Lecture 5 - Restoration of LWD Flashcards
What size does in-stream wood have to be to be considered large woody debris?
> 10 cm in diameter and > 2 m long
How does large wood get recruited to a channel? (4)
Stream bank undercutting
Wind throw
Slope failures
Tree decay
What are the best trees for large woody debris? (3)
Cedar because they can last for ~200 years
Spruce and fir are next best
Why should large woody debris be a certain size?
The larger the wood, the more stable it is in the stream channel (long pieces can catch on the banks if moved and heavier pieces require higher flows for mobilization)
How does large wood influence habitat diversity and complexity? (6)
Provide slow back water areas during high flows and floods (prevents juveniles from being swept downstream to dangerous larger rivers)
Provides cover from predators and temperature
Large wood forms pools in lower order streams
Retains spawning gravels
Provides substrate for invertebrate prey
Helps retain nutrients in the stream (eg. Spawned salmon and OM)
What percent of wood is lost from streams per decade in the PNW?
10% per decade
When did it become illegal to log streams to their banks in B.C.? (2)
1988 on the Coast and 1995 in the Interior
How long will it take until B.C.’s rivers will return to large supplies of wood?
75-150 years
What are the consequences of decreased large wood? (5)
Loss of cover and structural complexity
Reduced cover protection from predators
Reduced varieties of current velocities and other hydraulic features
Less trapping of gravel and nutrients
Less habitat for salmonids and invertebrates
Why do younger forests has a higher rate of loss of large wood? (4)
May be second growth - trees are predominantly maple and alder which decay more rapidly
More trees might be recruited but the overall volume of wood is smaller because the trees aren’t as big
This makes them faster to decay (even decay-resistant wood) and highly mobile
How long would it take for large wood to recover to pre-logging levels?
> 250 years
What is the ideal situation rather than habitat restoration?
Habitat protection!
How is buried large wood important?
Buried large wood that has accumulated below the gravel surface provides important structure and nutrient supply to the subsurface hyporheic zone
How does large woody debris differ from small streams to larger rivers? (4)
In small (<10 m) and intermediate (10-20 m) streams LWD may be found in complex arrangements or individual pieces scattered along the channel
In larger streams (3rd to 5th order), LWD has less continuous accumulations but may have “drift” jams that block out the flow
These can redirect the stream course and create floodplain channel features that can become important overwintering habitat for juvenile salmonids
Rivers (5+ order) are characterized by infrequent, but occasionally massive accumulations of LWD
How do interior rivers serve in a reverse way to coastal rivers?
Off-channels are a refuge from high flows on the coast but the main channel acts as a refuge from freeze up in the winter in the interior
What are some negatives of too much large wood in a stream? (3)
When the presence of debris jams completely blocks upstream spawning migrations
Possible impairment of water quality
Presence of floatable debris pieces that can move during storms and pose threats to life, property, or aquatic habitat downstream
What stream orders have the most frequent disturbance regimes and thus the greatest habitat complexity?
Stream orders 3-5 have the greatest disturbance regimes and thus the highest effects of large wood and highest habitat complexity
When is suitable habitat generally limiting in streams? (3)
At the parr stage for species that spend more than one summer in freshwater
This is because fry defend much smaller areas (eg. 2%) than parr
Territory size required is proportional to fish length and body weight
What are some simple large wood designs for streams <20 m? (4)
Single root wads in the bank
Paired trees with root wad in between
Single log in-channel
Single log bank protection
What are some advanced designs of large wood? (4)
Full-spanning log jams (small streams only)
Lateral log-rootwad-boulder complexes
Woody debris catchers (eg. Triangular debris catcher)
Woody debris reefs
Triangular debris catcher (4)
Triangular debris catcher emulates nature by creating scour holes and collecting smaller pieces of large woody debris
Design uses the force of the river to load the structure against the stream bank, ensuring long-term longevity
Consists of 2 logs attached to trees or stumps on the bank, both ballasted by a common anchor boulder in the stream
More stable than single log
Where should advanced structures be placed? (2)
In geomorphic settings where there is either:
Historical evidence of log-jam structures
Where a systematic survey and experience says an area is suitable
What are the key forces acting on large woody debris in a stream? (3)
Water velocity
Drag
Buoyancy
What is the safety factor that had been built into design charts for large woody debris placement? (2)
Safety factor of 2.0 for the single log jam, and safety factor of 1.25 for the triangular log jams (because they are inherently more stable)
