Gualala River Sediment

1
Gualala River Sediment Information (http//www.rrr
aul.org, data after Kris Gualala,
http//www.krisweb.com)
This chart shows the density of roads in miles
per square mile for all Gualala River Calwater
planning watersheds. The highest road densities
tend to be in the northern and western basins
where industrial timber harvest is the primary
land use. Road densities over three miles per
square mile are known to yield sediment levels
that impair salmonid spawning (Cedarholm et al.,
1984). The Gualala TMDL TSD (NCRWQCB, 2001) found
roads to be a major source of sediment.
CDFG habitat typing results are displayed in this
map as habitat frequency by length for the
western Gualala Basin. Note dry reaches in
Robinson Creek, Dry Creek, lower Rockpile Creek,
the Wheatfield Fork and the lower South Fork
Gualala below Big Pepperwood Creek. When sediment
supply is so high that pools fill or streams lose
surface flow, it diminishes carrying capacity for
salmonid juveniles.
This map shows the risk of shallow debris slides
in the Little North Fork Gualala River basin
based on the Dietrich and Montgomery (1998)
shallow landslide stability model (SHALSTAB).
Highest risk slopes show in red and orange
colors. The road network associated with logging
crosses unstable slopes, which then poses an
elevated risk of debris sliding. This pattern of
disturbance is linked to sediment contributions
from some of these locations in high intensity
storms particularly since 1995.
Long-term electrofishing data sets collected by
CDFG in the lower Little North Fork show samples
dominated by steelhead young of the year but with
yearling and two year old fish present. Coho were
present only in 1988, when hatchery coho were
being planted. Steelhead have decreased in the
lower Little North Fork index reach, particularly
since 1992 in number and density, but biomass has
not shown a similar trend. This is consistent
with diminishing habitat as the Little North Fork
aggraded and pools filled.
This chart shows a comparison of results of 1994
and 2001 habitat typing surveys in the North Fork
Gualala River, Little North Fork and Doty Creek.
The proportion of pools in Doty Creek greater
than two feet deep increased between 1994 and
2001, while the proportion of deeper pools
decreased in both the Little North Fork and North
Fork Gualala. The North Fork shows the most
significant change in terms of loss of fish
habitat, with 22 fewer pools deeper than four
feet and six fewer pools between 3-4 feet. This
high rate of pool filling suggests high sediment
supply between 1994 and 2001.
Clear cuts in unstable, inner gorge locations of
upper Little North Fork Gualala River in 1999.
Landslides, some with delivery to streams, are
highlighted with arrows. Disturbance of unstable
inner gorge areas has been recognized as a major
source of sediment contributed to streams (FEMAT,
1993).
This image shows the Little North Fork Gualala
River just upstream of the North Fork. This large
amount of gravel is plugging the Little North
Fork channel and reducing pool frequency and
depth. Hope (2001) also found that aggradation
was causing increased peak flows and a change in
ground water levels that were damaging to
riparian trees.
North Fork Gualala bulk gravel samples show that
fine sediment less than 0.85 mm rose to levels
unsuitable for coho and steelhead spawning in the
period from 1992 to 1997. The increase in fine
sediment levels, particularly in the Little North
Fork and McGann Creek indicate a substantial
recent supply of fines from upslope area in wet
years in this period. Data from Gualala Redwoods,
Inc.
This map shows the Dietrich and Montgomery (1998)
shallow landslide stability model (SHALSTAB) for
lower Rockpile Creek with timber harvests
over-laid. This pattern of disturbance increases
risk of sliding but removes large woody material.
Debris slides in un-cut areas deliver large
amounts of wood which buffer sediment impacts.