CORVALLIS, Ore. — Tree harvesting methods designed to protect streams from soil erosion and sedimentation can be effective in maintaining water quality, scientists have shown in a study in the Oregon Coast Range.
By following rules enshrined in the Oregon Forest Practices Act, research in the Alsea River watershed showed that a stream draining clear-cut slopes carried no more sediment after harvest than before. In fact, the clear-cut watershed had lower sediment concentrations than streams in two nearby uncut watersheds.
While the study shows what can theoretically be achieved, researchers are cautious about applying their results to actual harvesting activities elsewhere. The practices in this study may not represent the variety of conditions faced in forest management across the state, they said. For example, no new roads were constructed in the process of carrying out the study. That’s significant because previous studies have showed that road construction can be an important source of sediment.
The results of the study were published in Forest Ecology and Management, a professional journal.
“This and a number of other studies provide some very nice evidence that current best management practices are proving to be much more effective than historical practices,” said Jeff Hatten, lead author and associate professor in the College of Forestry at Oregon State University. Studies in other parts of Oregon and the West show that the impacts of such practices depend on landscape characteristics including geology, soil type, slope and historical landslides.
In the 1960s, the Alsea watershed was the site of one of the first comprehensive studies of tree harvesting and water quality in the nation. Located in southern Benton and Lincoln counties, the river empties into the Pacific at Waldport and supports runs of chinook and coho salmon as well as steelhead and cutthroat trout. Research results provided evidence for standards included in the landmark 1971 Oregon Forest Practices Act, among the first such laws in the United States to set rules to protect streams from impacts of tree harvesting.
In that study, forests were clear-cut above Needle Branch and Deer Creeks, and significant increases of sediment were recorded in each stream after harvesting. Another watershed, Flynn Creek, was left uncut as a control.
Tree cutting practices at that time included widespread burning of branches and other non-saleable materials. For the most part, trees were harvested down to the stream edge; few uncut vegetation buffers were left along the streams. Even where such buffers were left in Deer Creek, storm-driven road failures caused pulses of soil to enter the water.
By the time the latest project was begun in 2005, sediment concentrations in Needle Branch and Deer Creeks had returned to their pre-harvest states. Researchers began a new round of monitoring sediment, stream discharge (a measure of how much water is flowing per second) and precipitation. In 2009, after five years of data collection, the upper portion of Needle Branch was clear-cut. Similar harvest operations were conducted in the lower portions in 2014, and to meet annual clear-cut limits in the Oregon Forest Practices Act, the remainder was cut in 2015. No trees were cut in Flynn and Deer Creek, which were maintained as control sites for comparison purposes.
Over the course of the study, which ended in 2016, more than 4,400 water samples were collected and analyzed at the Forest Hydrology Lab at Oregon State.
“We found that there was no evidence of an effect of contemporary forest practices on suspended sediment concentrations,” the authors wrote. For all years, both the mean and the maximum sediment concentrations were higher in Flynn and Deer Creeks, where there had been no harvesting, than in Needle Branch, where trees had been cut.
Among the harvesting practices used in the study were 50-foot-wide buffers along fish-bearing portions of Needle Branch, which are required by law. No buffers were left along non-fish bearing stream segments. Residual materials were burned in discrete piles rather than broadly across the harvested areas. Tree-cutting equipment was not allowed in the stream channels.
Large pieces of wood that fell into the stream channel of Needle Branch were left where they lay. The researchers did not directly measure the impact of this material on sedimentation, said Hatten, “but there is evidence (from other studies) that large wood can increase the formation of pools and riffles, increasing sediment retention. There is also evidence that wood can cause channel widening and steepening.”
Other authors of the report included Catalina Segura and Kevin Bladon at Oregon State; V. Cody Hale at Nutter and Associates in Athens, Georgia; John Stednick at Colorado State University; and George Ice (retired) of the National Council for Air and Stream Improvement, a nonprofit research organization for the forest products industry.
The study was funded by members of the Watershed Research Cooperative at Oregon State University and the National Council for Air and Stream Improvement. More information about the study is available on the Watershed Research Cooperative website.