Stream Crossings and Water Quality

Study bridges data gap for North Carolina Piedmont forests

In many situations, the adage “dirt doesn’t hurt” is true. One important exception is when soils erode, and rain washes the sediments into streams.

Johnny Boggs, a biological scientist with the U.S. Forest Service Eastern Forest Environmental Threat Assessment Center, recently led a study on preventing stream sedimentation in forests. Sedimentation impacts water quality and can harm aquatic insects, fishes, and other stream inhabitants.

wood-brigemat
A wood bridgemat at a skid trail crossing in Duke Forest, Durham, NC. Photo by Johnny Boggs, USFS.

Forest managers use Best Management Practices (BMPs) to safeguard stream health. For example, forest operations such as thinning and harvesting often require heavy equipment. When equipment must cross streams, managers can refer to BMPs for guidance on installing crossing structures. The crossing structures minimize contact with the soil and prevent disruptions to streamflow.

Previous studies have examined the effectiveness of BMPs in protecting forest streams in North Carolina’s mountains and coastal plain, but data from the region in between—the Piedmont—have been lacking.

Boggs, along with Center research hydrologist Ge Sun and research ecologist Steve McNulty, measured Total Suspended Sediment (TSS)—that is, sediment floating in the water —to test the effectiveness of BMPs at stream crossings in Piedmont forests. Results were published in the Journal of Forestry.

bmp-stream-crossing
A steel bridgemat at a skid trail crossing in Duke Forest, Hillsborough, NC. Photo by Neil Williams, USFS.

“Given the variability in soils, topography, forest management, and previous land uses within and across regions in North Carolina, some streams might experience more sediment production due to crossing sites than others,” says Boggs. “It’s important to keep improving our knowledge of how water quality is affected by stream crossings during forestry operations throughout the state.”

The researchers collected a total of 808 water samples upstream and downstream of crossing sites before, during, and after harvesting operations.

Sample sites included six Piedmont forests with a variety of soil types, watershed sizes, and road and trail slopes approaching each crossing.

Crossing structures included wood and steel bridgemats on temporary skid trails. Crossings on permanent roads included a culvert with a two and a half foot diameter and a steel bridgemat.

Researchers visited the sites at least every two weeks over harvesting periods that ranged from 4 to 27 months. They measured streamflow every ten minutes, which provided information about average streamflow as well as how much streamflow increased after rain.

Listen to a brief audio clip by author Johnny Boggs describing this publication. • Text Transcript

The researchers found that the concentration of sediments floating in the water samples was similar on both sides of the crossing sites.

In fact, TSS concentrations from both average streamflow and stormflow samples were below thresholds that may cause shifts in sensitive macroinvertebrate communities, like mayflies, stoneflies, and caddisflies.

Results from the study assure forest managers that BMPs applied at stream crossings are part of sustainable operations that can provide forest products for people now and in the future while simultaneously protecting forest ecosystems.

“Local sedimentation inputs to streams are some of the biggest current and future challenges for land and water managers,” says Boggs. “These resource managers can incorporate TSS data from this study into their decision support system to help estimate sediment concentrations and exports from stream crossings, haul roads, and skid trails in Piedmont forests and to further refine state BMP guidelines.”

To learn more about stream crossings for forestry operations, visit the North Carolina Forest Service website.

Read the full text of the article.

For more information, email Johnny Boggs at jboggs@fs.fed.us.

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