Pristine streams are known for their unkempt appearance, with large and small trees strewn about, sometimes piled up together in the water. The effect of all this wood is to create pools, riffles and hiding places for salmon, all considered beneficial to fish.

Efforts to improve salmon streams damaged by past logging and other human activities commonly include the addition of carefully placed logs, tree roots or “woody debris” to mimic this natural system. But a new report raises questions about the value of adding wood to streams — at least in the way it has been done in many restoration projects.

The “one big message,” Bilby said, is that scientists don’t fully understand how wood in streams benefit fish populations, despite the many restoration projects that routinely add wood to streams.

The report summarizes preliminary findings in 13 “intensively monitored watersheds” that have been carefully studied for nearly two decades. Researchers throughout the Northwest were able to document how some habitat projects — such as culvert removal and dike setbacks — have clearly benefitted salmon populations. But the results are mixed for several other habitat treatments, including the addition of wood to streams.

“Some IMWs clearly saw an increase in habitat quality with the application of these wood treatments, but there were a lot of them that saw no change in habitat quality associated with these treatments,” said Bob Bilby, lead author of the report published by the Pacific Northwest Aquatic Monitoring Partnership.

Intensively monitored watersheds is the name given to a coordinated, long-term research effort to identify changes in habitat and salmon populations as a result of specific restoration projects. The IMW effort was launched in the early 2000s, when officials realized that almost nobody was measuring the benefits of stream-restoration projects, at least with any degree of confidence. Funding agencies and organizations wanted assurance that the millions of dollars being spent on habitat improvements would pay off in the long run.

IMW projects were eventually developed in numerous watersheds from Northern California to Southern British Columbia in Canada. Beginning in 2005, experiences and outcomes from the restoration efforts were shared through the Pacific Northwest Aquatic Monitoring Partnership, supported by federal, state and tribal organizations.

“The idea was to concentrate a lot of restoration treatments in a single location and to have the monitoring infrastructure in place to be able to detect and quantify the response if, in fact, one occurred,” Bilby said during an online presentation of the new report during the Salish Sea Ecosystem Conference in April.

Location of the 13 intensively managed watersheds (IMWs) that are part of a long-term stream restoration monitoring program in the Pacific Northwest. Four IMWs are in the Puget Sound region. Map: Courtesy of Aspect Consulting

Participating in the report were 13 IMWs, including four in the Puget Sound region: Skagit River Estuary in northern Puget Sound, Hood Canal streams along the western side of the Kitsap Peninsula; the Elwha River on the north coast of the Olympic Peninsula; and the Deep Creek/Twin Rivers, west of the Elwha, called the Strait of Juan de Fuca complex. Other Washington streams involved in the project are the Lower Columbia in Southwest Washington, the Methow River in the North Cascades, and Asotin Creek in Southeast Washington.

The report documents the effects of specific restoration efforts, grouped into a dozen habitat treatment types — including engineered log jams, vegetation plantings, culvert removal, dike removal, road abandonment, flow augmentation, nutrient addition, and others.

“We saw a wide variety of responses to the application of restoration treatments,” Bilby said, “but one response really stood out, and that was the great degree of variation in the response to wood addition, which is one of the most commonly applied restoration treatments in the region.”

A Washington Conservation Corps crew builds a post-assisted log structure in the North Fork of Asotin Creek by driving wooden fence posts into secure wood debris. Stephen Bennett/ Eco Logical Research (CC BY 4.0)

The most extensive analyses were conducted in Hood Canal, the Strait of Juan de Fuca and the Lower Columbia River, the latter involving creeks named Abernathy, Germany, and Mill. But restoration projects in these three major areas failed to show measurable benefits from the addition of wood to the streams. 

“The reason for this is not fully understood,” Bilby said, adding that three hypotheses have been proposed:

• It might be that overall habitat in the selected streams is declining so rapidly that any improvements from the addition of wood “simply didn’t register.”
• Perhaps the large variation in conditions from year to year acted to mask any positive responses from the treatments.
• It is also possible that the specific type or extent of wood treatment was not appropriate for the location where the wood was placed.

The “one big message,” Bilby said, is that scientists don’t fully understand how wood in streams benefit fish populations, despite the many restoration projects that routinely add wood to streams. This is not to say that adding wood has no benefit, he added, but projects observed so far don’t seem to show much effect on fish populations.

On the positive side, the report was able to document measurable benefits of other habitat improvements, Bilby said. Removal of barriers, such as culverts that constrain the upstream or downstream passage of salmon, was shown to be beneficial, as revealed by the increased number of young salmon heading out to sea, their rate of growth, the number of adults returning to the stream and other factors.

Such positive responses among fish populations were also seen by removing or relocating lateral constraints to the stream, such as dikes and levees installed to reduce flooding.

The new summary report drew its findings from a questionnaire completed by managers of the various IMW projects who reported on the results of the monitoring efforts. Three separate workshops brought together IMW managers and other experts who were able to share information coming from the various IMW projects.

Findings from the questionnaires and discussions were compiled into 12 “core messages” reflecting the findings with an eye to improving ongoing and future projects, according to Bilby. Altogether, about 75 percent of specific treatments showed a positive effect on habitat, while 23 percent showed no response and 2 percent showed a negative response.

In terms of measurable effects on the fish themselves, 53 percent of the treatments showed a positive effect, with no change for 44 percent and negative effects for 3 percent.

“Several treatment types, such as removal of fish-passage impediments like dams and culverts, were consistently associated with increased access to habitat and a positive fish response across IMWs,” the report states. “Similarly, enhancing fish access to floodplain or tidal delta habitat by removing barriers or encouraging beaver colonization increased abundance and growth of salmon and steelhead at most IMWs where this treatment was evaluated.”

At the Bridge Creek IMW in Central Oregon, experts observed strong benefits to juvenile steelhead from increased beaver activity, which was encouraged by “beaver dam analogs.” These are special treatments, such as increasing pool habitat and reconnecting floodplains. Further evaluations in other IMWs are now testing the efficacy of encouraging beaver activity where feasible.

At the Skagit River estuary, some of the greatest biological responses were seen by removing levees and increasing delta habitat for juvenile Chinook. The improved habitat seemed to reduce competition for food, attract young fish for longer periods of time and increase their overall growth before heading out to sea.

While the need to restore habitat is often seen as a key factor in rebuilding salmon runs, researchers involved in IMW restoration projects also point out that factors beyond the watersheds — such as fishing and hatchery production — could be having a more powerful effect on fish populations. If so, measuring the benefits of habitat improvement becomes an even greater challenge. For example, it is not easy to measure fitness in fish or identify intrinsic factors that can help them survive during poor ocean conditions or other hardships brought on by climate change.

The report also acknowledges that some types of restoration projects can show benefits within a year’s time while others may take decades. Anticipated response times should be considered when setting up monitoring programs.

The report also identified management issues that need attention — such as the overall lack of an explicit adaptive management process that would allow IMW managers to learn and make further improvements as they go along. It would also allow for a more efficient way to translate findings for other restoration projects.

Those involved in the IMW program hope that the reported findings can help restoration experts design better projects in the future, according to Keith Dublanica, science coordinator for the Governor’s Salmon Recovery Office. With that in mind, Dublanica is helping to coordinate a deeper look at the IMW data with a specific focus on Washington streams, recognizing that there may be regional differences. The next review, which began this month and is scheduled for completion early next year, is supported with a grant from the state’s Salmon Recovery Funding Board.

Because of difficulties in measuring the factors that affect fish populations, one goal of the next evaluation is to determine whether the evaluation process itself is adequate or if it may be missing important elements that could be measured.

The review also is expected to revisit the issue of wood placement in an attempt to understand where and how wood can be added to streams to best help local salmon populations. While removal of dikes and reconnections of streams to floodplains often provided clear benefits, experts hope to use information from the IMW process to improve future floodplain-restoration projects.