Street to Stream

New environmental research challenges how municipalities think about development.

Clarksburg, Maryland is a small, unincorporated area in northern Montgomery County. Last spring, the Montgomery County Council made the decision to limit new development in Clarksburg’s Ten Mile Creek watershed, a tributary to Little Seneca Creek. The reason: Despite state-of-the-art efforts to control stormwater in previous developments, concern remains about the water quality impacts of additional hard surfaces such as roofs, parking lots and roads that prevent water from seeping into the ground, increasing surface runoff. The question is: will limiting such surfaces (referred to as impervious surfaces by scientists) be enough to protect the Ten Mile Creek and others like it?

“Probably not,” says Matthew Baker, an associate professor of geography and environmental systems at UMBC. Impervious surfaces are a useful index of the transformation caused by urban development, but they are not the only reason development creates problems in streams and estuaries. “One of the dangers of focusing on impervious surfaces is that it leads to the presumption that these kinds of surfaces are the only problem. Fortunately, the County has mandated additional protections in Clarksburg.”

Baker and Andrew Miller, a professor of geography and environmental systems at UMBC, recently co-authored a study with UMBC master’s student Dan Jones, Taylor Jarnagin of the Environmental Protection Agency, and Dianna Hogan of the U.S. Geological Survey. They analyzed detailed maps showing the shape of the land surface. These maps were developed from repeated aerial LiDAR (light detection and ranging) flights and captured terrain modifications during development in several tributary watersheds of Little Seneca Creek. Using these measurements, the researchers were able to assess how moving large amounts of earth might alter the way water travels to streams.

Jones and his co-authors found that earth movement during construction led to the creation of small ridges and swales where there had previously been none, as well as removal of existing hills and valleys. These modifications altered the direction that water flows across the land surface, moved or lengthened stream channels and subsequently shortened the distance required for runoff to reach streams.

It’s all about efficiency, says Miller. He explains, “Preparing land for construction typically involves engineering efficient drainage around roads, buildings and other structures, but when we add those modifications up across watersheds, we find they affect not only the way the land looks but how it functions.”

Baker and Miller expect that the terrain modification they describe will deliver more water more rapidly to streams during rainstorms, even without adding impervious surface. In Clarksburg’s special protection areas, Montgomery County has required that developers go to great lengths to capture and store excess stormwater until it can recharge groundwater, but such protections are costly and typically not included in new developments elsewhere.

Scientists from the county, federal agencies and universities are watching closely. “Lessons learned in Clarksburg are essential for helping us better understand how well new mitigation approaches work,” says Baker.

Both scientists and policymakers have called attention to the substantial impacts of impervious surfaces. “What we’re showing is that other aspects of development should also be considered,” Baker explains. “Controlling impervious surface alone may not be sufficient to protect streams of high conservation value.”

In the Ten Mile Creek watershed, Montgomery County faces the classic dilemma of wanting to preserve natural resources while still promoting economic development. Can development here be done in a sustainable way? As it becomes clear that impacts on habitat and living organisms occur for many different reasons, and with even very modest levels of development, that task has become more challenging.

Homepage photo: courtesy Marlayna Demond.