3.6 Large Woody Debris
Large woody debris plays an important role in streams by shaping channel morphology, storing sediment and organic matter, and providing habitat for aquatic species. Woody debris is also important in floodplains and riparian areas by providing cover for terrestrial and riparian-associated species such as small mammals and amphibians. The purposes of a LWD study are often to
- describe historical and current characteristics of LWD;
- evaluate how natural disturbances (e.g., fire, floods) affect LWD characteristics and processes under reference and current conditions; and
- identify the ongoing effects of hydroelectric projects and other land uses (e.g., forest management, roads, etc.) on LWD characteristics such as recruitment, storage, and transport.
The magnitude of project effects on LWD is a function of the amount of LWD trapped in project reservoirs, the potential mobility of that wood, and the distribution of potential depositional zones downstream.
A field inventory of LWD is often conducted in project reaches, and where feasible, in control reaches. Study reaches should be selected using photogrammetry and divide into sub-reaches based on channel confinement and channel gradient. Inventories commonly tally LWD (size criteria vary by geographic locale, relevant tree species, and study objective) that are wholly or partially within the bankfull channel. Detailed data on LWD characterization (e.g., decay class, recruitment mechanisms, association with jam, sediment storage, influence on channel morphology, habitat value) may be colleted for "key pieces" that are of sufficient size or shape to alter channel morphology or for all LWD in a study reach. Mapping LWD locations onto high-resolution, aerial photographs allows for spatial analysis of LWD characteristics.
- current and historical LWD inventories,
- LWD characteristics between project-affected reaches and control reaches, and
- current LWD inventories and published LWD data from streams with similar geographic locales, comparable size (bankfull width and/or drainage area), and comparable vegetation management histories
can be used to assess project effects on LWD frequency and volume.
Additional data on the volume of LWD removed from reservoir booms or intake structures on annual basis may be available from project operators. On larger reservoirs where expansive "rafts" of LWD may accumulate, aerial photographic analyses can be used to quantify the volume of LWD stored in reservoirs. In some cases where multiple series of aerial photographs are available over a relatively short time period, the annual change in LWD accumulation within a reservoir can be assessed.
Developing conceptual models of LWD dynamics based on geomorphic terrain, channel network position, and degree of project influence provides a framework for assessing project effects. Conceptual models typically characterize the predominate input mechanisms of LWD to a reach, the quantity of LWD stored, LWD residence time, and the primary ecological and geomorphic functions of LWD. As part of the conceptual model, a wood budget may be constructed to estimate LWD accumulation, transport rates, and volumes of LWD removed at project reservoirs on an annual basis. Conceptually, a wood budget uses a mass balance approach to analyze the input, output, depletion, and changes in storage of LWD in a channel network. Simplified wood budgets may be developed to estimate annual LWD recruitment, transport, and delivery rates under reference and current conditions. The wood budgets are intended to characterize long-term LWD dynamics and trends over several-decade time scales. The primary parameters of the wood budgets are LWD inputs from stream channel and reservoir margin hillslopes, and from the fluvial transport of the stream channel inputs to downstream reaches.
Listed below are advantages and disadvantages of large woody debris study approaches:
- Assuming similar methodologies were employed in historical surveys, comparing current field surveys with historical LWD surveys can provide a temporal and spatial analysis of how LWD changes through time and what effects the project has on LWD trends.
- Comparing control with project-affected reaches can help assess the project's effects on LWD.
- Conceptual frameworks of LWD dynamics facilitate assessing project effects on LWD and allow for integration of the LWD data into other relevant studies such as geomorphic and aquatic habitat studies.
- A wood budget can provide a quantitative metric for assessing how LWD loading changes through time as wells as quantify volumes removed from project reservoirs.
- Historical LWD data are often collected using a wide variety of methodologies and criteria, which makes comparing the data difficult.
- Finding representative control reaches for LWD field inventories can be difficult depending on the network position of the project facilities.
- Multiple land use practices often influence LWD dynamics, including timber harvest, road development, and fire suppression, which are often difficult to separate from a hydroelectric project's effects on LWD dynamics. This often limits comparisons with current field and historical field surveys as well as complicates finding representative control reaches with similar land use practices.
- Given the scope and time frame over which many LWD studies are conducted, it is unfeasible to monitor and collect many of the parameters that are necessary for calculating a wood budget. Thus, multiple assumptions must be made and data extrapolated from published reports on other streams in order to calculate a wood budget, which increases the uncertainty in wood budget results.
- LWD trends and dynamics are most influences by episodic events (i.e., large floods or fires) and often fluctuate on a multi-decadal to century time-scale, which can limit the ability of current and historical field surveys that may characterize 20 years or less to asses long-term trends or project effects on LWD.
The following references are recommended for additional information on LWD assessment:
Benda, L., D. Miller, J. Sias, R. Bilby, C. Veldhuisen, and T. Dunne. 2003. Wood recruitment processes and wood budgeting. Pages 49-73 in S. V. Gregory, K. L. Boyer and A. M. Gurnell, editors. The ecology and management of wood in world rivers. American Fisheries Society Symposium 37. American Fisheries Society, Bethesda, Maryland.
Benda, L., and J. Sias. 2003. A quantitative framework for evaluating the mass balance of in-stream organic debris. Forest Ecology and Management 172: 1-16.
Lassettre, N. S. 1999. Annotated bibliography on the ecology, management, and physical effects of large woody debris (LWD) in stream ecosystems. Prepared for California Department of Forestry, Sacramento. Department of Landscape Architecture and Environmental Planning, University of California, Berkeley.
Martin, D. J., and L. E. Benda. 2001. Patterns of instream wood recruitment and transport at the watershed scale. Transactions of the American Fisheries Society 130: 940-958.
Montgomery, D. R., and H. Piegay. 2002. Wood in rivers: interactions with channel morphology and processes. Geomorphology 51.
Naiman, R. J., E. V. Balian, K. K. Bartz, R. E. Bilby, and J. J. Latterell. 2002. Dead wood dynamics in stream ecosystems. Pages 23-48 in P. J. Shea, J. W. F. Laudenslayer, B. Valentine, C. P. Weatherspoon and T. E. Lisle, editors. Proceedings of the symposium on the ecology and management of dead wood in western forests. General technical report, PSW-GTR- 181. USDA Forest Service, Pacific Southwest Research Station, Albany, California.