It was a flood of biblical proportion. In January, 1996 warm temperatures melted four feet of snow in the Susquehanna River basin. A heavy rain added to rising river levels to carry millions of tons of sediment into reservoirs behind three downstream dams. These dams became giant nutrient banks. The sediment trapped behind the Conniwingo Dam alone contains 50 times the annual nutrient flow of the Susquehanna. Another storm, like the one in 1996, could wash these sediments into the Chesapeake Bay with disastrous results for water quality and aquatic life.
All text and images courtesy of Watershed Radio. More information can be found on their website.
Flood of 1996
In January 1996 a blizzard struck the Mid-Atlantic region, depositing a record amount of snowfall. Within two weeks of the blizzard, warm and extremely humid air entered the region, followed by a major rainstorm. The combination of warm, humid air and heavy rainfall melted the snow at an unprecedented rate. In just over one day, the combination of two to five inches of water from snowmelt and two to five inches of rainfall created a major flooding throughout the Mid-Atlantic region, including the Chesapeake Bay watershed and many of the Bay’s tributaries.
Sediment and nutrients flow into the Chesapeake Bay
As a result of the flooding, large amounts of nutrients and sediment entered the Bay. Sediment, which is topsoil, sand, and minerals washed from the land into water, is a natural part of the environment. Too much sediment, however, clouds the water and coats the leaves of underwater plants, shielding the plants from the sunlight they need for growth. In addition, the sediment also carries other pollutants with it. Phosphorus and toxic contaminants, such as trace metals and pesticides, attach to sediment particles and travel downstream and in to the Bay.The flood of 1996 also brought many nutrients to the Bay. Two primary nutrients,nitrogen and phosphorus, occur naturally in the environment as a result of the decomposition of plant material, deposition of atmospheric nitrogen, and erosion of phosphorus-containing rocks. They have always been present in the Bay, providing nourishment to plants and animals. Similar to sediment, it is the excessive amount of nutrients that is harmful to the Chesapeake Bay. When there are plenty of nutrients in the water, algae grow very fast, and the abundance of algae will cloud the water and prevent the underwater grasses from receiving sunlight. And when the algae die, they are decomposed by bacteria that use up the oxygen dissolved in the water, leaving nothing for other plants and animals.
Monitoring the flow
Researchers at the U.S. Geological Survey (USGS) monitored the amount of freshwater, nutrients, and sediment that entered the Chesapeake Bay as a result of the flood. They found that during the January 1996 flood, 4.9 trillion gallons of water entered the Chesapeake Bay, almost three times as much as in an average January month. The USGS also reported that, as a result of the flood, 5.3 million pounds of phosphorus (six times the average amount) , 63 million pounds of nitrogen (three times the average amount), and 7.5 billion pounds of sediment (17 times the average amount) entered the Chesapeake Bay from its nine major tributaries, the majority coming from the Susquehanna, Potomac, and James Rivers.
It could have been worse…
Fortunately for the Chesapeake Bay, the flood occurred in the winter, a time whenunderwater grasses and many living organisms were dormant in the Bay and when farmland, rich in nutrients, was frozen, which prevented the rain from causing excessive runoff of nutrients into the Bay. The high amount of sediment transported to the Bay during the flood came from streambank and river-channel erosion.
Not only in 1996
Pollution of the Bay by sediment and nutrients is not only a problem when there is a major flood. Every day, the rivers in the Chesapeake Bay watershed carry sediment, nutrients, and other pollutants from all parts of the watershed down to the Chesapeake Bay. The erosion of farmland, shorelines, and construction sites, as well as the runoff from cities and suburbs contributes to the sediment that gets washed into the Bay, and excess nutrients enter the Bay from “nonpoint” sources such as airborne pollution and runoff from city streets and fertilizer-laden farmlands, as well as from “point” sources such as sewage-treatment plants.
To reduce the pollution of the Bay, we all need to step in and help. Farmers can plant more vegetation buffers along streams, developers can develop stormwater management programs, home owners along the Bay and its tributaries can use vegetation to reduce shoreline erosion, people can take good care of their septic systems, and we all need to become responsible citizens of the watershed.
Part of this story is based on a Fact Sheet about the January 1996 Flood, credit of the U.S. Geological Survey.
About the flood of 1996
Flood Forecast and Warning in the Susquehanna River Basin. This website includes video images of the flood of 1996.
January 1996 Floods Deliver Large Loads of Nutrients and Sediment to the Chesapeake Bay. Fact Sheet by the U.S. Geological Survey.
Nature’s Threat to the Bay. Article in The Washington Post by Susan Q. Stranahan. Sunday, April 8 2001.
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