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August Hurricanes Raise Water Levels in Southern Maryland

U.S. Department of the Interior
U.S. Geological Survey

Wendy McPherson (
Maryland-Delaware-D.C. Water Science Center
5522 Research Park Drive
Baltimore, MD 21228
FAX: (443)498-5510

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Updated: September 3, 2004

Rainfall from Hurricanes Bonnie and Charley in August caused water levels to rise in Maryland. Streamflow levels in southern Maryland are above normal and an observation well in Charles County is at its highest August level since 1979 according to hydrologists at the U.S. Geological Survey (USGS). In other areas of Maryland and Delaware, August water levels were normal to above normal. Total freshwater flow to the Chesapeake Bay was more than twice the normal amount for August.

Status of Streams and Wells

The map below shows the location of the wells and streams used by the USGS to monitor water conditions in Maryland, Delaware, and the District of Columbia. In July, water levels were normal in western Maryland, normal to above normal in the Piedmont and southern Maryland regions. Water levels ranged from below normal to above normal in eastern Delaware.


Rainfall in August ranged from below normal to above normal across Maryland and Delaware. Rainfall was 3.74 inches, or 1.03 inches below normal, at Baltimore-Washington International (BWI) Airport, and only 2.30 inches (second consecutive month) at Hagerstown, Maryland, according to preliminary rainfall data from the National Weather Service. However, summer rainfall was above normal and temperatures were not excessive in August. Rainfall in Wilmington, Delaware was 1.82 inches above normal (5.33 total inches), and has been above normal for several months. In Somerset, Wicomico, and Worcester Counties, Maryland, August rainfall was nearly 3 times normal. Rainfall at Washington D.C. was 1.65 inches above normal, with 5.09 inches.

Chesapeake Bay

Monthly mean streamflow into the Chesapeake Bay during August averaged 46.2 bgd (billion gallons per day), which is 113 percent above normal. Flow has been above normal for the last 2 months: See graph at The Susquehanna, Potomac, and James Rivers are the largest rivers in the Chesapeake Bay watershed and usually contribute about 90 percent of the freshwater streamflow to the Bay (see table below). In an average year, approximately 50 percent of the freshwater flow comes from the Susquehanna River, 25 percent from the Potomac River, and 15 percent from the James River. The remaining 10 percent comes from surrounding tributaries and smaller streams. This August, more freshwater entered the Bay from the Susquehanna River (68 percent) than normal, while the Potomac and James Rivers contributed less water than normal. The Potomac River contributed 10 percent, the James River 8 percent, and 14 percent was from other sources in August. More information about USGS studies to help with the protection and restoration of the Chesapeake Bay and its watershed can be found at


Streamflow in Maryland and Delaware ranged from normal to above normal levels in August. Rainfall in western Maryland was below normal and although streamflow levels rose, they were at normal levels. Streamflows in the central Maryland Piedmont region were normal to above normal. Southern and eastern Maryland received more than double the normal rainfall and streams responded with large increases, such as on the Pocomoke River (see hydrograph below). Current and historical streamflow data can be monitored on the web at:

Five-year hydrograph of monthly mean streamflow on the Pocomoke River shows streamflow was below normal in July (orange) and above normal (blue) in August. The large increase in flow was in response to rainfall associated with Hurricanes Bonnie and Charley. Normal monthly mean flow is shown by the white band. Five-year monthly streamflow hydrographs from the USGS stream-gaging network can be viewed on the USGS website at:

Daily streamflow on the Potomac River near Washington, D.C. averaged 3.3 bgd in August, which is 24 percent above normal for the month of August. More information on the Potomac River is available at:

Groundwater-Unconfined or Shallow Aquifers

Abundant rainfall throughout the summer caused groundwater levels to rise in many of the wells used by the USGS to monitor unconfined or shallow aquifer response to climatic conditions in the bi-state region. Groundwater levels were in the normal to above normal range during August, and the observation well in Charles County reached the highest August level since 1979. For 5-year hydrographs of groundwater levels for the climatic indicator wells, visit:

Groundwater-Confined or Deep Aquifers

Water levels in the deep confined aquifers continue to decline because the wells are pumped at higher rates than the rate at which deep groundwater is recharged. Because confined aquifers are deep, water levels in confined aquifers respond slowly to climatic conditions. The network of confined aquifer wells has been reduced because of diminishing funds. Several wells will no longer be measured on a monthly basis and the web pages will be updated when data is collected. Limited data for confined aquifer wells can be viewed at: Real-time water-level data can be viewed at:

Reservoir Storage

Storage of the Baltimore reservoir system dropped 2 percent, to 96 percent of capacity in August. The Baltimore reservoirs (Loch Raven, Liberty, and Prettyboy) have been nearly full since May 2003. Storage in the Triadelphia and Duckett Reservoirs on the Patuxent River, which serves Montgomery and Prince Georges Counties, dropped 7 percent to 89 percent of capacity in August.

Water Monitoring

The USGS has been collecting National streamflow data for 120 years, since 1884. Streamflow monitoring began in Maryland on the Potomac River at Point of Rocks, Maryland in 1895 and continues today. Streamflow and groundwater levels are used to assess current water conditions and can be used to predict the potential for flooding and drought conditions. These USGS data have been provided to State and local water resource managers and are critical for making appropriate decisions on water regulation. For more information on streamflow and groundwater levels in Maryland, Delaware, and the District of Columbia, visit Water Watch at:

The real-time streamflow stations used in this analysis are operated in cooperation with the Maryland and Delaware Geological Surveys, the Maryland State Highway Administration, the U.S. Army Corps of Engineers, the Maryland Department of Natural Resources, the Maryland Department of the Environment, Baltimore County, Baltimore City, and other agencies. The long-term observation wells used in this analysis are operated in cooperation with the Maryland and Delaware Geological Surveys and the Interstate Commission on the Potomac River Basin. The real-time wells are operated in cooperation with the Maryland and Delaware Geological Surveys, the Interstate Commission on the Potomac River Basin, and Calvert County, Maryland. The USGS publishes data for 137 streamflow stations, 393 observation wells, and 4 springs across Delaware, Maryland, and the District of Columbia.

The USGS serves the Nation by providing reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.

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