Keeping the Chesapeake Bay Clean: Mason Study Team Monitors Forest Buffers in the Watershed

Posted: December 11, 2006 at 1:00 am, Last Updated: November 30, -0001 at 12:00 am

Chesapeake Scene
Forest buffers along the banks of the Chesapeake Bay are not only scenic, but they are vital for maintaining water quality and controlling flooding.
Photo from National Oceanic and Atmospheric Administration Library

By Lynn Burke

You can tell the health of a body of water by the quality of the vegetation on its banks. Aside from serving as a home to wildlife – and a place to take a break on that canoe trip – these vegetative areas play a vital role in maintaining water quality and reducing flooding.

Riparian forest buffers, as they are called, are one of nature’s tools for cleaning water. These buffers slow storm water runoff from adjacent land, which means less nutrients – especially phosphorus and nitrogen from nearby farms – and less silt enter the waterways.

Located within the Chesapeake Bay watershed, George Mason University has long been home to a number of research studies aimed at improving the health of the bay and its tributaries. One of the latest efforts by Mason’s Environmental Science and Policy (ESP) Department is directly focused on a crucial goal of the Chesapeake Bay Program’s restoration effort: to significantly increase riparian forest buffers by 10,000 miles by 2010.

Looking at Changes in the Forest Buffer

“Less nutrient load helps reduce the chance of algal blooms, which can negatively affect the level of dissolved oxygen in the water needed to maintain a rich and diverse population of aquatic organisms,” says John Wilder, an ESP doctoral graduate with the Engineer Research and Development Center, Topographic Engineering Center of the U.S. Army Corps of Engineers.

“Slowing the overland storm water runoff also helps reduce the amount of sediment entering the waterways. Other pollutants are trapped by the buffer, helping to preserve the water quality of streams, rivers and eventually the bay itself.”

Chris Jones
Chris Jones, principal investigator on the study.

“This project provides actual baseline information on the extent and change in riparian buffer land use,” says freshwater ecologist Chris Jones, ESP chair and principal investigator for the study.

“Nobody really knew the extent of current riparian forest, nor could they effectively track changes. Some people have tried to use satellite imagery for this purpose, but the resolution was too coarse to pick up the narrow bands of land adjoining streams.”

Wilder, who is also an affiliate professor in ESP, came up with an approach that uses aerial photography and geographic information systems (GIS) to overcome this shortcoming. The use of aerial photography results in a higher-resolution image because the sensor is closer to the ground.

“A satellite pixel is nearly 30 by 30 meters; our imagery had pixels that were 1 by 1 meter,” says Wilder. “Because the buffer being analyzed is only 100 feet wide, the satellite imagery gives no margin of positional error.”

According to Wilder, if a pixel shifts just a tiny amount from its true location, which happens throughout an image that has been stretched to a map projection, the cover of that spot is mischaracterized. Even more error is then introduced when change detection is attempted because it is impossible to perfectly align two images taken at different times.

Chesapeake Bay scene
The U.S. Forest Service commissioned a Mason team to measure the effect of population growth on riparian forest buffers.
Photo from National Oceanic and Atmospheric Administration Library

“By using high-resolution imagery and direct photo interpretation, we were able to substantially increase the accuracy of the analysis,” says Wilder.

Buffers Declining in Maryland, Virginia and Pennsylvania

The aerial photography approach was tested on selected watersheds in Prince Georges and Frederick Counties in Maryland and Lancaster County in Pennsylvania. Because of the success at these sites, the U.S. Forest Service commissioned the research team, which includes Mason graduate students trained by Wilder, to repeat the technique in several other locations.

The team recently finished work on Henrico County, James City and Spotsylvania County in Virginia; Prince Georges County in Maryland; and Dauphin County in Pennsylvania, examining the effect of population growth on riparian forest cover.

Aerial photo mosaics of permanent and semipermanent drainages in these locations from 1993 to 1994 and 2002 to 2003 were analyzed to determine the change in aerial coverage for different land use and land cover classifications throughout a 100-foot-wide digital buffer zone.

Overall, during this period, riparian zone development (defined as residential, commercial and industrial, developed open space and transportation infrastructure) increased in the five study sites by 8.5 percent. The riparian zone forest cover in these sites declined by 2.5 percent. The findings were presented at the Chesapeake Bay Program Forestry Workshop held in Annapolis.

“Quantifying the nature, magnitude and spatial location of land use and land cover change in the forested riparian buffer zone is the first step in developing management strategies that can help to mitigate water quality problems,” says Wilder.

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