Public Release: 

Rivers Deliver Record Flow to the Cheseapeake Bay in 1996

US Geological Survey


Total freshwater inflow into the Chesapeake Bay during 1996 was the highest ever recorded, according to the U.S. Geological Survey.

USGS scientists said that rivers and streams carried a combined average flow of 87.5 billion gallons of water per day (bgd) into the Bay during the past year, about 1.7 times the long-term normal. Previous record high was 85.1 bgd in 1972, the year of Hurricane Agnes.

The high flows also carried increased amounts of sediment, nutrients, and other chemicals that affected the water quality and living resources of the Bay. The environmental effects could have been much worse, however, according to preliminary information collected by the agencies involved in the cooperative Chesapeake Bay Program.

"The repeated high waters and floods of 1996 caused obvious and serious economic damage throughout the upstream Chesapeake basin. Less well understood are the environmental effects of those record inflows to the Bay itself," according to Scott Phillips, coordinator of the USGS Chesapeake Bay effort, Towson, Md.

"Although the record freshwater inflows damaged and caused localized losses to vegetation and oyster beds, the total impact on the Bay ecosystem probably did not match the damage from the previous record flow year of 1972," Phillips said.

"Two primary factors -- the timing of the large storms and the success of management actions to reduce nutrients entering the Bay -- lessened the potential effects of the high flows," the USGS spokesman said.

These favorable factors include:

  • The highest flows in 1996 hit in January and December, when underwater plants and grasses are not growing. The grasses are an important habitat for crabs and food for waterfowl. In 1972, the high flows came in the summer, which is a very productive time for grasses and other living resources in the Bay.
  • Information the USGS is collecting in cooperation with the Maryland Department of Natural Resources and the Virginia Department of Environmental Quality indicate that nitrogen and phosphorus concentrations have decreased since the 1980s in most of the major rivers draining to the Bay, due to implementation of nutrient management actions. As a result, the amount of nutrients entering the Bay, though large, was less than it would have been under previous management practices.

Chesapeake Bay Program agencies reported that high flows in 1996 affected the Bay in several ways:

  • The increased flows decrease the salinity of the Bay and make the water fresher. The unusually low salinities in 1996 resulted in localized oyster mortalities in Maryland and the Potomac River. The low salinity, however, may help keep the oyster diseases, MSX and Dermo, in check, allowing surviving oysters to grow.
  • The large amount of nutrients brought in by the rivers caused a large spring algal bloom. This bloom, along with high sediment inputs, decreased the amount of light available to underwater grasses and plants, which are important as nurseries for young and larval forms of creatures in the Bay. High flows in 1993 and 1994 resulted in slightly decreased plant growth for the next two years. More losses may result in 1997 from the high 1996 flows.
  • The algal blooms contributed to low dissolved oxygen counts in the bottom waters of the Maryland portion of the Bay, but not record lows.
  • Fish and crabs are not expected to show long-term effects from the 1996 high flows.
  • As a result of high water levels in the nontidal portion of the Potomac River all during the growing season, submerged aquatic vegetation disappeared almost completely between Harpers Ferry and Great Falls. There was also extensive loss of submerged vegetation in many Potomac tributaries such as Conococheague Creek.
  • High turbidity and very low available sunshine also decreased the amount of submerged aquatic vegetation in the tidal Potomac River. The plant hydrilla was particularly affected -- most of the large bed on the shoal under the Wilson Bridge did not survive. Submerged vegetation above the Wilson Bridge, such as the newly appearing vegetation around Roosevelt Island and existing beds near National Airport, declined significantly.
  • The freshwater inflow into Chesapeake Bay was higher than normal during every month except April, and in January and December reached the highest levels ever recorded since USGS measurements began in 1951.
  • Susquehanna River in 1996 averaged 41.9 billion gallons a day (bgd), which is 1.6 times normal flow and the second highest annual flow behind 1972 (43.0 bgd).
  • Potomac River in 1996 averaged 22.3 bgd, which is 2.3 times normal flow and the highest ever recorded.
  • James River in 1996 averaged 10.8 bgd, which is 1.6 times normal flow and the third highest on record (1972 was the highest at 12.2 bgd).

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