Bill Weihbrecht stood atop a 5-foot dirt cliff that served as a bank to the South Branch of the Codorus Creek, and peered into the chocolate brown water.
Months earlier, he had sought to measure how rapidly the bank of the 10-foot-wide Pennsylvania stream was eroding. So Weihbrecht, an environmental consultant, pounded 3-foot “pins” into the streambank. As more dirt eroded away, more of the pin would be exposed, allowing him to measure how fast the dirt cliff was collapsing into the water.
A few months later, the pins were no longer in the bank — they were lying in the bottom of the stream. “We never, ever thought we would lose three feet of soil in the two years we were doing the study,” Weihbrecht said, “but we lost it in 10 months.”
Weihbrecht calculated that 862 tons of dirt were washed into the stream from an 1,800-foot section of streambank.
That sediment joins countless millions of tons of clay, sand, soil and bits of gravel slowly washing down creeks and rivers toward the Bay.
The Bay Program estimates that more than 5 million tons of dirt enter the Chesapeake and its tidal rivers each year. That’s enough to cover a football field to a depth of more than 1,700 feet — or more than three times the height of the Washington Monument.
It comes from a host of sources: washing off the watershed and down its rivers, eroding from shorelines along the Chesapeake or flowing in from the ocean.
Once in the Bay, it causes a variety of ills, such as smothering oyster beds and other bottom dwellers. Phosphorus, as well as many contaminants, latch onto sediments for a ride downstream. The buildup of sediment in shipping channels increases the need for dredging.
Increasingly, though, sediment is blamed for clouding the water and blocking sunlight from reaching critical underwater grass beds. Recent studies suggest that in many places, sediment is playing a larger role than algae in blocking sunlight from reaching underwater grass beds, which provide important habitat for juvenile fish and crabs, as well as waterfowl and other species.
To help clear the water for grass beds, the Bay Program later this year will establish — for the first time — sediment reduction goals for major tributaries when it sets new nutrient limits for phosphorus and nitrogen.
Part of the sediment goal is straightforward, noted Rich Batiuk, associate director for science with the EPA’s Bay Program Office. A large amount, but not all, of the phosphorus associated with runoff is bound to particles of sediment. “As we set phosphorus goals, you are essentially setting a sediment goal along with that,” Batiuk said.
The question is how far beyond that level the goal will go. The answer is shrouded by scientific questions.
A special Bay Program Sediment Workgroup will issue a report later this year summarizing key issues. But some of its likely conclusions, presented at various meetings in recent months, paint a sobering picture.
No one is certain where much of the sediment comes from or to what extent it can be controlled — and at what cost.
Different parts of the Bay are affected by sediment from different sources. Some sources, such as shoreline erosion, result from natural processes such as sea level rise, and others, such as sediment from the ocean, can’t be controlled at all.
Controlling sediment runoff in the watershed is not likely to yield quick results — and certainly little by the 2010 cleanup deadline — because dirt moves slowly down rivers and streams.
Scientists have an even poorer idea of what happens when sediment reaches the estuary. Where it moves, and how long particles maintain their ability to be suspended in the water — blocking light — before permanently settling to the bottom remain unanswered questions.
“With sediment, we’re almost back to where we were in the early 1980s, when we were just starting to tackle nutrients,” said Scott Phillips, who oversees U.S. Geological Survey programs related to the Bay.
“There is a need to enhance the scientific information — to understand sediment sources, their delivery to the Bay and their relation to water clarity — so we can help agencies begin to formulate sediment reduction strategies.”
Sediment happens. The creation of the Delmarva Peninsula — and the formation of the Chesapeake Bay — is a byproduct of sand and soil washed down the Susquehanna River.
“Sediment is a natural process,” said Mike Langland, a USGS geologist who co-chairs the Sediment Workgroup. “You are never going to control 100 percent of it, and you shouldn’t even try it.”
On the landscape, wind and rain work to loosen particles of sand and clay, and the force of gravity pulls them downhill into the local stream. In the natural world, dense vegetation, as well as obstacles such as fallen trees and limbs, help to slow but not halt that movement.
As land was cleared and developed, more sediment began heading for the Bay. But much of it has not shown up in the estuary — yet. Scientists believe that much of the sediment loosened by past activities is still working its way down more than 100,000 miles of rivers and streams in the Bay watershed.
“It really is not known how long it would take a particle that was eroded in New York to reach the upper Bay,” Langland said. “There are some numbers out there that are suggestive of travel times on the order of a mile per year.”
If so, it could be a matter of centuries before particles from the headwaters complete their journey. Sediments from even closer areas, such as those that Weihbrecht watched wash into the Codorus — a bit more than 100 miles from the Bay — may take decades to finish the trip.
That means his efforts may not help clean up the Bay until close to the 22nd century.
Complicating the picture further is that the Codorus sediment wasn’t the product of erosion from the land. Rather, it resulted from increased runoff and old sediment accumulations in the stream that altered its natural hydrology.
When the Bay watershed was mostly forested, little rainfall actually entered streams as runoff; most soaked into soil and entered through groundwater. Development and land clearance dramatically changed that, speeding rain into the nearest waterways. That high-powered runoff, combined with growing amounts of sediment, dramatically changed stream hydrology in many places, causing fast, powerful, sediment-laden surges of water to head downstream, where they hit banks like a sandblaster.
The result is streams that eat away at their own banks. “Everyone thinks it’s all coming off the land,” Weihbrecht said. “But that’s not what we saw. It’s coming from within the stream.”
Some suggest that half or more of all the new sediment being transported down waterways comes from streambank erosion, but geologists say no study provides a good watershedwide figure.
If much of the erosion is occurring in the stream, rather than running off the land, the implication is huge. “We have some tricks up our sleeves when it comes to holding it in place on land, but once it starts moving in the stream, unless you are talking about major efforts, it’s essentially in there and is going to work its way down the system,” Batiuk said.
Reshaping the streambeds to halt streambank erosion and allow the waterway to adjust to its new hydrology is far more expensive than controlling runoff. For just 4,000 feet of the Codorus, it cost $200,000 which was funded through Pennsylvania’s Growing Greener program, according to Weihbrecht.
Of the 148 miles of stream he surveyed in the South Branch of the Codorus for the local chapter of the Izaak Walton League, Weihbrecht found that 10 miles were severely eroded, and 56 were moderately eroded. Altogether, he estimated the streams were adding 37,000 to 78,000 tons of sediment to the stream annually. Stream restoration that would halt additional erosion, he said, would cost between $25 and $75 per foot — or as much as $400,000 per mile.
Sediment from the Bay’s 64,000-square-mile watershed is a problem mostly in relatively restricted, freshwater areas of the upper Bay and its tidal tributaries. When riverflow meets saltwater from the ocean, an effective trap for sediment particles results, creating a natural area of turbid, churning water known as the estuarine turbidity maximum zone.
Little sediment from the watershed passes through that area, except during powerful storm events such as hurricanes and floods. But below the turbidity maximum, other factors create sediment problems — and ones just as problematic as those found upstream.
In the midsection of the Bay, shoreline and marsh erosion caused by sea level rise and — on the Eastern Shore — land subsidence, are major sources.
“We really can’t stop any of those processes,” said Jeff Halka, a geologist with the Maryland Geological Survey and co-chair of the Sediment Workgroup. “Sea level rises bring wave energy further and further in. I really don’t think that there is anything that we can do in the immediate future to control sea level rise or waves generated by storms.”
Worse, sea level appears to be rising rapidly around the Bay. The current rate is about 1.3 feet per century, about twice the worldwide average, allowing waves to sweep past natural shoreline buffers such as marshes.
Simply “hardening” the shoreline to halt erosion has its own drawbacks. When waves hit solid surfaces, they bounce back — potentially stirring up more bottom sediment and hurting chances for grass recovery. Further, solid walls prevent the formation of beaches, which are important habitats for horseshoe crabs, terrapins and other Bay species.
Halka said some alternatives may help, such as building offshore breakwaters to reduce wave action before it reaches the shoreline, and helping to maintain beaches. But like stabilizing streams, fighting Mother Nature with artificial structures would prove a costly — and ongoing — job if attempted over large areas.
Yet in terms of helping the Bay, that may be a worthwhile fight, at least for some areas, officials suggest. Unlike controlling runoff in the watershed, efforts to manage shoreline erosion could show quicker results. “To get an immediate impact, you want to focus closer to the estuary than trying to work upstream,” Langland said.
Harder to deal with is sediment from the ocean, which is a major contributor in the lower Bay and near the mouths of lower Chesapeake tributaries, such as the James. Although some estimates suggest the ocean could be the source of 12 percent of the sediment entering the Bay, it’s unclear how important the ocean-borne sediments are, Halka said.
The sediments that cause water clarity problems in the Chesapeake are “fine” sediments — usually bits of soil or clay that measure less than .04 millimeters — which can remain suspended in the water for a longer period of time. Once settled, they are more easily stirred up so they can cloud the water again.
Halka said some evidence suggests that most of the ocean sediment consists of larger, coarse material such as sand, which quickly settles to the bottom. If so, it may pose less of a problem, which would be a good thing — the ocean sediment is totally uncontrollable.
Eventually, fine sediment will combine with other particles, sink to the bottom and be buried. But no one knows exactly how long this takes to happen, or precisely where and how sediment moves around in the Bay once it arrives, according to Larry Sanford, a scientist with the University of Maryland’s Center for Environmental Science.
One reason for this uncertainty is that newly arriving sediment particles are resuspended and redeposited numerous times before they are finally buried. Also, sediment moves much more during storms than it does under relatively calm conditions. Storms are easy to miss with the normal Bay monitoring effort, and hard to sample in any case.
Sanford has been reviewing monitoring data from the Upper Bay, and so far has seen only a “weak” relationship between the amount of sediment entering from the Susquehanna and turbidity in the water.
It’s possible, he said, that most of the sediment entering the Bay under normal conditions is quickly buried — regardless of the amount coming in — with roughly the same amount of material left to be resuspended in the water.
“You can do a lot of things in the watershed to try to control sediment, but we really don’t know how much of that is actually going to impact turbidity in the water column of the estuary,” Sanford said. “If that is the case, then the whole issue of trying to reduce water column turbidity by reducing loads becomes a lot more problematic.”
That won’t be known without further work. But, he cautioned, even if the “background” levels of sediment remain the same under normal conditions, that could change during severe storms. During extreme events, he said, the backlog of sediment that has built up over time may be stirred up, magnifying the impact of the storm.
That could be compounded, he said, if the reservoir behind Conowingo Dam on the Susquehanna fills with sediment — something projected to happen in the next two to three decades — allowing huge storms to wash more sediment downstream and cover a wider area. As a result, controlling sediment to prevent buildup behind the dams would be good, he said.
“Anything we can do decrease the impact of huge events by maintaining storage capacity behind the dams is a good thing,” Sanford said. “But that doesn’t mean that trying to control everyday loads coming over the dam will have much of an impact on everyday turbidity in the estuary.”
While there is agreement that efforts to control sediment are worthwhile, the uncertainties around the issue may mean the sediment goal that emerges this year will almost certainly be more vague than the nutrient goal.
Batiuk compared the development of a sediment goal with the original 40 percent nutrient reduction set for the Bay in 1987. The goal offered no detail — such as trying to focus where or how reductions should take place — but helped launch nutrient control efforts throughout the region.
“The key thing is to get a forward-looking sediment goal that doesn’t get too prescriptive,” he said.
Indeed, many scientists caution that controlling sediment alone may not bring back the grass beds. The sharp decline of grass beds during the last three decades — a time when nutrient levels were increasing — suggests that nutrients and sediments likely acted in tandem to stress the beds.
Today, though, simply controlling nutrients may not allow grass beds to return in many areas because local sediment conditions may have gotten worse when the “underwater meadows” disappeared. Just as forests on the watershed help hold sediment in place, underwater grasses help sediment stay put. When they are lost, wave action more easily stirs up the dirt, allowing it to cloud the water and hampering the ability of grasses to recolonize the area.
Further, large grass beds reduce the energy in waves before they reach the shore. That, in turn, reduces the power of the waves, and their ability to erode the shoreline. “If you’ve ever watched a wave go through a grass bed and see how it gets dampened, it’s amazing,” Batiuk said.
In addition, scientists suggest the settling ability of fine sediments may have also been affected by the loss of filter feeders, such as oysters and menhaden. When they take up sediment while in pursuit of algae, the material is “repackaged” and expelled in larger particles more likely to sink to the bottom — and stay there. “Those biodeposits are harder to erode and are not as susceptible to resuspension,” Sanford said.
Yet as murky as the sediment issue is, one thing is clear. Trying to get dirt to stay put is generally a good thing. Whether it’s protecting islands and marshes from eroding into the Bay, or protecting streams from eroding in the watershed, sediment control provides local benefits.
“Any sediment reduction is a good thing,” said Langland, of the USGS. “Anything you do to control sediment is going to eventually have some sort of a positive impact somewhere. The key word is eventually.”
As Weihbrecht stood on the eroding banks of the Codorus, he pointed out how sediment and fine gravel covered the rocky bottom of the stream, eliminating habitat for fish eggs and aquatic insects. While brown trout lived just a few miles upstream, none would venture here.
“It’s a compounding situation,” he said. “Once a stream gets into a condition like this, they can’t fix themselves, at least not for thousands of years.”