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Record Low Water Flows to Chesapeake Bay in June
Released: 7/2/1999

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Jim Gerhart 1-click interview
Phone: 410-238-4201 | FAX: 410-238-4210




Water use from Potomac River during June 1999 was one third of river flow

As the Mid-Atlantic region continued to experience prolonged dry weather, flow into the Chesapeake Bay and from its three main tributaries dropped to record low volumes for the month of June, according to U.S. Geological Survey hydrologists. Freshwater inflow to the Bay was less than 74% of the previously recorded low set in 1964, because flows from the Potomac, Susquehanna, and James Rivers all set new low flow records.

June 28 was the lowest flow day recorded for the month. During that day, almost half of the total Potomac River flow was used for water supply. (Total flow is computed using data from the USGS gage at Little Falls and municipal withdrawal figures.) Average daily flow in the Potomac in June 1999 was only about 24% of the flow in June 1998, and withdrawals for water supply were 25% higher than withdrawals in 1998. Consequently, unusually high proportions of river flow (33%) were diverted out of the river into public water supplies last month. The normal diversion rate in June is about 5% of flow.

The numbers follow impacts.

Current and potential impacts of these low flow conditions include:

· Increased stress on fish in streams. The local movement of fish may be impeded in small streams as riffles become too shallow for them to cross or if the level of dissolved oxygen in the water drops due to localized algal blooms. Fish have to move around in streams in order to adjust to short-term fluctuations in water conditions, such as daytime temperature increases and pollutant inputs.

· Conversely, improved water-quality conditions in the Bay. Smaller amounts of nutrients and sediment were carried into the Bay this spring, and there have been fewer algal blooms than usual. Consequently, levels of dissolved oxygen were above average in the bottom waters of the Bay this spring. Higher dissolved oxygen levels are beneficial to bottom-dwelling organisms, including crabs, oysters, and fish.

· Healthier underwater grasses. Low amounts of sediment and algae in the water allow better light penetration and encourage the growth of submerged aquatic vegetation (SAV).

· Higher salinity levels in Bay waters and in coastal low-gradient streams, especially those on Maryland’s Eastern Shore and in most of Delaware. Higher than usual salinity levels in Bay waters could encourage the growth of oyster diseases and increase the abundance of jellyfish during the summer.

· Freshwater may not be as available to some farmers who irrigate by pumping from streams.

· 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 may 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.

The Numbers:

The USGS maintains a long-term stream gaging network across the country. Here is specific information for Maryland and Delaware.

One of the most severe hydrologic droughts on record in the Mid-Atlantic region took place in the mid-1960’s, according to the U.S. Geological Survey (USGS). June 1999 river flows into the Chesapeake Bay and its tributaries approached and, in some cases, fell below mid-1960’s levels. At the same time, ground-water levels have fallen to near or below mid-1960’s levels. The low-water conditions are the result of a precipitation deficit that dates back to the late spring of 1998.

Hydrologists at the USGS report that the previous lowest daily average river inflow to the Chesapeake Bay for June was recorded in 1964 at 15.2 billion gallons per day (bgd). Last month, the flow was 11.2 bgd, only 28% of the average flow for June (40.2 bgd). In June 1964, average daily inflows to the Bay at the mouths of the Susquehanna, Potomac, and James Rivers were 7.7, 2.7, and 1.6 bgd; the June 1999 flows were 4.8, 2.2, and 1.4 bgd.

Daily average June flow at the Potomac River at Washington, D.C. was about 1.5 bgd, about 30% of the average flow (about 4.9 bgd). The rate of flow varied from a high of about 2.1 bgd on June 3 to a low of about 1.0 bgd on June 28. The previous lowest June flow in the Potomac was set in 1969 at 1.7 bgd.

The lowest flows ever measured by USGS on the Potomac River were in September of 1966 (see graph on p. 3-- note logarithmic scale!). The decreasing trend in flow rate in 1999 is parallel to the trend set in 1966. The average daily flow rate in the Potomac in June 1999 is already lower than the June 1966 rate (1.9 bgd). If the precipitation deficit continues, it is possible that the river may continue to set record low flow levels as the summer progresses.

Diversions from the Potomac River for water supply in June averaged about 505 million gallons per day (mgd)--25% more than June of last year. On June 28, the lowest flow day recorded for the month, the average flow rate over the dam at Little Falls was about 519 mgd and diversion for water supply (taken out above the dam) was about 498 mgd--almost half of the total flow in the river for the day. Diversions from the Potomac are made in seven places above the Little Falls dam and are usually about 5% of the total river flow at this time of year.

The contents of the Baltimore reservoir system decreased to 77% of average. Last year at the end of June, the contents of the Baltimore reservoir system were 86,660 million gallons (8% above the average June volume); this year, the reservoir system held 61,510 million gallons at the end of June.

Ground-water levels throughout large parts of Maryland and Delaware are at or near the lowest levels recorded during the mid-1960’s. For example, the water level measured by USGS in June in the Fairland, Md. observation well (near Washington, D.C.) was more than 4 inches below the previous record low level (set in 1986) and about 2 feet 5 inches below the long-term June average water level for that well. Ground-water levels in the Fairland well have been below average since August, 1998, and have been equal to or lower than 1965-66 levels since October 1998 (see graph on page 4). Ground-water levels are likely to continue to decline as long as the precipitation deficit continues, with the potential of reaching record low levels later this summer.

Drought updates for Maryland may be accessed on the web at http://md.water.usgs.gov/. For Virginia, see http://va.water.usgs.gov/. Newly available on the web from the USGS is a national map of daily streamflow information, color-coded for quick reference. The URL is http://water.usgs.gov/dwc/national_map.html. Real-time streamflow data, both tabled and graphed, for Maryland and Delaware may be viewed on the web at http://md.usgs.gov/rt-cgi/gen_tbl_pg. Other information on >water resources in neighboring states can be found on the USGS Chesapeake Bay web page at http://chesapeake.usgs.gov/chesbay/ under "Current Hydrologic Conditions."

Potomac River Water Supply Near Washington, D.C--1999


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