Woody Debris is a Salmon's Jam

By Emily Howe, Aquatic Ecologist

In Washington’s salmon-bearing streams, installation of log jams — a buildup of wood debris creating deep pools for salmon — has become a priority method to recover salmon habitat and reset geomorphological processes that currently suffer from the legacy effects of a century’s worth of logging. But as we learned this month on the Washington coast, not all jams are created equal. 

In sub-freezing temperatures, a team of scientists scouted a 2-mile length of Shale Creek, where The Nature Conservancy owns timberland and riparian forest. Our aim is to accelerate both coastal rainforest and stream restoration by integrating terrestrial forest and aquatic ecological processes. Terrestrial and forest systems are not simply next to each other on the landscape, but rather they are dependent upon one other. They are inextricably linked to one another by the flow of materials across the land-water divide. 

Salmon in the Skagit River. Photo © Julie Morse / TNC.

In coastal tributaries, like Shale Creek, connections between riparian forests (trees on banks of a river) and streams are restricted by the legacy effects of stream cleaning and felling riparian trees. Termed “stream cleaning," the practice of stripping large wood out of streams was used in order to transport felled timber from the upper reaches of the watershed to mills downstream. Large wooden splash dams were constructed in high elevations, with logs collected in the temporary reservoirs. Once the winter rains fell, loggers would dynamite the dams, sending the logs and enormous flows raging down coastal tributaries. Any large wood in the stream, such as a log jam, would have impaired the transport of logs to the mill. Hence, coastal streams throughout the Pacific Northwest bear the legacy effects of stream cleaning. 

But what are those legacy effects? And how can we identify them? And once they are identified, what can we do? 

Putting Together a Puzzle

It takes a team of keen-eyed, trained scientists to fit together the puzzle pieces of a landscape — a forester, a hydrological engineer, a geomorphologist, a fisheries-habitat biologist and an aquatic ecologist, to be exact. By walking together, we were able to identify locations where the river no longer interacts with its floodplain and where upslope timber practices have led to landslides. We were looking for places where landslides have impacted stream channel depth, substrate and location and riparian forests have changed in age and species composition. The stream channels we were looking for lack habitat diversity (such as pools, riffles and glides) and we were interested in how salmon species respond to stream conditions by the placement of their nests (redds). It turns out that installation of logs and log jams can facilitate the biological and geomorphological processes that tie all these components together into a functioning ecosystem.

Emily Howe and Kyle Smith make observations as they travel along Shale Creek. Photo © Kyle Smith / TNC

But part way through the day, we realized that we had scrambled through three natural jog jams, each spanning the width of the river. Superficially, they looked similar: about the same size, containing a conglomeration of wood sizes, difficult to navigate, evidence of side channels, etc.  However, the geomorphology team had a distinctly different take. 

The first jam, our team members told us, needed immediate attention in order to prevent negative feedback loops that would further cause the stream bed to sever connections with the floodplain. We noticed the second jam could use some intervention, but was not as destructive as the first. The third jam, was performing beautifully. 

What Type of Log Jam?

What made one giant log jam good, but another bad? 

A log jam in the Central Cascades. Photo © Zoe van Duivenbode / TNC

The first jam had caught the debris from an upslope landslide, likely catalyzed by a recent cut on the steep slope above. The river was finding its new path, and because water is lazy, it was taking a shortcut by cutting a trough through the recently deposited sediments. The process, called "headcutting," can lead to channel incision that restricts floodplain connectivity. Without stable wood installed in strategic places, the river is likely to continue to cut downwards, severing connectivity with the floodplain. The problem with headcutting is that it begets more headcutting and incision, preventing the system from recovering on its own. This was a bad jam. 

In contrast, the third jam exhibited the processes we aim to recover in the coastal tributaries of the Pacific Northwest. This large jam exhibited a series of side channels that dissipate the energy of the river across a wide space. The jam successfully redirects flows to the floodplain and side channels, slowing down flows to facilitate sediment retention and rebuild stream-bed elevations, creates pools and riffles and prevent further headcutting. This was a good jam.

Armed with this new information, and observing the relationship between salmon redds and large wood, we now have a clear path forward for enhancing stream ecosystem health on Shale Creek.  Not only will we address major geomorphological processes, as described in the large jams, but we will also tend to the smaller types of wood installations that promote habitat complexity for salmon. Those are the short-term actions that will kick off long-term recovery, defined as more natural interactions between streams, floodplains and riparian forests. That’s why we are simultaneously planting robust tree species, such as spruce and cedar, in the riparian corridor.  Unlike alder, which crumble to pieces within four or five years, these trees decompose so slowly that when they finally fall into a creek, they can remain there for centuries. And that’s the way it should be.

It may take decades for full recovery to be realized in these coastal stream systems. But with this integrated approach, we expect success.

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