Map of the Umpqua River watershed in western Oregon, that drains to the Pacific. (Photo Credit: DEMIS Mapserver/FIS)
Landslides created upstream habitats for coho salmon: study
Tuesday, February 12, 2013, 22:50 (GMT + 9)
A study of the Umpqua River basin in the Oregon Coast Range helps explain natural processes behind the width of valleys and provides potentially useful details for river restoration efforts designed to improve habitats for coho salmon (Oncorhynchus kisutch).
Coho salmon thrive in broad, flat valleys that contain multiple auxiliary channels to the main river. These valleys formed after large landslides altered the landscape, said study co-author Joshua J Roering, professor of geological sciences at the University of Oregon.
The network of secondary channels, while often devoid of water in the dry season, fill and provide a calm, safe haven away from swiftly running currents for the fish during periods of heavy rain and storms. They now have been identified as a critical component of salmon habitat.
While geologists and hydrologists long ago documented how river-channel width and depth varies, little had been done to understand how valleys beyond the channels form, and why they may be narrow or wide, Roering said. The study -- appearing online in the journal Geology -- combined on-the-ground observations and a remote-sensing technology known as airborne Light Detection and Ranging (LiDAR).
"What we've learned here is that if you introduce variable geology and geomorphology -- big landslides, messiness that happens in the world -- the valleys tend to narrow and widen, and fish appear to love that. They seem to respond to the heterogeneity that is so inherent in most real landscapes," Roering said.
Roering used LiDAR in previous research, including a project in which he and colleagues discovered a dry lakebed that formed some 22,500 years ago by a landslide and later drained in northern California's Eel River basin. That paper -- published 22 November 2011, in the Proceedings of the National Academy of Sciences -- also connected landslides to a divergence of genetics in two present-day steelhead trout populations occupying the river.
Christine May, lead author of the new paper and a biologist at James Madison University in Harrisonburg, Virginia, said that Roering's work on landslides and her own previous observations led to the newly published research.
May said: "It became apparent that there was a strong linkage between the occurrence of large landslides and the location of highly productive salmon habitat."
For the study, they used LiDAR provided by the Oregon Department of Geology and Mineral Industries, focusing on Harvey and Elk creeks -- part of the Umpqua River basin where Roering previously documented ancient landslides. Researchers measured the widths of the valley using the high-resolution topographic data and showed how widths varied based on watershed-mathematical formulas used to plot and map the terrain.
To understand the activities of the salmon, researchers applied topographically derived stream-layering techniques and a formula created by Burnett in 2007 for estimating the "intrinsic potential" of each stream to provide rearing habitats for coho salmon.
They found that valleys downstream of landslides are narrower than would be expected, which suggested that valley constriction had subverted valley widening. Deep-seated landslides, the authors noted, caused wide valley segments to occur higher in the channel network than would otherwise be expected.
Off-channel streams perched in mountain drainages, they said, increase habitat diversity and lead to a greater abundance of salmonid productivity.