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Well-Managed Watersheds Key to Restoring Bay
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The forests of a watershed such as Mattawoman protect the water and the climate by sequestering greenhouse gases.

The adage “you are what you eat!” applies equally well to the Chesapeake Bay: thousands of tributaries feed the Bay whatever is served by their watersheds. Hence proper management of local watersheds is key to restoring the Bay’s integrity, as is well known. Less obvious is that a well-managed watershed also reduces the production of the greenhouse gases responsible for global warming.

Because a watershed managed for water quality necessarily preserves forest cover, the greenhouse gas CO2 (carbon dioxide) continues to be sequestered in wood, as discussed below. Curbing sprawl development is critical for forest preservation and also yields very significant reductions in CO2 emissions because of reduced demand for transportation and for heating and cooling of large houses. Preserving forests also improves quality of life because of green space, decreased traffic congestion and air pollution, and increased recreational opportunities.

To realize these benefits, we must provide attractive housing opportunities in mass-transit-oriented and pedestrian-friendly communities. The planning community has long understood the advantages of this shift, as well as the excess costs of sprawl to the taxpayer.1 Enlightened watershed management reduces costs, enhances quality of life, and, as current scientific evidence shows, abates global warming and improves the health of our aquatic habitats.


Mattawoman Creek Threatened by a Sprawling Greenhouse

A local watershed that serves as a poster child for the plight of the Chesapeake Bay, and epitomizes the connection between land use and global warming, is that of Mattawoman Creek, a 95-square-mile watershed that drains to the lower Potomac River at Indian Head, Maryland, at the frontier of Washington, DC’s rapidly expanding urban gradient.

Mattawoman is regarded by state fisheries biologists as “the best, most productive tributary in the Bay.” It is celebrated as the Chesapeake’s most productive nursery for migratory fish; as the center of the Potomac’s $30 million dollar bass fishery; and for its outstanding biodiversity. But sprawl development and the associated loss of forest are primary threats to its continued health.2 In addition, it has the misfortune of lying in the path of the Outer Beltway scheme, presently disguised in proposals for the ICC as well as, in Mattawoman’s watershed, the western Waldorf Bypass. The growth fostered by such a bypass, and by Charles County’s proposed Cross County Connector extension, would serve as a cheerleader for a new Potomac River crossing at Chicamuxen, at Mattawoman’s mouth.

Mattawoman exemplifies the connection between global warming and land use because its watershed is being rapidly converted from forest, a CO2 sink, to sprawl, a CO2 source.


The impact of greenhouse gases

When distributed in our atmosphere, a greenhouse gas like CO2 traps heat much as a pane of glass does in a greenhouse: it is transparent to sunlight but blocks Earth’s radiant heat from escaping into outer space. In addition to the documented increase in global surface temperatures, the price of disturbing our atmosphere with excess greenhouse gases is predicted to include altered patterns of storm frequency and intensity, new rainfall distributions with attendant drought and desertification. In two words: climate change.

And disturbing the atmosphere we are. The obfuscations of climate change nay-sayers have recently been put to rest. A 2006 report by the National Academy of Sciences (NAS), an independent body of top U.S. scientists tasked to evaluate technical issues for our government, states, “In the judgment of most climate scientists, Earth’s warming in recent decades has been caused primarily by human activities that have increased the amount of greenhouse gases in the atmosphere.”3 This conclusion was reinforced this spring by the Intergovernmental Panel on Climate Change (IPCC), which in a politically tempered report stated that CO2 has “increased markedly as a result of human activities since 1750 and now far exceeds pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change…” 4 

The three most significant greenhouse gases, in order of importance, are water vapor, carbon dioxide, and methane.5 Earth is hospitable to life only because these gases assist sunlight in warming the globe. But the balance between heat input and the loss mediated by greenhouse gases is not immutable. Human activity is pouring prodigious quantities of CO2 into the atmosphere (see Fig. 1), which is leading to global warming. The NAS reports, “The recent rapid rise in both surface temperature and CO2 is one of the indications that humans are responsible for some of this unusual warmth.”6

Exurban sprawl-development is increasingly recognized as a significant contributor to excess CO2 emissions, because it depends so highly on burning fossil fuels for extended vehicle trips and for heating and cooling large houses. For a mostly forested watershed like that of Mattawoman, sprawl is a double-edged sword: we not only lose the natural CO2 uptake, or sink, afforded by forest, but we replace it with a CO2  source, sprawl.


Deforestation = More CO2

Forest is the best land use for water quality.7 This is justification enough for the preservation of woodlands that protect a Chesapeake Bay resource as stellar as Mattawoman Creek. Protection of water quality, like soil creation, oxygen and food production, and climate regulation, is one of the many “ecological services” that nature provides us. In fact, per acre, the ecological services provided by forests have the highest value of all dry-land habitat types.8 

With respect to climate change, a growing forest converts CO2 in the atmosphere to wood through the chemistry of photosynthesis, and hence acts as a “sink” for CO2. Thus, the biomass of a forest sequesters CO2 from the atmosphere. The vast majority of Mattawoman forests, as in the rest of Maryland, are second growth, meaning they have a century or more of growth before CO2 removal from the atmosphere may be balanced by CO2 emissions (due to decaying tree fall, for example).9 

Estimates for northern hardwood forests arrive at sequestration rates in excess of two tons of CO2 per acre per year.10 Our mid-Atlantic forests, with larger trees and a longer growing season, sequester even more. So preservation of our forested land, which helps decelerate the rapidly increasing CO2 levels, is a worthy endeavor.

The practice in our locale of clear-cutting for subdivisions and other development exacerbates CO2 emissions, because a substantial fraction of our woodlands are simply burned. This converts the carbon bound in wood and roots back into CO2. In addition, vast quantities of carbon bound in forest soils begins a conversion to CO2 once the overlying forest is cleared, and is further degraded during grading operations.11 

Finally, there is the question of “albedo,” which refers to the fraction of incident sunlight that is directly scattered and reflected from Earth back to space. Snow is very reflective (has high albedo), and so has a cooling influence. In contrast, asphalt is highly absorptive, contributing a warming influence. The effects of albedo for various landscapes are presently a subject of scientific research, but the Earth’s forests appear to play a significant role.

When compared with bare ground or many croplands, a forest is more absorptive.12 But compared with the urban environment, broadleaf forests such as those in our region appear to be more reflective.13 Therefore, the replacement of forest with more absorptive urban landscapes potentially represents yet another contribution to global warming, in addition to loss of forest sequestration, CO2 emitted by burning, and conversion of soil carbon to CO2.


Sprawl = Even More CO2 

Exurban development sprawls over the landscape at a rate that exceeds population growth, especially in our region.4 Consider, for example, that in the twenty-five years preceding 2020, more land will be converted to housing in the Chesapeake Bay region than in the past three and one-half centuries, according to the Maryland Office of Planning.

New highways through lightly populated areas use taxpayer funds to subsidize development. The ensuing sprawl constitutes a recognized “growth inducing impact” of highways. And sprawl is recognized as a significant contributor to global warming because of its large demands for fossil fuels. The burning of fossil fuels, such as oil and coal, is the primary source of CO2 emissions.15 In this mix, houses and cars are not minor players (see Fig. 2 below), and it would be wise to reduce these contributions. The United States, with 5 percent of the world’s population, accounts for 23 percent of the global CO2 emissions from fossil fuel.16

Consider the workday commute of about 25 miles (one-way) to Washington, DC by a vehicle from the area that would be opened to development by Charles County’s Cross County Connector extension. Using government data,17 one can compute that a single average vehicle (with a consumption of 20 miles per gallon) would contribute 6 tons of CO2 to the atmosphere every year.

An alternative is pedestrian-friendly housing complexes, as noted earlier. In this scenario, the transportation emissions of CO2 engendered by the sprawl model would be significantly curtailed.

Like transportation emissions, CO2 emissions from electricity production to heat and cool exurbia are also disproportionately large. From 1970 to 1990, house size increased from a national average of fewer than 1,500 square feet to greater than 2,000, and it continues to climb.18 In Montgomery County, average house size now exceeds 2,500 square feet.19 In two similar domiciles of 1,700 and 2,200 square feet, the larger house emits more than two additional tons of CO2 annually.20 Quantitative results vary depending on many details, but it is clear that larger houses emit more greenhouse gases.

Electricity is provided primarily from coal-fired power plants. Therefore, additional benefits of reducing electricity usage are less mercury emission and preservation of cherished landscapes like the Appalachian Mountains. Whole mountaintops  are being shoved into stream valleys to reveal coal seams in a mining process called mountaintop removal.

An important step we can take to turn these destructive processes around—and at the same time improve our quality of life—is starting locally, and placing watershed health on the balance scale. Presently, the ethics of decision making is geared to facilitate sprawl, with its attendant contribution to climate change. We need to add weight to the other side of the scale, by recognizing the value and benefits of protecting our watersheds. How else to save a creek that is feeding the Chesapeake such a healthy diet that Maryland’s Department of Natural Resources has stated, “Mattawoman represents as near to ideal conditions as can be found in the northern Chesapeake Bay, perhaps unattainable in the other systems, and should be protected from over-development.”    

Figure 1 

 Figure 1


Figure 2

Adapted with permission from the Mattawoman Watershed Society,




1 For development alternatives to the sprawl model, including transit-oriented development with a regional context, consult the Coalition for Smarter Growth, For extensive additional discussion, consult

2 U.S. Army Corps of Engineers, Baltimore District. “Mattawoman Creek Watershed Management Plan,” Charles County, MD, August 2003.

3 “Understanding and Responding to Climate Change: Highlights of the National Academies Reports,”

4 Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change: Summary for Policymakers,

5 “Understanding and Responding to Climate Change: Highlights of the National Academies Reports”

6 Ibid.

7 K. Mountford, “Past is Prologue,” Bay Journal. 16, no. 6 (Sept. 2006), “About Forests,” Chesapeake Bay Program, click on links to “habitat” at

R. Hanmer, director, Chesapeake Bay Program, “Money Doesn’t Grow on Trees—Or Does It?” Bay Journal. 16, no. 10, p. 25,

8 R. Costanza et al., “The Value of the World’s Ecosystem Services and Natural Capital,” Nature 387 (1997): 256, table 2.

9 See, for example, studies conducted by Brown University, summarized at, especially

10 Ibid.

11 Ibid.

12 S. Gibbard et al., “Climate Effects of Global Land Cover Change,” Geophysical Research Letters 32, p. L23705 (2005).

13 M. Lin, “Urban albedo impact using NCAR single column CAM2/CLM2,” presentation at annual meeting, American Meteorological Society 2005, Urban heat-island discussion, Dr. David Sailor, Portland State University,

14 F. K. Benfield, M. D. Raimi, and D. T. Chen, Once There Were Greenfields: How Urban Sprawl Is Undermining America’s Environment, Economy and Social Fabric (New York: Natural Resources Defense Council, 1999), as excerpted at Sierra Club analysis of sprawl and population growth,

15  “Inventory of U.S. Greenhouse gas emissions and sinks: 1990-2004,” U.S. Environmental Protection Agency report USEPA #430-R-06-002 (2006),

16 Ibid.


18 Eben Fodor, “Better Not Bigger.” Average house size, based on 1995 statistics from U.S. Census Bureau. Jerrit-Jan Knapp et al., National Center for Smart Growth, “Measuring Patterns of Urban Development: New Intelligence for the War on Sprawl,”

19 Knapp et al., “Measuring Patterns of Urban Development.”

20 The amount of CO2 emitted per kilowatt hour (kWh) of electricity consumption depends on locale. In Maryland, the figure is 1.4 pounds of CO2 per kWh, according to the U.S. Dept. of energy. See Data from an energy-savings calculator developed by the Lawrence Berkeley National Laboratory. See


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