Taking a break from digging holes for new plants at a recent NOSC Caldero event, I wandered over to the edge of the newly completed Dungeness River side channel. The massive log “nests” all along the bank inspired questions about logjams and why they are so common in stream and river restoration projects.
Often called engineered logjams by restoration practitioners, the large entangled clusters of tree trunks and root balls are fastened together or interwoven and anchored along the edge of a stream or side channel.
I had started reading a book called Deadwood: The Afterlife of Trees by Ellen Wohl (2022) prior to attending the January NOSC work party at Caldero. In the book, Wohl traces the hypothetical life of three old-growth trees in different regional watersheds, including a western red cedar on the Olympic Peninsula. That book and some follow-up readings gave me a whole new level of appreciation for the complexity and time scales involved with these structures.
Why are logjams important?
Logjams are an integral part of stream and floodplain processes. In nature, a tree with roots still attached falls into a river during a high-water event and is carried downstream to a point where it is wedged against the bank. Another floating tree catches on the first one. Wood pieces pile up on each other. Pools form beneath and downstream of a logjam and become rest areas for spawning salmon. The pools and quiet waters also collect smaller wood debris, grow microbes and algae that feed aquatic insects, which in turn nurture juvenile salmon. Sand, silt and clay settle in and around the ‘jam’, partially burying it. Maybe a side channel forms or the main channel heads off in a new direction through the floodplain, leaving the logjam to become part of the floodplain forest.
Engineered log jams are designed to replicate the function of naturally occurring log jams. These restoration constructs are created in streams, rivers and floodplains where large, mature riparian trees are no longer falling into the river or stream due to a history of logging or land clearing upstream.
How long do they last?
Engineered log jams will not last forever. The hope is that they provide habitat services for long enough that upstream riparian forests grow the larger trees needed to continue feeding the natural formation of log jams. The goals, scope and site conditions of each restoration project are diverse and so, too, are the associated engineered logjams. They can be huge and complicated with trees flown in by helicopter for a large river project or they can be relatively small, but still carefully constructed along a streambank in a smaller project.
Streams, rivers and floodplains are dynamic systems, however, so engineered and natural logjams are continually changing form and evolving. A logjam may exist for a few decades or a century, or be disassembled and “reassembled” downstream by large storm events within a relatively short period of time.
I love trees at all stages of their life cycle—living, dead, standing snags or lying on the ground covered in mosses and nurturing the next generation. But I never really thought about how they function in logjams. What happens after sediment accumulates in and around the jam, partially burying it? Decomposition still happens, although often not as quickly. Microbes still break down the wood, and invertebrates still feed on the fungi and bacteria, but in streams dissolved oxygen and nutrients come from stream water rather than soil.
Time and Time Again
How long might these buried logs endure? Wohl describes how studies of the Queets River floodplain reveal, “on average any portion of the floodplain has a 300 to 500-year period of stability between when it is deposited and when it is next eroded…” Eventually the meandering river returns to the site of a logjam it had left behind and erodes the soil around it. If still somewhat intact, the logs are re-exposed and moved downstream before being trapped in another logjam, covered with soil, then eventually eroded and moved downstream again.
In addition, Wohl reports, “Over half the large conifers that the Queets introduces to the channel through bank erosion are deposited in more than three locations and move more than a mile downstream during a period of hundreds of years before finally disintegrating.…Some of the logs have an even more punctuated journey: 10% of the trees that fall into the channel have 7 distinct places of burial and subsequent erosion over a distance of more than 7 miles.” I had never thought about logjams existing on a time scale of centuries!
The size of a river or stream, rainfall patterns, and the specifics of that particular tree all influence how long a given log might last once it has fallen in the water, of course, but some version of this is happening in almost every stream with downed wood. The new Caldero Side Channel logjams still have their “new construction” look, but give them time. Lots of time. They’re just entering a new life, the beginning of a long “afterlife” that will nurture the Dungeness River and its inhabitants for many generations.
References and Additional Readings
Dead Wood: The Afterlife of Trees by Ellen Wohl. 2022. Oregon State University Press
Engineered Log Jams:
The Residence Time of Large Woody Debris in the Queets River, Washington
5000-year record of disturbance and vegetation change in riparian forests of the Queets River, WA (USGS)
Articles about how buried logjams support old-growth trees in floodplains:
Logjams associated with development of old-growth forest patches in floodplains:
“The Floodplain Large-wood Cycle Hypothesis: a mechanism for the physical and biotic structuring of temperate forested alluvial valleys in the North Pacific coastal ecoregion” Brian D Collins, David R Montgomery, Kevin L Fetherston, Tim B Abbe, 2012. Geomorphology
Photo by Ælfhild Wiklund
