Urban Development

The watershed of the Bay is large, encompassing 168,000 square kilometers and including parts of the states of New York, Pennsylvania, West Virginia, Maryland, Delaware and Virginia and all of the District of Columbia. It is, therefore, greatly influenced by human activity on the land. Water quality in the Bay has suffered due to increases in the discharges of sewage, agricultural fertilizers and animal wastes, and through deforestation and land development.

The coastal watersheds in the mid-Atlantic are among the most highly developed in the United States and the declines in the water quality and aquatic ecosystems of the Chesapeake Bay estuary are related to the high levels of impervious surfaces within its watershed. Impervious surfaces, including asphalt, concrete and other hard surfaces that water cannot easily penetrate, can have deleterious effects on the hydrological functioning of a watershed and contribute to increased levels of nitrogen, phosphorous, sediments and other pollutants. Urbanization is taking place at a rapid pace in the region, much of it in outlying areas, characteristic of low density "sprawl."

In the Washington, DC area, we have mapped changes in the built environment between 1986 and 2000 (Figure 1 and right sidebar). Developed land increased 64% during this time period, from 2,544 to 4,167 square kilometers, and much of this new development is taking place in outlying counties.

Figure 1: New development near Washington, DC between 1986 and 2000.  This image is focused on northern Virginia, one area of rapid change in the region. The background is a Landsat satellite image, where shades of green indicate vegetation (i.e. forests), light colors indicate bare agricultural fields, and black is water.

When looking at change at the small watershed scale (Figure 2), impervious surfaces in several watersheds in the Washington, DC area have doubled between 1986 and 2000. Most watersheds have more than 10% of their area impacted by urban development.

Figure 2: Increases in developed area in 19 small watersheds in the Washington, DC area between 1986 and 2000.

We found that rates of change across the Chesapeake Bay watershed are comparable to what has been observed in the Washington, DC area.  Between 1990 and 2000, there has been a 61% increase in developed area, from 5,177 to 8,363 square kilometers. These dramatic changes have implications for resource land loss and stream health.

Access these data here:

ftp://ftp.whrc.org/GIS-RS/cbw/urb_development

©Woods Hole Research Center, 2007