Proper forest road construction helps keep streams healthy, according to study
By Chris Branam
Researchers measure the degree of hydrological connectivity of an unpaved road. Photo courtesy Ivan Arsimendi.
Logging roads in the uplands of the northern Oregon Coast Range aren’t sending enough sediment into streams to harm fish and aquatic insects, according to a new Oregon State University study.
Ivan Arismendi, an aquatic ecologist in OSU’s College of Agricultural Sciences, and his colleagues investigated whether current road-building practices were sending excessive sediment into the water. They sampled five streams above and below unpaved roads in the Trask River watershed in the northern Oregon Coast Range from 2010-2013.
They found that roads in these watersheds were contributing only minimal levels of silt to the streams—not enough to be “biologically significant” for aquatic life, Arismendi said.
Their findings, he said, suggest that current road-building practices are solving an important environmental challenge associated with logging in the steep, wet forests of the Oregon Coast Range.
An excess of fine-grained suspended sediment in forest streams has long been known to harm fish and other organisms and degrade the aquatic ecosystem. The silt clogs the gills of fish, makes it harder for them to see and suffocates their eggs. Even relatively small changes in suspended sediment concentrations can adversely affect aquatic biodiversity, Arismendi said.
Decades ago, forest roads were built by bulldozers dumping unstable material down steep slopes. Early forest roads and their ditches also delivered water and sediment directly into streams.
By the 1980s, forest management practices were being refined to mitigate and minimize stream sedimentation. Road builders hauled away the displaced soil instead of leaving it to be washed downhill. The practice now, said Arismendi, is to route forest road runoff to the hillslopes instead of into the stream.
The research team included scientists from OSU, the U.S. Forest Service Pacific Northwest Research Station, Weyerhaeuser Co., and the Oregon Department of Forestry. They sampled before and after road construction and after forest harvest and hauling.
They tested whether the differences between paired samples from above and below road crossing exceeded various biological thresholds. They predicted there would be significantly higher suspended sediment and turbidity, which is a measure of cloudiness in water.
The team was surprised to find only minimal increases, said Sherri Johnson, a scientist with the U.S. Forest Service Pacific Northwest Research Station and co-author of the study. They did find that concentrations and transport of suspended sediment seemed to be highly influenced by local conditions, such as one case of a streamside tree uprooted in a windstorm.
The research has the potential to provide scientists, policy makers, and resource managers with an expanded understanding of the effects of contemporary forest road practices on fine sediment in streams, Arismendi said.
“A main objective of Oregon’s Forest Practices Act is to protect and maintain healthy streams,” said Mark Meleason, a riparian and aquatic specialist with the Oregon Department of Forestry and co-author of the study. “This study found that best management practices applied to the management of several roads within active timber sales was effective in reducing fine sediment delivery to streams.”
Liz Dent, chief of the Oregon Department of Forestry’s State Forests Division and co-author of the study, said, “ This research is one of several studies that has been published from the Trask Watershed Study – a 10-year, multi-million-dollar study on the effects on contemporary forest practices on the aquatic environment.”
The Trask River watershed is located near Trask Mountain. The streams draining the study area flow into the Trask River, which flows into Tillamook Bay. The watershed is owned and managed by the Oregon Department of Forestry and Weyerhaeuser Co. with a small portion belonging to the Bureau of Land Management.
The research was funded by the Oregon Forest Industries Council, the National Council for Air and Stream Improvement Inc., the Oregon Department of Environmental Quality and Weyerhaeuser Co.
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