that money would come from, he said. But then, he noted, there is only one sure thing about defusing the Susquehanna’s sediment “time bomb.” All actions have big price tags: Restoring destabilized streambanks throughout the basin, by some rough guesses, would cost hundreds of millions of dollars.
“No matter what you do,” Swartz said, “it is going to be expensive.”
Impacts of Increased Sediments
The Bay Program’s Scientific and Technical Advisory Committee conducted a workshop last year to examine impacts of increased sediments from the Susquehanna River on the Chesapeake Bay.
According to the scientists, the consequences included:
- Increased amounts of phosphorus reaching the middle portions of the Chesapeake Bay.
- Increased turbidity in the Bay and faster sedimentation everywhere in the Upper Bay, especially in navigation channels, which would increase the need for channel dredging.
- Adverse impacts on the recovery of underwater grass beds because the sediment would reduce the amount of light reaching the plants.
- Benthic (bottom-dwelling) organisms would suffer increased mortality and reduced reproduction. Those that aren’t killed would have to spend more energy to keep from being buried. Young oysters are especially sensitive to sediment deposition.
- Fish might be impacted as increased sediment could affect their feeding, clog gill tissues and smother eggs. Siltation could also result in habitat alterations, and increased turbidity may change the abundance of planktonic prey important for larval and juvenile fish.