Land development poses a number of threats to both the habitat and water quality of streams. Planning and development practices can reduce, though not entirely eliminate, many impacts, which include:

Wetlands and forests, which act as natural filters, may be lost to development. Even if they are not directly disturbed, development may increase runoff to these areas or alter their hydrology.

Population growth increases the burden on sewage treatment plants. Growth in areas not served by sewers usually must rely on septic systems which generally are less effective at nutrient removal.

Sediment runoff increases as more ground is disturbed. Sediment can smother bottom-dwelling organisms in streams and the Bay, and cloud the water, preventing sunlight from reaching grass beds that provide important habitat for fish and other aquatic species.

Increased nutrient runoff occurs as fertilizers are often placed on lawns and other developments at rates that exceed those used on crop land. The excess is washed away with the rainwater and ultimately may find its way into streams or groundwater. Nitrogen oxides landing on paved surfaces also add to the nutrients contained in runoff. Excess nutrients cause algae blooms that cloud the water and also deplete water or oxygen needed by aquatic organisms.

Rain falling on roofs, roads and parking lots is directed into streams faster than would take place when rain falls on grass or wooded areas. That results in a sudden "pulse" of water that erodes streambanks, increasing the amount of sediment in the water. Runoff water is often heated by the pavement and can raise the temperature of a stream enough to make it unsuitable for use by many native species.

Sprawled development that forces people to spend more time in their cars can add to air pollution problems. About a third of the nitrogen oxides - a major source of nitrogen pollution to the Bay - stems from auto exhaust, and growth in automobile use in coming years is expected to offset some of the new pollution reductions expected under the 1990 amendments to the Clean Air Act. Tailpipe exhaust also contributes toxics to the air, including several polynuclear aromatic hydrocarbons (PAHs), which are among the Bay Program's "Toxics of Concern" - those chemicals which pose the greatest threat to Bay organisms.

Stormwater runoff from developed areas is one of the major sources of toxic pollution to the Bay and its tributaries. Contaminants in stormwater runoff include pesticides, heavy metals, chemicals leaking from automobiles and other contaminants