Adding Wood Structures to Streams Promotes Fish Recovery,
Wood structures added to mid-sized streams to create pools for juvenile fish help in recovery efforts for salmon and steelhead, but wood structures could be added to streams at a faster pace if the cost was lower, according to studies published in October.
One study that evaluated the effectiveness of adding well-placed wood into the Entiat River in Washington found that pools created by the wood structures attract juvenile chinook salmon and steelhead.
"Our central thesis is that in-stream habitat restoration structures do show higher occupancy by juveniles (young-of-the-year) of both steelhead and chinook," said Karl Polivka, research fish biologist with the Pacific Northwest Research Station, U.S.D.A. Forest Service. However, by September of each year of the study, there was no difference between restored and unrestored reaches of the stream in the presence of juvenile fish.
A second study looking at wood structures added to streams on the northern California coast touted a method known as "accelerated recruitment wood loading."This method works only in medium and small streams, but is nearly 80 percent less expensive than the typical anchored structure, such as those used in the Entiat River and also used in many recovery programs.
That, according to Jennifer Carah, ecologist in The Nature Conservancy California water program, offers the opportunity to do more restoration in streams at a faster pace and with less money.
"Coho salmon in California are critically imperiled so there is a strong impetus to achieve as much habitat restoration as possible in priority watersheds in the short term and with limited resources," Carah said. "We found that unanchored wood loading techniques are both effective at creating salmon habitat and can be implemented for about 20 percent of the cost of traditional techniques."
Unanchored wood loading techniques does not use engineering or hardware for in-stream log structures. The method of installing wood to create pools, cover, shading and slow areas that was studied by Polivka in the Entiat River were engineered and used hardware to anchor the wood structures in place.
"Juvenile salmon and steelhead occupancy of stream pools treated and not treated with restoration structures, Entiat River, Washington," was published in the Canadian Journal of Fisheries and Aquatic Sciences. Polivka's co-authors are Ashley Steel, supervisory statistician with U.S.D.A. Forest Service and Jenni Novak, resource specialist 2 at the Cascadia Conservation District in Washington.
"Low-cost restoration techniques for rapidly increasing wood cover in coastal coho salmon streams," was published in the North American Journal of Fisheries Management. In addition to Carah, authors are Christopher Blencowe, owner and registered professional forester, Blencowe Watershed Management; David Wright, fisheries biologist, Campbell Timberland Management; and Lisa Bolton, project manager, North Coast Coho Project, Trout Unlimited.
During Polivka's five-year study, they found similar rates of occupancy by fish in all natural pools and relatively higher occupancy at restored pools. So, the increased occupancy in restored sections of the stream actually represented an increase in total stream capacity rather than just a redistribution of fish, but it required extensive sampling in unrestored areas to prove this, he said.
Regardless of location in restored or unrestored sections of the Entiat River, pool size, depth and velocity of the current had the most impact on juvenile fish occupancy, according to the report.
However, the finding of increased occupancy in restored sections of the river was not consistent within a season.
"It declined to no difference between restored and unrestored habitat by September of each study year," Polivka said. "There were also some years in which the positive effect of structures was not apparent for one species or the other." In fact, steelhead occupied the natural pools at a higher density than they did the restored pools.
The reason for the seasonal change was a drop in pool size and depth as stream flows declined over the summer, according to the report. By September chinook salmon occupied both restored and unrestored pools. Steelhead tended to occupy unrestored habitat anyway, as well as reaches of streams with faster currents.
His study occurred after the restoration structures had already been placed within the river, so there was no pre-treatment monitoring, Polivka said. At other sites he has studied, but not reported on "pre-treatment monitoring shows that overall population changes might dampen the higher occupancy observed [in this study] at restored habitat."
The study of low cost restoration techniques begins with the assumption that adding wood to streams lacking woody structures for various reasons, such as logging or floods that have scoured wood out of streams, will increase the amount of useful habitat for juvenile coho salmon along the Northern California Coast.
The study installed the low-cost wood structures in six watersheds and 72 kilometers (about 45 miles) of streams, concentrating all the installations into small to medium streams.
Carah said there is growing enthusiasm for using accelerated recruitment wood loading techniques in Northern California and believes the technique could also be used for other salmon species in other areas, such as the Columbia River basin, but only following the technique guidelines and only in smaller streams.
"Traditional techniques for adding wood to streams (engineering wood jams and anchoring wood using hardware like rebar, steel cables, etc. ) can be expensive and are rarely implemented at a large scale" due to the expense, she said.
The report says that there may even be advantages for going the less expensive and more natural route:
The accelerated recruitment approach calls for directional falling of trees already along the river bank and/or the placement of whole trees or parts of trees using heavy equipment from alongside the stream. Wood length must be 1.5 to 2 times the width of the stream where it is placed to increase the likelihood of retention, the report says. No hard anchoring is used.
The treated areas resulted in an increase in the number of pools and pool area, wood cover and wood volume. Retention of the added wood was fairly high, ranging from 73 percent to 100 percent (the mean is 92 percent) across the two to six years of the study, according to the report.
The cost of this approach ranged from $95 to $511 per installation, with an average cost of $259. That's 22 percent of the cost of the traditional anchored technique, which costs $748 to $1,625, averaging $1,169 per installation.
"These new techniques are not appropriate for use in all contexts," Carah said. Exceptions would be large rivers, or rivers with a lot of downstream infrastructure. However, where the technique is appropriate, it "can increase the pace and scale at which habitat is restored for salmon due to their lower cost."
She said there is no formal plan to step up the installation of wood into streams using this less expensive technique, but there is enthusiasm to put more wood into streams to increase habitat for juvenile coho salmon.
An earlier study of 91 restoration projects that also used unanchored wood in coastal and Columbia Basin streams, found that after six years most of the projects had increased overwintering rearing capacity for salmon and biologists saw an overall increase in the surface area of pools along with an increase in gravel. After six years of monitoring, the amount of large wood added to the stream had declined by almost 75 percent, but still had not fallen back to pre-treatment levels.
Also see, CBB, April 5, 2014, "Study of 91 Stream Restoration Projects Shows Most Increase in Over-Wintering Capacity for Salmon".
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