Now it's official: A report released today (8/30) finds the Conowingo Dam is losing its ability to prevent pollution from reaching the Chesapeake Bay.
The U.S. Geological Survey reports that the reservoirs behind Conowingo and other dams on the lower Susquehanna River are nearly full of sediment and are increasingly failing to trap it as it washes down river. The 94-foot-high hydroelectric structure at Conowingo is just the last and largest of several dams on the river.
Scientists have long warned that the sediment buildup behind the dams would eventually max out, allowing more water-fouling silt and phosphorus to get through. But as recently as last year, officials had said they expected the Conowingo would continue trapping sediment and the phosphorus that attaches to it for another 15 to 20 years.
But as The Baltimore Sun reported earlier this month, a USGS scientist taking another look at the dams' sediment-trapping ability has concluded that they're already deteriorating significantly, with extreme weather events like Tropical Storm Lee last September scouring out large amounts of sediment and flushing them into the bay.
Sediment can cloud the water, blocking out sunlight needed by the underwater grasses that provide food and shelter for fish and crabs. It also can smother oysters and other shellfish. Additionally, phosphorus, which binds to the sediment particles, can feed the algae blooms that cause a massive "dead zone" to form in the bay every summer.
Driven by the tropical storm and heavy spring rains, the Susquehanna dumped record amounts of phosphorus and sediment into the bay last year, the USGS reports, more than in any year since monitoring began in 1978.
The dams' declining ability to trap sediments apparently were reponsible for at least part of that. Since 1996, according to the report, the average amount of phosphorus getting to the bay from the Susquehanna has increased 55 percent, while sediment has jumped 97 percent. Yet another USGS report found recently that nutrient and sediment levels in the river upstream of the dams were down 25 percent overall.
"It has been understood for many years that as the reservoirs on the Lower Susquehanna River fill with sediment, there will be a substantial decrease in their ability to limit the influx of sediment and nutrients, especially phosphorus, to the Chesapeake Bay," Bob Hirsch, research hydrologist and author of the report, said in a news release accompanying the report. But his recent analysis shows increases in pollution "are not just a theoretical issue for future consideration, but are already underway."
As the Chesapeake's largest tributary, the Susquehanna plays a major role in the bay's health, furnishing nearly 41 percent of the nitrogen, 25 percent of the phosphorus and 27 percent of the sediment getting into its waters.
Hirsch warned that the dams' declining ability to trap sediment and phosphorus were already undermining the multi-state bay cleanup effort.
"The findings of this USGS study increase the urgency of identifying and implementing effective management options for addressing the filling reservoirs," Bruce Michael, director, resource assessment for the Maryland Department of Natural Resources, said in the release.
Officials have been studying the problem for the past year and in the next year or two hope to identify possible remedies, which may include dredging sediment from behind the dams to restore their trapping ability, or requiring further reductions in pollution from Pennsylvania and New York to offset the dams' failure to continue trapping it. Either is likely to be costly.