Floods carry pollutants into bay Near-record amounts of nutrients, mud come from tributaries

January 23, 1996|By Timothy B. Wheeler | Timothy B. Wheeler,SUN STAFF

Floods in the mid-Atlantic region have been dumping near-record amounts of mud and polluting nutrients into the Chesapeake Bay.

"A big storm event like this scours out sediment and carries proportionally more than a normal flow," said Robert Magnien, director of tidewater ecosystem assessment for the Department of Natural Resources. The weekend's floods probably will dump more mud and nutrients into the bay than will be washed in during the rest of the year, he said.

The last time flooding was this severe -- during Tropical Storm Agnes in June 1972 -- the bay suffered drastic losses of underwater grasses, crabs, fish and shellfish. The nutrients spawned huge algae blooms, which combined with the mud to kill grass beds by depriving them of sunlight. Underwater grasses are considered essential because they provide food and shelter for crabs, fish and waterfowl.

At its peak about 6 p.m. Saturday, the Susquehanna River poured through the Conowingo Dam at the rate of 6.8 million gallons per second. The river flow at the dam in June 1972, during Agnes, was 8.5 million gallons per second.

Normal January river flow at Conowingo is 290,000 gallons per second, according to the U.S. Geological Survey.

As the Susquehanna flows, so goes the upper Chesapeake Bay. The river, with a drainage basin of 27,100 square miles in New York, Pennsylvania and Maryland, is the bay's largest tributary. It supplies half the fresh water entering the bay.

The Potomac River, the bay's second largest tributary, also experienced near-record flooding, with a peak flow of 2.6 million gallons per second measured at Washington about 1 p.m. Sunday.

That is nearly as high as it was during Agnes in 1972, and exceeds the river's flow during the floods that devastated the river's upper drainage basin in West Virginia and Western Maryland in fall 1985.

Normal January flow for the Potomac is 96,000 gallons per second.

While the flooding this time approaches Agnes in intensity, Dr. Magnien said yesterday it should do much less ecological damage to the bay, because it hit during winter when many plants and animals that might be hurt are dormant.

"The good news is this hits the bay at a time that [bay grasses] are not growing, and phytoplankton, or algae, are not growing very quickly because of low light and temperature conditions," he said.

The die-off of grasses that began shortly after Agnes continued for more than a decade, as the bay's man-made pollution worsened, until the state's launched a cleanup effort in 1983. Grass beds since have recovered, though they still are not as extensive as in the early 1970s.

The weekend flood was caused by Friday's brief but intense rain, which boosted the melting of 30 inches of snow on the ground.

River flows that peaked Saturday night or Sunday are dropping rapidly now. The Geological Survey reported that by noon yesterday, flows were about half their weekend highs. The Susquehanna was at 3.5 million gallons per second, and the Potomac at 1.3 million gallons per second.

Unless more heavy snows and rains follow, the bay's nutrient pollution should dissipate before spring.

"By the time this is all over with and we can calculate the amount of water associated with it, this will be less water than Agnes," said James Gerhart, district chief for the U.S. Geological Survey in Towson.

Nitrogen -- one of the two nutrients affecting the bay's water quality -- should be washed down the bay and out into the Atlantic Ocean by the time the waters warm, Dr. Magnien said.

The other nutrient -- phosphorus -- generally adheres to particles of mud, and it should settle to the bottom as the water clears up. Some will get back into the water in spring and summer, but the impact of such a gradual release will be less severe, Dr. Magnien said.

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