Click here for part 1


by Jody Argo Schroath  

Last month, we introduced you to the Conowingo Dam, the Chesapeake Bay's very own 800-pound gorilla in the room. We looked at its construction in 1928, its capabilities and its structure, rising about 100 feet and reaching nearly a mile across the lower Susquehanna River. For all intents and purposes, our gorilla is a fine sturdy fellow, maintained in tip-top condition by owner Exelon Corporation, and going stronger than ever. But now it's time to take a good look at the gorilla's dark side,a dark side that could forever alter the very life and structure of the Chesapeake Bay, which gets half of its fresh water from the Susquehanna. Plant and animal life could suffer or disappear altogether; channels could clog, dead zones could spread over most of the Bay. We've known about this for years--that sooner or later the millions of pounds of silt that wash down the Susquehanna every day will overwhelm Conowingo Dam's storage capacity and will flow unimpeded into the Bay. We've done some studies to confirm it. But we haven't even begun to do anything about it. Too costly. Too complicated. But there's more. Our gorilla holds a time bomb that could go off at any time--in the form of another Hurricane Agnes-- and decimate large portions of the Bay in a single blow. It's time to take a good look at our gorilla's Mr. Hyde. 

Dark Side of the Gorilla: Diagnosis
As the Susquehanna River works its way nearly 500 miles downstream from upstate New York, it picks up millions of tons of sediment--runoff from city sewer systems, from hundreds of farms, mall parking lots and abandoned coal mines. A great deal of this sediment makes its way into the Chesapeake. While the Susquehanna contributes 50 percent of the Bay's fresh water, it also contributes 25 percent of its sediment and the lion's share of its nutrient load: 66 percent of its nitrogen and 40 percent of its phosphorus. That's a lot . . . but it could be worse. Before the Susquehanna reaches the Bay, it runs into a mighty roadblock: the Conowingo Dam. The dam traps 60 to 70 percent of the sediment, 40 percent of the phosphorus and 2 percent of the nitrogen. As a result, millions of tons of sediment and nutrients have accumulated behind the Conowingo, as well as the two smaller dams immediately upstream from it--Safe Harbor and Holtwood.

In the last decades, however, the storage ponds behind both Safe Harbor and Holtwood have filled to capacity, leaving the Conowingo as the Bay's last defense. A 2009 study by the U.S. Geological Survey (USGS) estimated that over its lifetime the Conowingo has accumulated 174 million tons of sediment (not to mention 670,000 tons of nitrogen and 130,000 tons of phosphorus). But Conowingo's storage capacity too is reaching its limit--estimated to be 204 million tons. That leaves room for only an additional 30 million tons. So the crucial question is: When will the Conowingo's storage pond reach its capacity? The arithmetic is straight forward: The Susquehanna currently transports downstream an average of 3 million tons of sediment a year, about 2 million of which settles out behind the dam. That means that the Conowingo could reach its capacity for storing sediment--also called its "steady state"--in 15 to 20 years, depending on a number of variables. It is here that we get a glimmer of good news. The amount of sediment flowing downstream is decreasing dramatically--from more than 100 million tons per year in the 1930s to the current level of 3 million tons per year. Between 1996 and 2008, it was even lower than that, with a total of only 14.7 million tons deposited behind the dam. This was due in large part to lower-than-average rainfall, better soil management practices upstream and a few "scouring" events. The last, as its household-cleaner name implies, occur when the rush of water coming down the river is so terrific that it flushes some of the stored sediment with it through the dam and into the Bay. Scouring events follow large rainfalls and snowmelts upstream . . . or large hurricanes, such as 1972's Hurricane Agnes, the mother of all Susquehanna scouring events. (We'll have more to say about scouring events a little bit later.) 

Meanwhile, this decrease in sediment flow may buy us more time. If the amount of sediment continues to decrease, say to 2.5 million tons a year, steady state might not be reached for another five years beyond the estimated 15 to 20. Beyond that, if the statistically predicted number of scouring events occur, an additional five years might be gained, the USGS study says. Finally, if recent regulations that establish best practices for land management survive and are put into practice, sediment will be further reduced in the next few years. But will that be enough? No, says Michael Helfrich, the Lower Susquehanna Riverkeeper and a latter-day John the Baptist for the Conowingo sediment issue. "If we were able to completely stop sediment from entering the Susquehanna today," he says, "it wouldn't be enough. There is already enough sediment in the system, working its way downriver to fill up the remaining storage room. Sediment would continue to flow up to 50 years after . . . best management practices were put into effect."

When the Conowingo's storage capacity reaches steady state--and right now that seems inevitable--"as much material that is coming down the river will go [through] the dam and come eventually into the Bay," explains Dan Bierly of the U.S. Army Corps of Engineers. And this, he continues, "is a great concern, not just from the dredging aspect, but from the environmental aspect." Specifically, according to the USGS, this will mean a 250-percent increase in sediment entering the Bay, a 2-percent increase in nitrogen and a 30- to 40-percent increase in phosphorus. According to the Chesapeake Bay Program's Scientific and Technical Advisory Committee, the added sediments and nutrients would dramatically increase algae blooms, smothering underwater grasses, which are vital habitat for blue crabs and young fish. Further, it would increase turbidity and clog navigation channels. And it would clog the gills of fish and cover their eggs.

All that doesn't even reflect the evils contained within the sediment itself. A core sample of sediment taken from the Conowingo Pond in 2006 by the USGS for the Susquehanna River Basin Commission found that in addition to the expected sand and soil, the sediment contained significant amounts of coal, some toxic metals, silver at lower levels, PCBs, methane gas, phosphorus, antibiotic-resistant bacteria and even low levels of radiation. 

The Big Scour: A Time-Bomb Scenario
All of the calculations and predictions above assume a fairly even flow of water and sediment--an average. The Susquehanna, however, is many things, but never average. From season to season and year to year its flow and sediment rate changes dramatically with meteorological events such as rain, drought and snow melt. Several times a year, usually in the spring, the flow increases to such a degree that Exelon is forced to open some of the dam's spill gates to ease the pressure from the rising water behind the dam. Occasionally, the flow is high enough to scour some of the sediment laid down earlier. Then the turbidity of the water increases with the added sediment, muddying the Bay as far south as the Chesapeake Bay Bridge. Very occasionally, the scouring is severe enough to produce a measurable effect on the health of the Bay. The worst such event in the history of the Conowingo Dam came in 1972, when Hurricane Agnes dumped 8 to 10 inches of rain over the Mid-Atlantic and Northeast, sending a battering ram of water and debris down the Susquehanna that nearly overwhelmed the dam, and subsequently the Bay.

Over the course of four days in June, torrential rains from Agnes washed an estimated four years worth of sediment out of New York and Pennsylvania toward the mouth of the Susquehanna. When it reached the Conowingo reservoir, it scoured another eight years worth of sediment from that repository. Altogether an estimated 30 million tons of sediment washed through the spill gates of the Conowingo and into the Chesapeake. It was the biggest single "catastrophic" event ever recorded on the Bay. The silt laid down by Agnes killed nearly all of the underwater grasses in the upper Bay. It has taken three decades for them to recover. (For a more personal look at what it was like inside the dam during Agnes, see the sidebar below.)

Agnes may have been the worst scouring event in the dam's history, but it certainly wasn't the only one. A massive ice-jam flood in January 1996 dumped nearly as much sediment into the Bay as Agnes, but it had considerably less effect on its plants and animals because it took place before the beginning of the growing season. More recently, in late September 2004, there was Hurricane Ivan, which moved across the northern states, dumping torrential rain as it went. The flow through the Conowingo Dam reached nearly 550,000 cubic feet per second, the highest since Hurricane Agnes. Photos taken by a NASA satellite show the sediment spreading well below the Chesapeake Bay Bridge.

More scouring events will occur. That's a certainty. What's not a certainty is when, how many and how severe. A Hurricane Agnes is a rarity, but not an anomaly. At some point, there will be another Agnes. Or worse. It's a certainty that keeps Riverkeeper Helfrich awake at night, because when that day comes, all hell is going to break loose on the Bay. In the years since Agnes, millions of tons of additional sediment have built up, so that when that next "catastrophic" event comes, Helfrich says, it will certainly scour even more sediment than Agnes did. USGS hydrologist Mike Langland agrees. "There is more there to be scoured," he told Bay Journal's Karl Blankenship in 2009, "and it is probably in areas that would be more rapidly scoured--closer to the dam."

Disarming the Gorilla: Can It Be Done?
The short answer to that question is: We don't know yet. It's not as if the problem of steady state and catastrophic scouring events has just cropped up. It has been discussed and studied for years. Yet it's still a problem without a solution. There are a number of ideas that have been kicked around during a number of studies, but there has yet to be a study to determine the best and most cost-effective solution. Why not? One reason is that studies are expensive and none of the political entities involved has wanted to spend a lot of money to be told they are going to have to spend a great deal more money fixing the problem. Because the one certainty is that all of the possible solutions anyone has come up with so far are expensive . . . very expensive . . . mind numbingly expensive. 

So there are some ideas but no solution. For example, a Sediment Task Force convened in 1999 by the Susquehanna River Basin Commission made some recommendations, including reducing sediment with stream buffering and best land management practices and asking the Army Corps of Engineers to study the feasibility of dredging the storage basin. A few years ago, Congress assigned the Corps the task of looking into sediment reduction plans. "Congress has told us to undertake a study to consider the material behind the dams and decide what to do with it," the Corps' Dan Bierly said at a 2002 public meeting. But the Corps has had a hard time finding a partner willing to share in the expense of a study. Their first proposed study found no takers. Recently they have returned with a less ambitious proposal (with a lower matching-funds requirement).

In 2009, the USGS did a bathymetric study of the Conowingo reservoir to measure its capacity, its current level of fill and the chemical makeup of the sediment. Other groups--often with much the same participants--have held other meetings and other studies in other years, beginning in the mid-1990s. But all of these have focused more on determining the exact nature of the problem and the timetable for reaching steady state than on finding the best way to fix it.

Not that the problem lacks for suggested mitigations. First and foremost among these is the call to stop the sediment from entering the Susquehanna at all. And indeed recent EPA policy has laid out a timetable for best land management practices for the Susquehanna Basin Watershed and even more recently set regulations for total maximum daily loads (TMDLs), which would limit the amount of runoff and pollutants permitted for each grid of land per day. However, the TMDLs have raised a furor among some farmers, and the Farm Bureau has sued the EPA to stop their implementation.

The suggested mitigation most often heard is to dredge the sediment and then relocate it. If it could be done, it would certainly open up room in the storage basin. But the proposal brings with it a lot of baggage. For example, how would you capture the sediment--which is fine and easily stirred up into the water above? And then, if you did manage to capture it, what would you do with it? First, you would have to find a way to transport it, probably by train or by truckload. And if so, how many train- or truckloads would it take? How much would you need to dredge? At least 2 million tons a year, just to keep up with the yearly load. Then, where would you take it? Suggestions have been made that include everything from abandoned quarries to the Aberdeen Proving Grounds. But each of these has problems of its own. The dredged sediment would quickly fill all of the quarry basins available, and Aberdeen has unexploded-bomb issues that would first have to be overcome. Furthermore the sediment is polluted, and the pollutants could leach right back into the soil and eventually back into the river. Several proposals have called for reusing the sediment. One interesting idea is to burn it in a kiln constructed nearby to expunge the pollutants before turning it into aggregate for cinder blocks or other construction material. The kiln, of course, would produce destructive greenhouse gases, but would produce a sellable product.

An idea that perhaps carries more weight and a lower price tag is something called sand-bypassing. Sand is actually good for the Bay. The catch basin behind the dam contains plenty of sand--and thanks to its bathymetry study, the USGS even knows where the sand deposits within it. Sand-bypassing would involve moving the sand (in a manner yet to be determined, of course) through the dam and out the other side, making more room in the storage pond. Happily, the river bottom for some distance in front of the dam is short of sand, since the dam prevents the natural bottom shift that would otherwise occur. The sand also could be used on beaches and for rebuilding islands.

Helfrich's half-serious solution would be a lot less expensive. "We should pray for a series of scouring events that would be big enough to move a lot of the stored sediment, but not big enough to damage the Bay."

When all is said and done and a solution is finally devised, who's going to pick up the tab for putting it into action? The line is a very short one. The Susquehanna River states of Maryland, Pennsylvania and New York are all up to their eyebrows in financial "scouring" events of their own. None of them has yet stepped forward. Nor has dam owner Exelon, which holds fast to the argument the dam has long been a help rather than a hindrance. "Anytime it rains," says Mary Helen Marsh, director of environmental operations for Exelon and former general manager of Conowingo Dam, "cows, buildings and everything else ends up in the river behind one of the dams. Holtwood, Safe Harbor and Conowingo are actually holding back that sediment and giving those three states some time to address the root cause. . . . Our position is that sediment is a watershed issue."

With no one stepping forward to accept responsibility and no study yet even offering a solution, it's little wonder that Helfrich is pressing for a study to look at the cost of doing nothing. "The cost of fixing the problem may seem expensive," he says, "but when the states see the cost of not fixing the problem, it will seem like a bargain."

Final Thoughts: Tick . . . tick . . . tick . . .
Early in March of this year, a passing storm dropped more than two inches of rain over much of the Susquehanna River's 27,100-square-mile drainage area--an area that lay deep in late winter snow. By March 8, rain and snowmelt had reached the Conowingo. The water levels and flow rose dramatically--as they often do in the spring. This time, however, the flow continued to increase, and by the morning of March 12, Exelon opened 26 of its 53 spill gates. Just before noon, the flow reached a peak of 485,000 cubic feet per second (cfs). The average flow of water through the Conowingo that day was 414,000 cfs--well below Agnes's peak of 1,130,000 cfs, but the highest since Hurricane Ivan in 2004. And this was only a rain storm.

The silt-heavy water roared through the open gates like 26 freight trains, sending iridescent plumes of spray shooting high into the clearing skies, higher than the 100-foot-high dam. Downstream the water roiled and boiled, yellow and brown and ochre. Waves of water broke against the steps leading down to the fishing platform and swirled up the trees.

All along the observing platform above, stood an appreciative audience. From early that morning--a Saturday--cars had arrived, full of families eager to see the sight. Children laughed and pointed. Adults held up cell phones to snap photos. 

Yet, if these folks had listened carefully, they might have heard more than the rush and tumble of water. They might have heard the Conowingo's ticking time bomb. We have 15 to 20 years--and perhaps more if we're careful and lucky--before the pond behind the Conowingo fills up. But until we find a solution to that problem, the time bomb will remain armed. That's the thing about time bombs--they can go off at any time. Another Hurricane Agnes, or worse, could come this summer or next. Tick . . . tick . . .