Flooding from Fran - Potomac River, DC, and Chesapeake Bay

Updated: Prepared Monday, September 9, 1996

Nearly all major streams and rivers in the Potomac River basin experienced serious flooding during September 6-9, according to the U.S. Geological Survey. Many of the streams draining to the Potomac and the river itself approached or exceeded the levels recorded during the January 1996 floods.

According to USGS scientists, the Potomac River at Washington, D.C., had a peak stage of 17.81 feet and a flow of 313,000 cubic feet per second (202 billion gallons a day) about mid night. By comparison, the peak of the January 1996 flood produced a flow of 347,000 cfs (224 bgd). During Hurricane Agnes, the peak flow was 359,000 cfs, and the highest peak flow of record was 484,000 cfs in 1936.

Many of the streams and rivers flowing to the western portion of the Potomac river, such as the South and North Branches of the Potomac and the Shenandoah Rivers, experienced flooding close to or exceeding the January, 1996 levels, which was near a 100-year flood. Consequently, the mainstem of the Potomac river above Harpers Ferry, near Paw Paw, WVA, Hancock, MD and Point of Rocks, MD had flooding near the January levels as this water drained from the tributaries and flowed downstream. Further downstream, near Washington, D.C., the flood waters were not as large as the January floods, with the levels near a 20-year flood, meaning there is a 5- percent chance of a flood this size in any given year. River levels were not as high because the rains did not greatly affect the rivers closer to DC, such as the Monacacy river and Conococheague Creek.

The flooding the past several days on the Potomac only added to near-record amounts of water moving down the river during 1996. The Potomac river flow has been above normal during most of 1996; only February and April flows were about average for those months. This August, the river averaged about 6.7 times the normal flow for the month, the highest since 1931, when the USGS began to keep records. In addition, some sites have river levels equivalent to a 100- year flood, or one that has a 1-percent probability of occurring in any given year. In conjunction with the flooding in January and June, parts of the Potomac River basins have exceeded 100-year flood values two times during 1996.

The affects of Tropical Storm Fran caused two periods of flooding in the Washington DC area. Although high tides and winds caused high water Friday night in the area, the high water from the storm runoff and resultant river flooding did not hit until Saturday night and Sunday.

"The stream flows we measured this past weekend are similar to those measured in January and they are among the highest we've ever seen in the Maryland area", said Robert W. James Jr., head of the stream-gaging program for the Maryland-Delaware-D.C. office of the USGS.

The impact of the floods on the Chesapeake Bay is unknown at this point, although large amounts of nutrients--nitrogen and phosphorus--and sediment were mobilized and trans ported by the flood waters. During the January 1996 floods about 2.5 billion pounds of sediment, 20 million pounds of nitrogen, and 2 million pounds of phosphorus were transported into the Chesapeake Bay from the Potomac River, which is the second largest river entering the Bay. The largest river--the Susquehanna--did not experience the same degree of flooding this past weekend because of the northwestern track of Tropical Storm Fran.

The USGS has been collecting samples since this past Friday to determine the amount of nutrients, pesticides, and sediments that entered the Bay during this storm. The amount of sediment and phosphorus entering the Bay could be higher than in January because the frozen ground decreased the amount of sediment erosion that occurred with the large snowmelt.

The timing of the storm may reduce its potential affects on water-quality and living resources of the Chesapeake Bay according to Scott Phillips, USGS hydrologist. Phillips commented "the sediments decrease the amount of light in the Bay for plants and grasses, but the plants are starting to die off this time of year as part of their natural cycle. Additionally, the increased nutrients can cause algal blooms, but this is mostly a concern in the spring. Therefore, the adverse affect of this storm may not be as great as if it had hit in late spring or early summer".

The USGS stream-gaging program in Maryland and the District of Columbia is supported by funding from numerous Federal, State, and local agencies, including the U.S. Army Corps of Engineers, Maryland Department of Natural Resources, Maryland Geological Survey, Maryland State Highway Administration, City of Baltimore, and Washington Suburban Sanitary Commission.

During floods, flow data are used chiefly by the Corps of Engineers to manage water projects such as diversions, dams, locks, and levees. The National Weather Service uses the data for flood warning and flood-forecasting purposes and the Federal Emergency Management Agency uses the data as part of its nationwide floodplain mapping and flood-insurance program.

(For further details contact Jim Gerhart, District Chief (410 238-4201) or Bob James, Hydrologic Surveillance and Assessment Chief (410 238-4205) or see our Web Page at http://md.water.usgs.gov/

Table listing stage and discharge at selected stream-gaging stations

Images showing flooding near Washington, D.C.