- Large woody debris (LWD) consists of boles, root wads and large branches that
enter the stream through a variety of processes including senescence, wind throw, bank
failure, fire, and debris torrents brought on by flood events. Within the stream, LWD
provides significant benefits to aquatic organisms by contributing nutrients, providing
cover, and influencing stream flow to create and maintain quality habitat.
Harvesting of trees in riparian zones impacts the long term availability of LWD.
Effective management of forest resources adjacent to a stream should be planned in
accordance with a thorough understanding of the entire watershed. Knowledge of how
LWD is delivered and distributed in a particular type of stream can better enable forest
and stream resource managers to predict how such a stream would respond to
management activities within the riparian zone. Those who wish to, may then make
more informed decisions to provide for long term input of LWD to the streams.
The study of LWD in streams has become widespread over the last two decades,
but few projects have examined the delivery and distribution of LWD from a
quantitative perspective. This study reports the results of a descriptive quantitative
analysis of the delivery and distribution of LWD in Cummins Creek, OR. The study
examines how fluvial and hillslope processes compare as mechanisms to deliver LWD
to the stream while describing morphological characteristics of the channel associated
with the deposition of LWD.
Fluvial delivery is responsible for a substantial volume of LWD at discreet
locations along the mainstem, most notably at limited tributary junctions. Though
many pieces of wood may be delivered to their present positions by fluvial means, such
wood tends to be aggregated or situated outside of the active channel where it fails to
adequately interact with stream flow to create and maintain aquatic habitat. This
fluvially transported wood is more likely to stabilize within the active channel, however,
when it is trapped by larger pieces of LWD introduced to their current position in the
stream by hillslope processes.
Wood delivered to its present position by hilislope processes tends to be larger,
making it more resistant to fluvial transport. The stability of this non-transported wood
and its random dispersal along the mainstem makes it more effective in providing long
term benefits associated with LWD at more numerous locations on the mainstem. Most
of this wood is associated with pools, suggesting a relationship between stable LWD
and desirable aquatic habitat. Efforts to maintain a broad distribution of LWD in a
stream over time should begin with a management scheme designed to protect enough
trees in the immediately adjacent riparian zone to provide for continual input of stable