Dealing with growth has been considered by some to be the “Achilles heel” of the Bay Program.

The 1987 Bay Agreement warned “there is a clear correlation between population growth and associated development and environmental degradation in the Chesapeake Bay system.”

The agreement said state and federal governments “will assert the full measure of their authority to mitigate the potential adverse effects of continued growth.”

A panel of experts was appointed to study the issue, and in 1988 they reported that “unmanaged new growth has the potential to erase any progress made in Bay improvements.” It called for more assertive planning efforts by all three states to control the rate of growth, as well as preserve farms, forests and environmentally sensitive areas.

But it was not until the Chesapeake 2000 Agreement, which calls for a 30 percent reduction in the rate of sprawl by 2012, that the Bay Program set a quantifiable goal to deal with the issue. The commitment was teamed with a complementary goal to permanently preserve 20 percent of the watershed by 2010. Together, these commitments were designed to slow the rate of land conversion and ensure that some natural habitat and open space remain forever undeveloped.

The relationship between the Bay and its watershed is critical: While the Chesapeake covers about 4,000 square miles, its drainage basin covers 64,000 square miles — an area 16 times greater than the surface of the Chesapeake. What runs off the land can overwhelm the Bay.

As land is developed, though, its ability to filter water — and remove nutrients and other materials — is reduced. Instead of slowly filtering through the soils and into the groundwater, or slowly flowing through forests toward streams, it is often quickly routed into the nearest waterway.

That reduces the amount of water entering streams through groundwater (an important source of stream water during dry periods) and increases stream flows during rainfall. Wide fluctuations in stream flows stress fish and other stream-dwellers. The increased flow during storms erodes streambanks, smothering important bottom habitats with silt.

yield work by the Maryland Department of Natural Resources’ Biological Stream Survey found brook trout were absent from watersheds in which 2 percent of the land was covered with impervious surfaces such as pavement or buildings. As the amount of impervious surfaces increased, fewer species were found.

Sprawled, low-density development increases the distance people have to drive. That can offset expected air quality improvements from more stringent vehicle emission requirements. Nitrogen oxides from fossil fuel combustion, about a third of which come from vehicles, are a major source of nitrogen entering the Bay.

From 1970 to 1994, the population of the watershed grew by 26 percent, but the number of miles traveled within the watershed increased by 105 percent.

Low density developments are more likely to use septic systems, which generally offer a lower level of nutrient removal than many wastewater treatment plants.

Because of such impacts, the Chesapeake 2000 Agreement noted that “difficult choices” lay ahead in managing growth, but offered a vision of the future: “We acknowledge that future development will be sustainable only if we protect our natural and rural resource land, limit impervious surfaces and concentrate new growth in existing population centers or suitable areas served by appropriate infrastructure.”