CORVALLIS, Ore. — Researchers have analyzed layers of sediment at the bottom of a lake in southwest Oregon to describe the history of watershed disturbances reaching back nearly 1,500 years.
Like chapters in a book, the annual layers provide a glimpse into the natural and human factors that have affected Loon Lake since it was created by a landslide on Lake Creek. Using a 23-foot-long core taken from the deepest part of the lake in 2013, scientists have documented earthquakes, the pulse of regional climate cycles as well as fires and storms that swept through the watershed in the 1700s and 1800s.
Starting in the 1930s, human activities, such as land clearing, development, logging and road building replaced natural events as a major source of the eroded soil and bits of leaves, wood and other materials that gradually settled onto the lake bottom. After the adoption of erosion-control practices in logging operations in the 1970s, the rate of sediment accumulation declined.
Scientists reported their results in Earth Surface Processes and Landforms, a professional journal. The lead author was Kristin Richardson, a master’s student in water resources science at Oregon State who now works as a hydrologist for the U.S. Forest Service in Montana.
“We were very excited to find that the layers in the recent period were annually deposited. It’s unusual to find this kind of annual resolution in lakes at this latitude, elevation and climate in North America,” said Richardson.
A popular recreation spot, Loon Lake lies 25 miles east of Reedsport in the Oregon Coast Range. It has a maximum depth of just over 100 feet. The lake’s steep sides and deep channel make it particularly useful for such studies because sediments tend to become concentrated instead of being deposited across a wide area.
Several of the layers mark milestones in the history of the region. A thick layer of sand and woody debris was dated near the year 670 and coincides with a large-magnitude earthquake that originated in the Cascadia subduction zone just offshore in the Pacific. It appears that a debris flow from hillsides beside the lake carried trees into the water.
“This layer caused us the most grief,” said Richardson. “We had difficulties coring through it the first time, and on the second deepest core, we also had difficulty. We decided to not go deeper for fear of getting the equipment stuck and not being able to retrieve the core at all.”
Other layers dated near the year 1700 reflect the last major Cascadia earthquake, but they are much thinner than the earlier deposit. The difference, said Richardson, may be related to conditions in the watershed, such as whether soils were waterlogged or whether other events such as fires or floods had made the land prone to sliding.
Starting in 1939, the cores reveal a steady increase in sediment washing into the lake, a possible result of road building, logging and storms driven by a wet period in a climate cycle known as the Pacific Decadal Oscillation, or PDO. Then 40 years later, the accumulation of sediment decreased markedly.
Watershed disturbances such as fires and earthquakes can expose and loosen soils for transport, the scientists added, but it may take a storm to wash materials from the land into the lake. While large-scale patterns of disturbance can be revealed in sediment cores, researchers are cautious in attributing specific events to the thickness or content of individual layers.
“We think part of the reason for the reduced sediment loads is that Oregon’s forest management law was having an impact,” said Hatten, co-author and associate professor in the College of Forestry. “We can’t separate the consequences of that law from other influences, such as climate, but when you combine these findings with studies in the Alsea, Trask and Hinkle Creek watersheds in Oregon, they suggest that regulations were having an effect.”
To determine the ages of the layers, researchers calculated radiocarbon ages of the organic materials prior to 1880, just before the onset of logging. The ages of sediment laid down in much of the 20th century were calculated by measurements of a radioactive element, cesium 137, which was released by atmospheric atom bomb tests starting in the 1940s.
“Sediment cores are commonly used in lake studies to understand how the environment changed. Here we’ve gone back close to the time when the lake was first formed,” said Hatten. “We’re seeing the results of all the processes that generated sediment flows into the lake, and we can only infer how much individual events or sources contributed to each layer.”
The sediment core is stored for further analysis in the Oregon State University core repository, one of the largest such facilities in the nation.
The study was supported by a grant from the U.S. Environmental Protection Agency.
About the OSU College of Forestry: For a century, the College of Forestry has been a world class center of teaching, learning and research. It offers graduate and undergraduate degree programs in sustaining ecosystems, managing forests and manufacturing wood products; conducts basic and applied research on the nature and use of forests; and operates more than 15,000 acres of college forests.