Drought and the Chesapeake Bay: Flirting with record low flow

July 30, 1999

Near-record low volumes of freshwater flowed into the Chesapeake Bay in July, according to the U.S. Geological Survey (USGS). Daily average river inflow to the Bay in July was 8.5 billion gallons per day (bgd), 37% of the long-term average flow for July (23.1 bgd).

USGS hydrologists report that the July low flow record to the Chesapeake was recorded in 1966 at 6.8 bgd, which was 29% of the long-term July average and 80% of the July 1999 flow.

River inflow to the Bay in June 1999 was a record low 11.2 bgd, only 28% of the long-term average flow for June (40.2 bgd). The previous lowest daily average river inflow to the Chesapeake Bay for June was 15.2 bgd, recorded in 1964. The record low flow rate into the Chesapeake for August was 6.0 bgd, set in 1966. Records for freshwater inflow to the Bay have been kept since 1951.

These extreme low flow rates into the Chesapeake Bay are the result of record or near-record low inflows from all its tributary rivers and streams, said Gary Fisher, a USGS surface-water specialist. The major tributaries to the Chesapeake Bay are the following.

Susquehanna River. Daily average inflow to the Bay at the mouth of the Susquehanna in July was 3.5 bgd, 42% of the total river inflow to the Bay in July.

Potomac River. Daily average flow at the Potomac River at Washington, DC gage (Little Falls) was about 0.9 bgd, 30% of the average flow for June. The Potomac River contributed 16% of the total river inflow to the Bay in July. [Why the Potomacs not setting records--below.]

James River. Daily average inflow to the Bay at the mouth of the James was 1.2 bgd, 14% of the total river inflow to the Bay in July.

Since there was very little rain in July, virtually all the flow in the Bays tributary rivers and streams came from ground water seeping into streams, according to Fisher. The prolonged dry weather has resulted in very low ground water levels across the region. A record low water level was recorded at the USGS index well in Montgomery County, MD on July 29.

Details, real-time streamflow, other state and national drought information, and definitions may be found on the USGS web site md.usgs.gov/drought.

Current and potential impacts of these drought conditions include:
The contents of the Baltimore reservoir system decreased to 68% of average in July. Last year at the end of July, the contents of the Baltimore reservoir system were 85,440 million gallons (9% above the average July volume); this year, the reservoir system held 53,320 million gallons at the end of July, 62% of capacity.

Slow recovery of water supplies, especially ground water. The months of rainfall well below average have depleted the water system in the region, so that it will take months of average rainfall to replenish the ground-water system and large reservoirs and lakes.

Increased stress on fish in streams. The local movement of fish will 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 or other phenomena. 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. Submerged aquatic vegetation (SAV) is flourishing in the Chesapeake Bay and its tributary rivers and streams, says, Nancy Rybicki, of the U.S. Geological Survey. Low amounts of sediment and algae in the water allow better light penetration and encourage the growth of plants.

Higher salinity levels in Bay waters and in coastal low-gradient streams, especially those on Marylands Eastern Shore and in most of Delaware. Higher than usual salinity levels in Bay waters could encourage the growth of oyster dis- eases. Jellyfish are in abundant supply already this 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 Potomac:
The USGS maintains a long-term stream gaging network across the country. Here is specific information for the Potomac River.

Low-flow records are being approached or set in many of the Potomacs tributary streams, but the flow in the main stem did not set July records because of the input from reservoirs.The lowest flows ever measured by USGS on the Potomac River were in September of 1966 (see graph on web page md.usgs.gov/monthly/poto.html . Current flows in the Potomac are not at record-low levels because regulations implemented since 1966 mandate that minimum flow conditions of 300 mgd below Great Falls and 100 mgd below Little Falls must be maintained at all times by releases from reservoirs upstream. Flow in the Potomac was unusually low this year between July 11 and July 29.

Withdrawals from the Potomac River for D.C.-area water supply in July 1999 averaged about 516 million gallons per day (mgd)--16% more than the average daily use in July 1998 and 11 mgd more than June 1999. The average daily water use diversion from the Potomac in July 1999 was 57% of the entire flow of the river. Diversions from the Potomac are made in seven places above the Little Falls dam and are usually about 17% of the total river flow at this time of year.
News Release
U.S. Department of the Interior
U.S. Geological Survey
Maryland-Deleware-D.C. District
8987 Yellowbrick Road
Baltimore, MD 21237
Gary Fisher 410-238-4259

US Geological Survey

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