Linda D. Zynjuk, Brenda Feit Majedi
The Blizzard of 1996 struck the Mid-Atlantic region in January, depositing a record amount of snowfall. Within two weeks of the paralyzing 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, two to five inches of water from snowmelt combined with two to five inches of rainfall, resulting in widespread major flooding throughout the Mid-Atlantic region, including the Chesapeake Bay watershed (Figure 1).
An excessive amount of nutrients can harm the Chesapeake Bay. 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. When an excess amount of nutrients exists in the Bay, algae grow at a very fast rate. An abundance of algae results in cloudy water, which prevents Bay grasses from receiving the sunlight they need to live. Additionally, when the algae die, they are decomposed by bacteria that use up the oxygen dissolved in the water. Dissolved oxygen is very important to the survival of living resources in the Chesapeake Bay, and if it is depleted, the plants and animals that make their home in the Bay may die.
Sediment is fragmental material derived primarily from the physical degradation and chemical decomposition of rocks. Sediment also is present naturally in the environment, and is transported to the Chesapeake Bay by its tributaries.
Sediment harms the Bay when present in excessive amounts. Excess
sediment enters the Bay from the erosion of farmland and construction
sites, as well as in runoff from cities and suburbs. Excess sediment
clouds the water and coats the leaves of plants, both of which deprive
them of sunlight needed for growth. In addition, phosphorus and toxic
contaminants, such as trace metals and pesticides, attach to sediment
particles that are carried downstream and deposited into the Bay.
HOW MUCH WATER,
NUTRIENTS, AND SEDIMENT ENTER THE BAY DURING AN AVERAGE
The USGS monitors the amount of freshwater that enters the
Chesapeake Bay each month. During an average January, approximately
1.7 trillion gallons of water enters the Bay (Figure 2). Most
of this water comes from the Bay's three largest tributaries-the
Susquehanna (53 percent), the Potomac (18 percent), and the James (12
As a result of the flood, 5.3 million pounds of phosphorus, 63
million pounds of nitrogen, and 7.5 billion pounds of sediment entered
the Chesapeake Bay from its nine major tributaries (Figure 3). The
majority of the nutrients and sediment that entered the Bay during the
January 1996 flood came from the Susquehanna, Potomac, and James
Rivers. These three major tributaries combined transported 94 percent
of the phosphorus load, 97 percent of the nitrogen load, and 97 percent
of the sediment load to the Bay.
HOW MUCH ABOVE
NORMAL WAS THE JANUARY 1996 FLOOD?
The January flood transported roughly three times the amount of
freshwater, six times the amount of phosphorus, three times the amount
of nitrogen, and 17 times the amount of sediment to the Chesapeake Bay
than is transported during an average January. Nearly one-half of the
phosphorus, one-quarter of the nitrogen, and all of the sediment that
is typically transported to the Bay in an average year entered the Bay
during the January 1996 flood.
WHAT EFFECTS WILL
THE EXCESSIVE AMOUNT OF WATER, NUTRIENTS, AND SEDIMENT HAVE ON THE
Although the January 1996 flood was one of the largest ever recorded
on the Chesapeake Bay, its impact is expected to be minimal because of
the time of year it occurred. The flood occurred in the winter, a time
when grasses and many living organisms were dormant in the Bay and when
farmland, rich in nutrients, was frozen, thereby preventing 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.
Monitoring streamflow and water quality during this event and other major hydrologic events on the Bay is important in understanding how natural processes such as floods affect the health and living resources of the Chesapeake Bay.
As part of the Chesapeake Bay River Input Monitoring Program, the USGS, in cooperation with the Maryland Department of Natural Resources and the Virginia Department of Environmental Quality, continues to monitor the amount of water, nutrients, and sediment entering the Chesapeake Bay and thereby contributes to the information needed to restore its health and productivity.
District Chief U.S. Geological Survey 208 Carroll Bldg 8600 Lasalle Road Towson, Maryland 21286 (410) 512-4800 Internet: email@example.com