A Primer on the Wood Regime for Stream Management
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A Primer on the Wood Regime for Stream Management
Dan Scott, PhD
Watershed Science and Engineering
Colorado State University Department of Geosciences
dan@watershedse.com
December 5, 2022
1 INTRODUCTION
The wood regime describes how wood behaves as it moves from forests to streams and downstream through
river networks. Because wood is one of the primary building blocks of forested riverscapes, understanding the
wood regime is crucial to stream management, planning, and restoration. This memo reviews the baseline
knowledge needed to consider the wood regime when assessing stream conditions for restoration,
management, and planning efforts. It also provides additional technical information that can facilitate a deeper
dive into components of wood dynamics relevant to stream management.
Key Points:
• Wood is an essential but transient component of forested riverscapes, often just as important to
ecosystem function as water, sediment, and biota.
• Wood in river corridors is lacking due to a history of wood removal, forest harvest, lack of forest
regrowth after disturbance, and stream simplification. These impacts have inhibited how much wood
gets to the river, the river’s capacity to store wood, and the beneficial functions that wood can provide.
• Wood in rivers can be described as a regime, analogous to the flow and sediment regimes. The wood
regime consists of the recruitment, transport, and storage of wood throughout the river network.
• The wood regime can be assessed at multiple levels of detail and spatial scale depending on assessment
objectives, watershed context, and budget.
• By assessing the wood regime, managers can target the root causes of wood deficiency and plan for
long-term, sustainable restoration of wood to the river corridor, not just to meet a target amount of
wood in the river, but to meet a target function of wood in terms of its geomorphic, hydrologic, and
ecologic benefits. Wood regime assessment can also inform the management of flood and geomorphic
risks posed by wood.
2 WOOD IS AN ESSENTIAL COMPONENT IN FORESTED RIVER CORRIDORS
Wood is a transient ingredient of forested rivers. Just like sediment, wood moves downstream and breaks down
over time. As it sporadically moves downstream, wood actively shapes the riverine ecosystem, whether it is
small, large 1,2, mobile, or immobile 3–7 (Figure 1). The functions of wood do not come simply from wood load, or
how the quantity of wood in the river corridor. Instead, wood function derives from how wood interacts with
flows of water and sediment, its stability during various flows, position in the river corridor, surrounding biotic
activity, and how it decays and breaks down.
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Figure 1: The wood regime works alongside the flow and sediment regimes to shape the ecologic and geomorphic
integrity of river corridors. Wood influences the four habitat characteristics listed around the margins. Modified from
Wohl et al. 8.
Broadly, wood slows down the flow of water, sediment, energy, and nutrients. This in turn diverts those flows
sideways, into the banks and onto the floodplain, and vertically, into the channel bed. By altering these flows of
water, sediment, energy, and nutrients, wood improves habitat, provides beneficial geomorphic functions, and
can make it easier for a river to absorb disturbances like fires and floods. These functions include, but are not
limited to:
• Retaining sediment, which can attenuate sediment pulses, for example, from wildfires 9–11.
• Retaining water, especially when wood drives flow into riparian forests, which has potential to
attenuate flooding and sustain summer baseflows, although the magnitude of these effects varies
widely 12–16.
• Reducing flow energy and erosion risk by widening and roughening channels 17,18
• Helping the river corridor absorb disturbances such as fires and extreme floods 10,19,20.
• Improving water quality by driving water into the sediment near the channel, or hyporheic zone 21–24,
which can reduce nitrogen, increase oxygen, and buffer water temperature 25–29.
• Diversifying patterns of flow velocity and depth, creating conditions favorable to fish feeding and
resting 30,31.
• Creating diverse habitat patches 32–34, providing the foundation for biodiversity 35.
• Providing refuge and cover for fish, both directly and by creating side channels, sustaining riparian
vegetation, and preventing anchor-ice formation 10,36–40.
• Providing substrate and nutrients for macroinvertebrates 32,41,42.
For additional information and references on the functions of wood in rivers, see the Wood Function section of
the Additional Information at the end of this document.
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3 WOOD IN RIVERS CAN BE DESCRIBED BY THE WOOD REGIME
The wood regime 8 describes the recruitment, transport, and storage of wood in the river corridor (Figure 2).
Recruitment describes how wood gets to the river, where it can then be transported downstream. Wood
typically reaches the river via banks eroding into streamside forests or floodplains with buried wood, the felling
of streamside trees by wind, fire, or insects, landslides, and transport down tributaries. Once it reaches the river,
wood begins its journey downstream, during which it will spend most of its time in storage, punctuated by brief
episodes of transport.
Figure 2: Wood recruitment, transport, and storage make up the wood regime and vary throughout river networks.
Arrow size corresponds to the magnitude of recruitment and transport.
Wood transport typically occurs during floods that at least fill the channel 43,44, and although most wood floats,
wood can also be dragged or bounced along the channel, depending on flow depth and velocity. Wood
recruitment and transport together determine wood supply, or the amount of wood delivered to any given point
along the river network. Wood supply to any given reach can come from either local recruitment (e.g., an
eroding bank recruiting a streamside forest) or transport from upstream.
Wood storage is the pattern and amount of wood present along the river network. As wood moves downstream,
it can deposit where there are landforms or vegetation present to trap it. Within a reach of the river corridor,
wood storage capacity can be described by how broken up and evenly distributed landforms are across the river
corridor, measures of spatial heterogeneity 45–48. Riverscapes with a mix of landforms that facilitate transport of
wood and those that trap wood tend to have the highest wood storage capacity and, if wood supply is sufficient,
the highest wood load.
4 WOOD IS LACKING IN MOST FORESTED RIVER CORRIDORS
Any river corridor with local or upstream forests was likely once adapted to having wood delivered to,
transported through, and stored in it 49. Multi-channel river-wetland corridors likely historically abounded with
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wood, not just individual logs, or even large wood jams (accumulations of many logs), but also large rafts of
wood, covering up to square miles of river in some of the largest river corridors 49,50. Today, there is much less
wood in river corridors than there likely was historically, and its functions have been severely diminished as a
result. Even many desert rivers, which today tend to be wood-starved, historically saw large wood rafts and jams
and massive wood fluxes 51.
In the United States, wood removal and forest harvest became the norm in many watersheds following
European colonization. Rivers were cleared of obstructions, including wood, to facilitate navigation and allow
splash damming to move logs from headwaters to processing sites 49,52,53. On top of directly removing wood
from both streams and forests, a combination of fire suppression and climate change are shifting forest
structure and presence 54,55, potentially limiting wood supply to rivers. These human effects have had three
notable impacts on the wood regime:
1. Forest reduction (timber harvest, stand-killing fires, insect infestations) and a lack of forest regrowth
decrease the amount of wood getting to the channel 47,56,57.
2. Removal of in-stream wood has reduced in-stream wood loads, reducing wood-provided habitat,
nutrients, and geomorphic effects.
3. Simplifying streams has likely reduced their capacity to store wood, or, alternatively, increased their
wood transport capacity 43. In a simplified stream, wood is more likely to move downstream as opposed
to being transiently stored.
In Colorado, wood used to be much more
abundant in rivers, and riverine
ecosystems have suffered due to its loss.
In the past, a walk down a river corridor
would involve going over and around logs
and wood jams (e.g., Figure 3), but today,
wood might be barely noticeable along
many Colorado streams. Please see the
Wood in Colorado Rivers section for more
information on wood loss specific to
Colorado and the Wood Removal and
Loss of Wood Loads in River Corridors
section for more references on wood
removal, both in the Additional
Figure 3: Wood jams were historically much more common in rivers.
Information section at the end of this Picture shows a wood jam on a side channel of the Poudre River, CO.
document.
5 ASSESSING THE WOOD REGIME SUPPORTS SUSTAINABLE RESTORATION OF WOOD FUNCTION IN
RIVER CORRIDORS
Just like water and sediment, wood moves downstream, although it can be transiently stored 6,58,59. Even when
securely anchored, wood naturally decays and breaks down, losing substantial structural integrity in a matter of
years to decades, depending on wood species and environmental conditions (see the How Long Does Wood Last
in Rivers — Wood Decay and Breakage section for more detail). Because wood is inherently transient, the
amount and function of wood in any part of the river network changes over time. This change is described by
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the wood regime, or how wood is recruited, transported, and stored. Understanding the wood regime, and thus
how wood changes through time, enables more informed and effective wood management and restoration.
Wood supply to any point on a river is determined by both wood recruitment (how much wood reaches the
stream) and transport (how that wood moves downstream). Understanding wood supply will determine if wood
restored to the river corridor could be replaced as it breaks down or if future wood supplementation may be
required. Alternatively, documenting the relative importance of different wood sources (e.g., local recruitment
from bank erosion, landsliding, tributaries, etc.) can inform how management actions might affect wood supply.
Understanding wood recruitment and supply can answer questions such as:
• Will wood placed as part of river corridor restoration be replaced as it decays or mobilizes downstream
(i.e., what is the likelihood of needing to supplement wood or adaptively manage a site in the future)?
• How will upland/streamside management actions affect the amount and function of wood in a given
portion of the river network (e.g., could armoring banks reduce crucial wood function downstream)?
Wood transport describes how far, during what flows, and the mechanisms by which wood moves downstream.
Once delivered to the channel, if logs only move a short distance downstream before being stored for long
periods of time on the floodplain, then upstream sources of wood recruitment probably can’t be relied upon to
sustain in-stream wood far downstream. This may occur on very large rivers that are well-connected to their
floodplains, or those in which overbank flows are frequent 60. On the other end of the spectrum, in small
streams, it may be that logs simply cannot move downstream, because they are far longer than the channel is
wide 43,59. Understanding wood transport can help answer questions such as:
• Is placed wood likely to reach infrastructure far downstream of a restoration project?
• Will wood mobilized from a restoration project make a noticeable impact on wood supply downstream
(e.g., the amount/characteristics of wood reaching a bridge), or does the river already transport enough
wood to make any contribution from placed wood negligible? (Answering this question can provide
evidence for whether a project will significantly increase wood-related risks to downstream
infrastructure.)
• Will restored forests upstream be able to provide wood supply to degraded downstream reaches?
Understanding wood storage is key to prioritizing wood restoration and setting expectations for restoration
goals. Wood storage data can also support wood budgeting, which can be a useful framework for tying together
multiple components of the wood regime to solve complex problems. Measuring wood storage patterns and
characteristics (i.e., whether wood is stored in jams or individual pieces, and whether such wood is on the
floodplain or in the channel) in relation to wood function can help guide restoration design aimed at replicating
those functions 47. Understanding wood storage can help answer questions such as:
• Where in a river network is wood function and wood-provided habitat most lacking (i.e., where is
restoration needed most)?
• Where along the river network is wood storage and function limited by wood transport versus wood
supply (also requires understanding wood recruitment and transport)?
• How long does wood typically last when it is stored (e.g., after a flood that deposits wood and creates
beneficial habitat, how long might that habitat last)?
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6 HOW TO ASSESS THE WOOD REGIME
A broad array of methods can be used to assess the wood regime. Rapid, cheap assessment (e.g., rapid field
surveys, mapping in publicly available imagery, or Google Earth) can often reveal important components of the
wood regime. More time-consuming methods can elucidate details that may be helpful for certain management
objectives and in certain watersheds. Basin-specific objectives and wood processes should always determine the
method and level of effort needed to assess the wood regime. Once questions are identified (e.g., how is wood
distributed through a river network, which reaches are most likely to receive wood supply naturally, where does
wood provide desired functions), assessments can be designed at an appropriate level of precision and cost to
answer them.
Assessing the wood regime can seem daunting. However, most management objectives, like sustainably
restoring wood for fish habitat, can be accomplished with only a simple understanding of key components of the
wood regime: wood recruitment, transport, and storage.
6.1 WOOD RECRUITMENT
At the most basic level, wood recruitment, or the delivery of wood to the stream, can be described by where
wood tends to enter the river network. Rapid assessments of wood recruitment can include:
• Field inventories of wood delivery sites, especially after floods
• Reviews of repeat historical imagery that can reveal locations of streamside forest erosion (e.g., Figure
4), landslides, and near-stream tree die-offs (e.g., from fire or insects)
• Maps of wood recruitment locations can show where wood tends to enter the stream. One can also
rank wood recruitment sites by how much wood is inferred to be entering the stream over time (e.g.,
low for a slowly eroding bank, high for a recently burned forest with numerous landslides),
acknowledging that wood recruitment rates tend to vary substantially through time.
If management objectives require it
(e.g., if in-stream wood conditions
will inform upland forest
management) or if available data
are insufficient, more detailed,
quantitative assessments may be
justified. Detailed wood recruitment
methods include:
• Conducting forest
inventories using LiDAR,
aerial imagery, or field plot
sampling before and after
wood-recruiting events 61,62. Figure 4: A tree falling, or being recruited, into the Poudre River, CO.
• Numerical modeling, when calibrated to field data, can help make predictions of recruitment in data-
poor areas or into the future e.g., 63,64.
These quantitative methods often require detailed, repeat remote sensing data such as LiDAR over large areas.
However, they can be very helpful in prioritizing restoration (e.g., looking for reaches near active wood
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recruitment sites, where placed wood may be sustained as it decays/breaks down or mobilizes) or identifying
opportunities where changes to forest management could benefit stream health (e.g., leaving an expanded
riparian buffer around actively migrating banks to allow for sustained wood recruitment).
6.2 Wood Transport
Oftentimes, the accurate amount of wood typically transported downstream is not relevant for stream
management (the design of wood trapping structures is a notable exception). Qualitative, rapid methods
include:
• Observing wood transport during
floods or inferring wood transport
from deposits of wood after floods
(e.g., Figure 5).
• Timelapse imagery, or streamside
videos taken during floods, are often
very helpful in describing how wood
is transported and approximately
how much wood is being moved
downstream 65.
• Direct observations of wood
transport after and/or deposition Figure 5: Wood deposit in Black Hollow, CO – the structure,
during and after floods can reveal distribution, and density of wood deposits reflects how much wood
how frequently wood is transported was in transport during a flood and how it moved downstream.
across floodplains e.g., 66.
• Describing in-channel wood by its decay status compared to wood that is actively being recruited from
streamside forests can indicate a stream’s transport capacity for wood: If wood entering the channel is
relatively fresh, but most wood stored in the channel is well-decayed, then transport capacity is likely
low, as well-decayed wood tends to break down and mobilize.
Even these low-cost methods can inform whether recruited wood tends to reach downstream sites, or whether
wood in transport is intercepted and stored on the floodplain after being transported only a short distance.
Many narrow, headwater streams tend to not transport wood at all 43, and simply mapping out those reaches
with too small of channels to transport wood can help inform expectations for wood supply lower down in the
river network.
If more detailed knowledge of wood transport is required (e.g., for design of wood trapping structures), more
costly methods might be required, which can include:
• Calibrated video-monitoring stations can be installed to measure wood flux during floods 67–69.
• Two dimensional hydraulic models can incorporate wood transport and deposition dynamics, especially
when they can be calibrated to real-world data 70.
6.3 WOOD STORAGE
Generally, the exact quantity of wood, or wood load, in a reach isn’t as important as relative variations in wood
load and, most importantly, wood function (e.g., providing habitat, storing post-wildfire sediment, etc.).
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Potential wood restoration projects can be prioritized by surveying wood load and function, then comparing
those to desired wood functions. Surveys of wood function should be specific to desired functions, but may
include measurements of sediment retention, pool scour, or organic matter retention at the scale of individual
wood pieces or wood jams. It is important to note that counting logs or measuring wood load alone does not
necessarily reflect wood function.
Rapid methods to assess wood storage can include:
• Field walks can, at the simplest level, geolocate wood jams (e.g., Figure 6) and individual logs throughout
long lengths of stream, optionally classifying logs by size class e.g., 6,71.
• Wood jams, which tend to provide the most function, can be mapped rapidly in aerial imagery.
• Wood function can be described qualitatively by answering yes/no questions about observed wood
structures (e.g., does wood produce scour pools, retain sediment, provide cover, etc.).
This type of rough estimate of wood
distribution and function through the
river network may reveal patterns of
wood storage — usually lower gradient,
less confined, and more complex
reaches will contain more wood than
more confined, simpler reaches 47. If
that typical trend isn’t apparent, then
it’s probably worth looking at wood
supply (i.e., wood recruitment and
transport) to determine why wood isn’t
being stored where you’d expect it to
be stored. For instance, if most wood is
stored on the floodplain, but in-channel
wood functions like overhead cover and
hyporheic flow are desired, it might be Figure 6: Stored wood on the North St. Vrain River, CO.
worth examining wood transport to determine if in-channel wood could be sustained along a given stream
segment.
If more precision is needed, field or remote sensing measurements of wood load may be required. Wood load
measurements scale in difficulty and cost from cheapest to most expensive from log counts 71, to area (i.e.,
mapping wood in aerial imagery) 48, to volume (i.e., measuring log lengths/diameters or wood jam dimensions
and porosity) 47,72, to mass (i.e., measuring volume and estimating wood density) 73. Wood area is often a good
proxy for volume when wood doesn’t tend to stack up vertically e.g., 48, but volume might be required where
wood jams are tall e.g., 66. Mass may be difficult to estimate accurately, depending on variability in wood species,
and is usually not relevant unless wood functions like carbon storage are of interest. Wood load is typically
expressed as a mass, volume, or area per unit length or area of stream. More precise measurements of wood
load can be combined feasibly with more precise measurements of wood function. For instance, backwater pool
volume can be measured alongside wood load, especially in streams where wood tends to be the dominant
backwatering obstruction e.g., 57.
WATERSHED SCIENCE & ENGINEERING · www.watershedse.com7 BRINGING IT ALL TOGETHER: USING THE WOOD REGIME TO INFORM STREAM MANAGEMENT
PLANNING
The wood regime should be a key component of stream assessment for any forested river network. Along with
sediment, biota, and water, wood helps build the physical template of a river. The stark lack of wood in many
streams, especially in Colorado, provides a substantial opportunity to restore stream process and function by
restoring wood to rivers (Figure 7). That restoration and the broader management of wood in rivers will only be
effective if informed by assessments of the wood regime.
The wood regime is a useful conceptual model that can help organize knowledge about wood. For high-level
planning and management efforts, a simple, qualitative description of wood recruitment, transport, and storage
may be enough to prioritize restoration projects or identify areas where wood may pose a risk to infrastructure
during floods. When projects move from prioritization to design, and details like whether wood will be sustained
from an upstream supply come into question, more intensive approaches become appropriate. Quantitative
wood budgeting 74–76, accounting for the inputs and outputs of wood to and from a reach, can be a useful
framework for putting numbers to the wood regime. Expanding on the wood budget, assessments of wood
storage capacity, in conjunction with other geomorphic assessments 77, can help guide restoration design and
hazard assessment. Whatever the level of detail needed, assessing the wood regime is key to informed stream
management in forested river corridors.
Figure 7: Placed wood on the North Fork Big Thompson River, CO. Flow is right to left.
WATERSHED SCIENCE & ENGINEERING · www.watershedse.com8 ADDITIONAL INFORMATION
The following sections provide additional information and references for those interested in learning more
about the topics discussed above.
8.1 WOOD RESTORATION
Wood restoration intended to enhance habitat, provide crucial geomorphic functions (e.g., retain sediment), or
manage risk (e.g., reduce flow energy to protect streamside infrastructure) is a common management action
that benefits from understanding the wood regime. However, there is a broad spectrum of wood restoration
techniques to choose from. The following paragraphs briefly discuss the array of options for restoring wood to
rivers
Wood restoration can be broadly categorized as either passive or active. Passive restoration generally involves
the cessation of activities that deplete wood from the river or impair forest regrowth, then letting forests
regrow and deliver wood to rivers over decades to centuries 8,57,78–80. Active wood restoration 81,82 involves
placing wood or structures meant to trap wood directly in the river corridor. Placed wood ranges in stability and
cost from well-anchored but expensive engineered log jams (ELJs) 83, to hand-built, generally cheaper, and
loosely anchored (usually with buried posts) structures such as beaver dam analogs (BDAs) or post-assisted log
structures (PALS; 84), to completely unanchored wood that the river can freely rearrange (Figure 8; 85).
Figure 8: Conceptual continuum describing the range of active wood placement methods and broad
generalizations about their relative characteristics. Middle photo courtesy of Ellen Wohl.
WATERSHED SCIENCE & ENGINEERING · www.watershedse.comELJs are expensive, meaning that hand-built or unanchored wood structures can cover larger areas for the same
cost. Whereas ELJs are static, and thus provide benefits in only one place, unanchored wood can move and
rearrange as channels themselves move over time, meaning unanchored wood can adapt to changing
conditions. However, because unanchored wood can move downstream, it can pose a greater risk to
downstream infrastructure or river users. For unanchored wood to sustainably restore ecological function, it
requires a thorough understanding of the wood regime, especially wood transport (whether wood will move
downstream) and supply (whether mobilized wood will be replaced). That understanding can enable restoration
over much greater extents of river corridors at lower cost (compared to ELJs) where risk can be appropriately
managed.
8.2 WOOD IN COLORADO RIVERS
All components of the wood regime vary substantially across the state of Colorado. The landscape transitions
from unforested alpine zones through subalpine, montane, foothills, and plains zones. Along this gradient, forest
characteristics, wood recruitment style and rate, wood transport capacity, and wood storage patterns vary
substantially. Even outside zones with high forest cover (e.g., desert rivers like the lower Colorado or Yampa, or
plains rivers close to the Front Range), cottonwood-dominated riparian forests and wood supplied from
upstream used to provide abundant, functioning in-stream wood. It is important not to discount the potential
for wood function in sparsely forested or desert rivers based solely on their modern characteristics — many
rivers, such as the Lower Colorado, historically had much more abundant wood to support the riverine
ecosystem 51.
Assessment of the wood regime in Colorado watersheds must account for this inherent variability in wood
dynamics. Wood recruitment, transport, and storage patterns may all vary simultaneously 86. Wood recruitment
rate and style will vary depending on forest characteristics and disturbance regime, which can vary between
forests at different elevations 57. Wood transport may be very low in narrow, headwater streams where logs
easily spans the channel 43,59, but past a threshold width to log size ratio, wood transport may pick up
substantially. Along rivers with well-connected floodplains, floods may transport lots of wood, but much of it
could be trapped in floodplain forests 66, thus reducing total transport distance. Simultaneously, flow regime
varies downstream and along elevation gradients 87, which can affect wood transport and resulting storage
patterns 88. Wood storage will likewise vary depending on decay and breakage, valley bottom geometry,
floodplain forest characteristics, and patterns of recruitment and transport.
8.3 WOOD REMOVAL AND LOSS OF WOOD LOADS IN RIVER CORRIDORS
The following papers provide more information on the effects of human actions on wood loads, as well as
comparisons of wood loads across regions. Wohl 49 reviews the history of wood loss from rivers. Wohl & Cadol 60
present wood loads in streams draining old-growth forests in Colorado. Multiple papers compare wood loads in
various regions 86,89,90. Ruffing 52 discusses the effects of wood removal in watersheds of southeast Wyoming
that are comparable to watersheds in Colorado. Generally, forest harvest decreases wood load: Specific to
Colorado, Livers et al. 57 and Jackson and Wohl 56 show the effect of forest management (unmanaged vs
managed) and age (old growth vs young) on wood load. Scott & Wohl 47 compare wood load between one
logged and one unlogged watershed in Washington State.
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The literature on the ecosystem services, habitat provision, hydraulic effects, and geomorphic effects of wood is
vast. The following papers provide more information on how wood affects river processes and forms. Wohl 91
discusses the gap between scientifically demonstrated and perceived wood function, and succinctly summarizes
wood and beaver influences in river corridors. Wohl & Scott 11 review wood influences on sediment retention,
while Montgomery et al. 23 more broadly review the geomorphic influences of wood in rivers. Wohl et al. 8
presents the concept of the wood regime. Collins et al. 37 discusses the cycle of wood jam deposition, floodplain
formation, forest succession, and subsequent supply of wood to the river. Kramer & Wohl 43 review wood
transport dynamics. Multiple papers discuss the importance of wood for aquatic organisms specific to Colorado
streams 92–96. Other reviews more generally synthesize wood influences on river processes 97–100. It is worth
noting that small wood (generally, pieces smaller than 10 cm diameter and 1 m length) also has important
effects on river processes 1,2, although the focus of wood restoration tends to skew towards large wood.
8.5 HOW LONG DOES WOOD LAST IN RIVERS — WOOD DECAY AND BREAKAGE
Wood can be lost by moving downstream or by breaking down. Wood breaks down through either decay, in
which bacteria and fungi decompose wood, or breakage, in which wood splits into smaller chunks. The rate at
which wood breaks down depends on species, frequency of saturation, burial, and other factors 101. Studies of
wood decay in streams indicate a median decay half-life of 18 years, with half-lives ranging from 0.6 to 462 years
102–105
. Decay rates vary by species, with hardwoods tending to decay faster than conifers. Breakage, or the loss
of branches or rootwads, physical abrasion of wood, and shortening of log length, may progress even faster than
decay 101, and both contribute to a loss of structural integrity and increased likelihood of mobilization.
Most logs present in streams are not very old. Boivin et al. 106 report measurements of logs in a large wood raft
— of 262 logs measured, the median time since death was 3 years, only 3 logs had a residence time over 100
years, and 64% of logs had a residence time less than 5 years. Iroumé et al. 107 measured decay of hundreds of
logs over up to 9 years and found that median decay was between 1 to 4% mass lost per year, depending on
species. Measuring 652 logs, Merten et al. 101 reported a loss of 1.9% of wood mass due to decay and 7.3% due
to breakage over just a single year. That being said, wood buried in floodplains, especially saturated
environments, may last for hundreds of thousands of years, again depending on species and soil conditions 108–
110
. Refer to Caza 111 and Harmon et al. 112 for wood decay rates in upland environments, which may apply to
wood on infrequently inundated floodplains or terraces.
In summary, wood naturally decays and physically breaks down in the stream environment. Unless it is fully
submerged or buried throughout the year, decay and breakage will significantly reduce wood mass and
potentially increase the likelihood of mobilization within a few years to decades after wood enters or is placed in
the stream. This fundamentally motivates the consideration of wood as a transient part of the river corridor, not
a permanent structure. If wood is used for a project with a design life over a few decades, then decay and
breakage should be considered. In many cases, an expectation should be set to either evaluate wood supply to
check that natural wood transport will replace lost wood over time or plan for subsequent artificial wood
additions as wood is lost.
Acknowledgements
Thanks to Abby Burk, Jackie Corday, and Ellen Wohl for motivating and providing helpful suggestions that
improved this memo.
WATERSHED SCIENCE & ENGINEERING · www.watershedse.com9 REFERENCES
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