Adding logs and other woody debris to rivers and streams is one of the oldest and most common measures to improve fish habitat. But debates continue over how much benefit logjams and other wood structures provide for fish and how much wood is natural or needed in a given river system.
A comprehensive research review by scientists from the Northwest Fisheries Science Center provides new clarity on the question. Large wood such as logs and root wads has always played a natural role in most river systems, the review found, and most studies have concluded that wood placed in rivers remains stable, improves habitat conditions and increases fish numbers – particularly for salmon and trout.
The question is important because thousands of habitat improvement projects across the country have added wood to streams, but with limited follow-up monitoring to measure success. Nowhere is this more evident than in the Columbia River Basin in the Pacific Northwest, where more than 2,000 projects since 1980 have added logs and other wood to rivers and streams to improve habitat and ultimately boost fish numbers.
“Considering how often habitat projects include woody debris, we don’t have a lot of information on how much wood is needed, how and where it should be placed or its benefits to key species like Chinook and steelhead,” said Phil Roni, lead author of the new review. “The good news is our review puts to rest some long-standing debate on natural wood function and questions about stability as well as physical and biological response to placed wood. Rather, we outline areas where we need additional information such as the long-term watershed- or population-scale response.
Wood improves habitat quality
Large-scale evaluations of habitat improvements not only in the Columbia Basin, but throughout the United States, Europe, Australia and elsewhere, are seeking to answer such larger and longer-term questions
But the review of research demonstrates that woody debris has always played a central role in river and streams, where it often improves habitat quality by creating pools and providing cover. Wood also increases the retention of organic matter and nutrients and helps create islands and new channels that provide additional refuge and habitat, especially for rearing juvenile fish.
In some areas streams and rivers have been cleared of wood to keep them free for navigation or because of an outdated perception that straight channels with little wood are preferred. But in fact removing wood can lead to habitat declines such as the loss of spawning gravel and pools for rearing.
“The extensive literature on the importance of natural wood in streams provides the rationale for wood placement in streams and in part explains the popularity of wood placement as a restoration tool to improve fish habitat,” the review found. “Studies on the physical response of placed wood, as well as studies on natural wood, have consistently shown that wood leads to increases in pools, cover, habitat complexity and other measures of aquatic habitat quality known to be important for fish.”
Most wood structures stable
The review also examined the literature for evidence of whether wood habitat structures placed in streams to improve habitat remain in place, without getting washed away. This has been an area of concern for decades. In most cases they do, research has found. The reviewed studies reported that more than 75 percent of wood structures placed in rivers and streams remain in place and provide habitat benefits for a decade or more.
However, healthy rivers naturally move wood around, so not all natural or placed wood should be expected to be static. The literature supports increasingly common “natural” placement of whole logs and trees in rivers rather than highly artificial log structures such as weirs. Logs and trees do not require “anchoring,” are generally stable, and function similarly to natural wood in rivers.
Studies have found physical habitat quality and fish numbers closely linked to the amount of wood present in rivers and streams. Few restoration projects have been monitored for their effect on fish habitat. But studies of the monitored projects reported significant increases in the frequency and depth of pools and improvements in habitat complexity and spawning gravels following the placement of instream wood structures.
How fish respond
Habitat improvements related to installation of wood structures frequently lead to increases in the number and density of coho salmon, Atlantic salmon and rainbow trout, based on studies that tracked the responses of fish. Of the 81 studies that examined the response of fish to wood structures, 68 reported a positive response in fish abundance, biomass or survival for at least one fish species and life stage.
Finally, most evaluations of fish response to wood placement have shown positive responses for salmonids, though few studies have looked at long-term, watershed-scale response or studied a wide range of species. Instead of continuing to debate the role of wood in rivers, the review authors concluded, scientists should focus on understanding where wood occurs naturally in different systems as well as how much, where and what type of wood placement should occur, and apply the information to guide and develop more natural and effective use of wood placement for restoration projects.