Lower Potomac River Basin Report

Lower Potomac River Basin Report

Based on the results of 1995 MBSS sampling, the major impacts to streams in the Lower Potomac River basin appear to be low dissolved oxygen levels, acid deposition, nitrogen enrichment, impaired instream habitat, fish migration barriers, and bank instability. The most likely reasons for many of these impacts are stream alterations which stem from agriculture and continuing urban sprawl.

Some of the specific findings include:

• Nitrate levels were elevated (greater than 1mg/l) in 12% of the stream miles in the basin. The primary sources of elevated nitrates in the basin are likely agriculture and acid deposition. Because these results represent primarily spring baseflow conditions, and by inference, groundwater concentrations, reductions in nitrate loading in problem areas of the basin may not be apparent for many years to decades until groundwater sources are purged of their elevated nitrogen levels.

• About one-third of the streams had dissolved oxygen (DO) levels less than the Maryland water quality standard of 5 ppm. This condition is likely due to farming practices in the basin and urban runoff that lead to over-enrichment and subsequent episodes of low DO.
  

• About 20% of the stream miles in the basin have limited or no buffering capacity, and an additional 60% of the stream miles in the basin may be subject to pH excursions during large storm events that are harmful to some aquatic biota. The generally sensitive nature of streams in the basin to acid inputs is also supported by the composition of the fish communities. The dominant fish species in the Lower Potomac River basin is the highly acid tolerant eastern mudminnow (38% of the total assemblage, or 1.3 million fish).

 

• In general, the riparian zone was healthy and about 59% of all stream miles had vegetated buffers greater than 50 meters wide. However, 62% of the streams had either fair or poor instream habitat, and highly unstable banks were observed at about 16% of the streams in the basin. Channelization and historical harvest of trees from the riparian zone are among the probable causes of unstable banks and reduced instream habitat.

• About 6% of the stream miles classified by the United States Geological Survey (USGS) as having year round flow were dry during MBSS sampling in 1995.

• Based on DNR’s fish Index of Biotic Integrity (IBI), about a third of the stream miles in the basin were in good condition, while 6% were in very poor condition. Physical habitat was an important influence on fish communities.

• About 3.4 million fish reside in non-tidal streams of the basin. A total of 41 species were collected in 1995, including the gamefish largemouth bass and chain pickerel. Nearly all of the estimated 5,100 largemouth bass and 27,000 chain pickerel were sub-adults. Thus, the streams in the Lower Potomac River basin may be important nursery areas which support fisheries in larger streams and the oligohaline portions of the Potomac River estuary.

• There are 34 known barriers to fish migration in the basin— this condition restricts use of historical spawning and nursery areas by migratory fish and contributes to the highly depleted condition of many migratory fish species in Chesapeake Bay.

For a copy of the Lower Potomac or one of our other basin reports, contact Ann Smith of DNR/MANTA at (410) 260-8611 or email her at asmith@dnr.state.md.us. A nominal fee will be charged to cover the costs of printing and postage.

to the MBSS Newsletter

Return to the Maryland Streams Page
Jump to the Maryland DNR Home Page.