Synopsis
Large woody debris (LWD) in the Carmel River includes significant branches,
trunks, and accumulations of dead wood lying in, or near, the active
channel. LWD plays myriad roles in the life cycles of aquatic and terrestrial
biota. Its utilization in the biosphere ranges from being a substrate
for microbes to serving as shelter and resting places for turtles and
endangered Steelhead Trout. Because of its diverse roles at various
levels of the food chain, it also strongly influences the riverside
terrestrial environment as well. LWD is also considered to be a factor
in channel shape, bank stability, bridge safety, and aesthetics. Few
studies have been undertaken to analyze the amount, location, and function
of LWD in the rivers of coastal California.
We present the results of a comprehensive inventory of LWD on the lower
Carmel River undertaken in summer and fall of 2003. The precise position
of every piece of wood falling within the definition of LWD was recorded
and plotted using GPS/GIS technology or was plotted using high resolution
aerial photographs; each piece of LWD was then assessed in several ways
to improve our understanding of the biological and physical function
of wood in the river. These data are summarized in graphical and tabular
format, and as GIS layers in an accompanying ArcMap 8 GIS project.
Our 2003 survey did not include the reach from Via Mallorca to the
Carmel Lagoon. We use data from a pilot study conducted in 2002 to fill
in that data gap. We recognize that this approach might introduce small
errors if wood in this reach moved during the subsequent 2002-2003 flows.
We believe that the combination of relatively low peak discharges in
winter 2002-2003 and relatively large average size of LWD in this reach
lowers the risk of introducing great errors. We feel confident that
the combination of 2003 data from the majority of the river and 2002
data from the lowest reach provides a relatively complete snapshot of
the LWD in the Carmel River at the beginning of the 2003-2004 rainy
season.
The 2003 survey reached to within 5 km (3 mi.) of San Clemente Dam.
This upper section contains a great surplus of LWD. The great amount
of time that would be required to catalog the LWD at a comparable scale
to the rest of the river put this reach outside the limits of the project
budget. For this reason we focused our efforts on the lower 23 km (14
miles) of river.
We conclude the following points from the 2003 inventory.
---A comprehensive survey was completed on 23 km (14 mi) of the Carmel
River. In this section of river there are 471 occurrences of large wood
or large wood accumulations. This figure leads to an average frequency
of 20.5 significant LWD pieces (or accumulations) per kilometer of river
(36.7 occurrences/mi).
---The density is not evenly distributed; the density decreases downstream
at an average rate of approximately 0.8 pieces/km (p<0.01) and shows
marked non-linear variability.
----71% of LWD pieces are between 15 cm and 30 cm in diameter. 84% of
the wood is between 1.5 m and 6 m in length. There is an increase in
LWD size downstream from Via Mallorca.
----Most of the LWD in the Carmel River has no significant impact on
bed or bank stability. Over 95 % of the LWD has either no impact or
favorable impact, either protecting the banks from scour or fostering
pool formation in the bed. Only 73of the wood was adversely stimulating
bank erosion.
----77% of the naturally occurring woody debris tends to be aligned
either parallel with the bank or pointing downstream. Only 9% were angled
upstream and 12% were perpendicular to flow.
----50% of the occurrences of wood were associated with aquatic fauna
sightings, commonly with multiple taxa per sighting. Fauna included
steelhead trout, stickleback, crayfish, western pond turtles, red-legged
frogs, and bullfrogs.
----7% (35 pieces) of the LWD surveyed in 2003 had been deliberately
placed in the stream for management purposes.
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