When Lord Calvert arrived in 1634, Maryland was 95 percent forested. Today it is only about 44 percent forested, with only about 80 acres of virgin “old growth” left, half of it in the Bay watershed. Regrown forests are often managed for timber production, which causes subtle but substantial adverse effects on streams by altering temperature, erosion rates and hydrology.

Also, because forestry management has historically viewed dead trees as wasteful and potentially harmful to forest health, streams today lack an abundance of large woody debris such as tree trunks.This debris provides habitat, alters channel morphology and bank erosion rates, and helps to delay the downstream passage of nutrients. The loss of woody debris has been, and continues to be, a major influence on stream conditions in Maryland.

Coal Mining & Quarries

Coal mining in the Appalachian Plateau has severely impacted many streams in that region. In streams along the North Branch of the Potomac, acid mine drainage, primarily from abandoned deep mines, has created a legacy of severe impairment in a number of streams as well as the mainstem river. The highly acidic water can kill all life in the streams. It also includes highly toxic chemicals. In 1929, the runoff from water used to fight a fire in a gob pile (coal mine tailings) at Crelin, WV, destroyed virtually all of the life in the Youghiogheny River for nearly 40 years.

To mediate the acid mine drainage problem, calcium is being added via automated dosers in several locations; the mitigative effects of these dosers cease when funds to operate them are withdrawn. Besides coal mining, the operation of sand, gravel and rock quarries, often near streams, may impact local streams.

Beavers & the Fur Trade

An early impact of colonization in Maryland was the elimination of beaver. The once-abundant rodents altered stream ecosystems by raising water tables, trapping nutrients, altering channel morphology and gradient, creating small openings in the forest, and adding woody debris to the water. As beaver were eliminated, stream channels became less sinuous, and habitat diversity was reduced. Today, beavers are back, thanks to reintroductions, but densities are still well below historical levels.


By 1776, colonists had established major cities such as Annapolis, Baltimore, Frederick and Hagerstown. Raw sewage often accumulated in ditches around the growing cities, contributing to such waterborne diseases as malaria, yellow fever and cholera. The health of the region’s rivers and bays also deteriorated.

Since world War II, the expansion of suburbs and the road network has sped stormwater runoff to streams. Today, there are almost no roadless watersheds in Maryland.

Roads and associated development increase impervious surfaces, which collect water and rapidly flush it into streams where it erodes banks and adds to sediment in the river. Roads and other solid surfaces heat water before it goes into the stream, often increasing stream temperatures by several degrees. Pavement reduces the amount of rain that seeps into the ground and enters streams through the groundwater. During dry periods, this results in stream flows far below their natural condition. Roads and other solid surfaces also collect a variety of contaminants and flush them into streams.

Water Usage

Streams have been tapped for power, drinking supplies and other uses. More than 1,000 manmade barriers to fish movement are known to exist in the state. These have restricted the abundance and distribution of such aquatic species as the American eel, once a dominant stream fish in many basins of the state.

As water use has increased, groundwater consumption has exceeded recharge rates in some areas, and many streams have unpermitted water withdrawal systems on them. Such withdrawals during low flow conditions in the summer frequently result in increased water temperatures and less physical habitat for stream dwellers.

Air Pollution

The combustion of coal and petroleum products releases nitrogen and sulfur oxides into the atmosphere. Because Maryland is situated within the “Belt of Prevailing Westerlies” it receives atmospheric pollution from the Midwest. In fact, the Chesapeake Bay airshed includes parts of 12 states.

While deposition is fairly even throughout Maryland, some places receive more than others because of geological differences, such as the Coastal Plain, Appalachian Plateau and portions of the Blue Ridge and Piedmont, where there is little natural buffering capacity. In contrast, the Valley and Ridge and parts of the Blue Ridge and Piedmont are well-buffered and resistant to acidification. Many fish and insects cannot live in acidic streams. Sudden increases in acidity — such as spring snowmelt — can be lethal to stream dwellers.


The soils, topography and climate of Maryland made it well-suited for farming. By the early 1700s, elaborate systems of ditches drained wetlands while forests were cleared for agriculture. This resulted in large amounts of sediment eroding into the water. Joppatowne, Port Tobacco and Upper Marlboro were closed as shipping channels became filled with silt.

In the 19th and 20th centuries, with the advent of larger farm machinery, hedgerows and stream buffers were removed — especially along smaller streams — to improve efficiency, increasing sediment loads even more. By the time erosion control became common practice in agriculture in the 1960s, streams had been significantly altered.

Agriculture also increases the amount of nutrients in the water. Today, nitrogen and phosphorus levels are elevated in most areas, especially near large poultry and hog operations. Limestone is added to cropland, especially on the calcium-poor coastal plain. The addition of nutrients and limestone has affected stream productivity, and in many cases has altered the biological community as well.