© NIKKI DAVIS
Ask Dr. Beth McGee
Beth McGee, Ph.D. is CBF’s
Senior Water Quality Scientist. If you
are curious about an issue facing the
Bay’s water quality, please send your
question to email@example.com.
This satellite image, taken on September 12, 2011, shows the
Chesapeake Bay and parts of Pennsylvania, Maryland, West Virginia,
Delaware, and Virginia. Sediment pollution that followed Hurricaine
Irene and Tropical Storm Lee stretched down the Susquehanna River to
the Patuxent River in Southern Maryland.
Q: In late August, Hurricane Irene hit the Chesapeake region bringing with it high winds and locally heavy rain. This storm was followed
by Tropical Storm Lee in mid-September. Collectively, these two
storms delivered nearly three feet of rain in some parts of the
Chesapeake Bay watershed. What effect will these storms have on
the Chesapeake Bay’s ecosystem?
A: The short answer is: we don’t know for sure, but ongoing and
future monitoring efforts will help answer the question. That said,
here is the latest thinking on potential effects:
The most visible effect was the enormous sediment plume, contributed by the large river systems, particularly the Susquehanna—
the Bay’s largest river. In a typical year, roughly three million tons
of sediment comes down the Susquehanna; two million of which
are trapped behind the Conowingo Dam in Maryland. During
Tropical Storm Lee, flows in the Susquehanna reached the second
highest since Tropical Storm Agnes in 1972. As a result, scientists
estimate that about four million tons of sediment, roughly a year’s
worth, was scoured from behind the dam and traveled downstream
into the Chesapeake Bay.
These sediments decrease water clarity, which shade the Bay’s
underwater grasses, and can bury oysters and other marine life living on the bottom of the Bay. There is some “good” news in that
the storm hit at a time when many of the bay grasses were already
starting to die back, so the impacts will likely be less severe than if
the storm had occurred in June—as was the case when Tropical
Storm Agnes struck, devastating the Bay’s grasses.
Impacts on oysters, already at a fraction of historic levels, may be
substantial. In addition to the potential for oyster reefs to be buried
by sediments, in the northern part of the Bay there is evidence of
high oyster mortality presumably due to the drop in salinity caused
by the influx of freshwater.
Lastly, along with all that sediment and freshwater, also came nitrogen and phosphorus—two of the pollutants responsible for the
Bay’s oxygen-starved “dead zone.” Runoff from sources such as farm
fields, animal feedlots, city streets, and suburban lawns all contributed to this large flood of pollution. To add insult to injury, many
sewer systems were overwhelmed, resulting in millions of gallons of
untreated sewage pouring into local waters and the Bay. Scientists
suspect that these pollutants may contribute to a more severe dead
zone next summer.
While there is nothing we can do to prevent extreme storm events
such as those we experienced this year, we can take steps to control
stormwater runoff that occurs during more typical rain events. By
doing so, we will help increase the Bay’s resilience and ability to
recover from these extreme episodes.
u To learn more about stormwater, visit cbf.org/stormwater.