Scientists collected data on stream temperature, aquatic species like fish and invertebrates, streamside vegetation, and channel morphology. Photo credit: Heather Kolowinski.
Wherever there are mountains and rainfall, there will be debris flows. Starting as a landslide and acting like an avalanche, a debris flow is an unconsolidated mass of loose material that travels rapidly down a stream channel. Ecologists Alex Foster and Shannon Claeson, with the Pacific Northwest Research Station, have firsthand knowledge of what a debris flow can do. “It’s this thick flowing slurry of sediments and boulders and uprooted trees. It easily can take out culverts and bridges,” said Foster. “At the landscape level, these are very common.”
Foster and Claeson, along with two colleagues, recently completed a comprehensive study of the aftereffects of two separate debris flows that hit two adjacent heavily forested streams in south-central Washington. They wanted to find out how recovery patterns differed between the two disturbed streams, as well as how aquatic species and habitat conditions compared before and after the debris flows.
Remarkably, they were able to conduct this unusual before-and-after and side-by-side comparison study because of blind luck: the debris flows just happened to tear through the site of their recently completed study of riparian buffers, so they had an established baseline of pre-disturbance data.
Foster and Claeson’s team had thought they were done studying these streams in late 2007. “We were finishing up with breaking down our equipment and packing up the study sites,” said Foster. “Then, this storm hit.”
The storm that struck in early December of that year was arguably one of the largest storms to hit the Pacific Northwest. Over three days, along with hurricane-force winds, nearly 6 inches of rain fell (12 percent of the yearly average for the area), leading to severe flooding and debris flows. The scientists leapt at the opportunity to study how these aquatic systems responded and recovered. “Every ecologist loves a good disturbance,” said Foster.
For the next six years, the team monitored the streams and their riparian zones to determine whether recovery rates of plants and animals, physical habitat, and water temperature differed, and if so, what factors affected resilience. Collecting data on so many components required a substantial amount of field work, so the team recruited volunteers, including many college interns who benefitted from a great learning opportunity.
The data they gathered told the story of a rapid transformation. The mature forest that had formed the riparian habitat along these streams had been completely obliterated by the debris flows, but revegetation occurred quickly. Other processes rebounded as well. “One of our key findings was how rapidly stream food webs recovered after the severe disturbance created by the debris flows,” they wrote in a 2020 article published in Ecology and Evolution. This has implications for species higher up in the food chain, like trout.
“U.S. Fish and Wildlife Service managers were really interested in how fast salmonids recolonized,” said Foster. “Trout can come back within a few months, if they can get access. We found that the debris flow actually increased distribution upstream for cutthroat trout because it eliminated barriers like sediment dams caused by centuries of trees that have fallen into the stream channel.”
While people often call these debris flows “catastrophic,” they are actually important to the long-term productivity and biological diversity of stream ecosystems. “They create habitat heterogeneity. They reset the stream channel,” said Foster. “It’s kind of a physical reset of the ecosystem. And we were surprised by how fast the recovery was and how resilient the system was to changes.”