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Chesapeake Bay and Potomac River are feeling the lack of April and May showers
Released: 6/3/1999

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Scott Phillips 1-click interview
Phone: 410-238-4252 | FAX: 410-238-4210




All-time record low river flows into the Chesapeake Bay were recorded for May, according to hydrologists at the U.S. Geological Survey (USGS). Flow into the bay has been below average for the last 10 months. Average daily freshwater inflow to the Chesapeake Bay in May was about 29.1 billion gallons per day (bgd), which is about 46 percent of the long-term average (62.6 bgd).

Lately, the Potomac River at Washington, D.C. has been hovering around the 10th percentile of streamflow, which means that historically (the last 68 years), the river has been this low on a daily basis only about 1 day in 10. Record low daily flows in some tributaries to the Potomac have recently been recorded. For example, flow in the Monocacy River near Frederick, Md. has been lower for the last 6 days (May 28 - June 2) than it has been on these dates since record-keeping began there 69 years ago.

Monthly flow rates in the Potomac have been below average for the last 9 months. Average daily flow in the Potomac River in May was about 3.8 bgd, which is about 38 percent of the long-term average (9.8 bgd).

The summer and fall of 1998 were extremely dry, according to the National Weather Service, and precipitation was slightly above normal at the beginning of 1999. USGS hydrologists have observed low to record low flow rates in the rivers flowing into the Chesapeake Bay since August of 1998. The increased precipitation in early 1999 resulted in normal winter increases in flow rates, although the flows continued to be below average during the winter and spring.

Rainfall in the Mid-Atlantic region in May was below normal again. The result has been continuing low-flow conditions in Mid-Atlantic rivers. The Maryland Department of the Environment issued a drought advisory for the State of Maryland in December that is still in effect.

Implications for the Chesapeake Bay and other resources:

· Water-quality conditions seem to have improved this spring as smaller amounts of nutrients and sediment have been carried into the bay.

· As a result of lower nutrient levels, there have been fewer algal blooms than usual this spring; consequently, levels of dissolved oxygen have been above average in the bottom waters of the bay, which benefits bottom-dwelling organisms and fish.

· Salinity levels in bay waters are higher than usual. This could encourage the growth of oyster diseases and increase the abundance of jellyfish during the summer.

If low-flow conditions persist, other impacts that may be noticed in the shorter term or become more obvious include:

· Small hydroelectric operations may have less water to use for the generation of electricity because required biotic-release flow levels downstream must be maintained.

· Recreational boating will be adversely affected, particularly canoeing on smaller streams, but also canoeing, commercial rafting, and tubing on larger streams. White-water recreation will be particularly affected as water behind dams is preserved for other uses.

· In low-gradient streams such as those on Maryland’s Eastern Shore and in most of Delaware, salinity levels will be higher further upstream, which could have general ecological impacts in the streams and adjacent wetlands. Freshwater may not be as available to some farmers who irrigate by pumping from streams.

· The local movement of fish may be impeded in small streams as riffles become too shallow or other controlling features become exposed. These movements are required by fish to adjust to short-term fluctuations in water conditions, such as daytime temperature increases or pollutant inputs.

Real-time streamflow data, both tabled and graphed, for Maryland and Delaware may be viewed on the web at http://md.usgs.gov/realtime . Other information on water resources can be found on the USGS Chesapeake Bay web page at http://chesapeake.usgs.gov/chesbay/ under "Current Hydrologic Conditions."


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