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Driving east on Highway 36, Grizzly Creek is located
between Hely Creek and MS Rainbow Bridge monitoring sites but
is actually a much closer to MS Rainbow Bridge. Grizzly Creek
is a direct tributary to the main stem Van Duzen River, merging
with the main stem in the heart of Grizzly Creek State Park, very
near to where Highway 36 crosses the stream. The monitoring site
is located in the redwoods with a sizable buffer zone of hardwoods
and conifers within the riparian zone around the stream. The site
is situated 40° 29.231' North and 123° 54.375' West, and
above the site is a catchment area with approximately 48.8 miles
of stream network. Elevations in Grizzly Creek range from 341
feet at its mouth to over 3,450 feet in the upper reaches, and
the monitoring site is approximately 350 feet in elevation. The
watershed is split between the state park where the stream meets
the main stem Van Duzen River and private ownership in the upper
timber holdings and timber harvests operated primarily by Humboldt
Redwood Company (formerly owned by PALCO), but also by Sierra
Pacific, Green Diamond (formerly Simpson Timber), and several
private landowners. Vegetation is predominantly mixed and evergreen
(Redwood) forest throughout. The catchment area of a monitoring
site is important when considering upslope factors that affect
water quality in the stream. This area represents the true watershed
that lies above the point where water from the stream is sampled,
and from which all rainfall and sediment are channeled into the
stream down to the point where water is withdrawn for turbidity
and suspended sediment analysis.
Stream discharge is directly proportional to the
size of the catchment area - the greater the area, the more water
is carried by the stream during storm events. The Grizzly Creek
catchment area is a bit larger than average compared with the
other areas within the project area. The stream drains an area
of approximately 49.1 square kilometers which is equivalent to
about 19.0 square miles. During the first year of sampling (HY07)
volunteers recorded a maximum discharge of 1,051 cubic feet per
second (CFS) and an average discharge of 196 CFS, a maximum turbidity
of 1,000 Nephlometric Units (NTU) and an average turbidity of
87 NTU. In HY08, there was a maximum discharge of 1,937 CFS and
an average discharge of 236 CFS, a maximum turbidity of 3,220
NTU and an average turbidity of 160 NTU over the winter sampling
season. Over the two seasons combined (HY07-HY08), the maximum
discharge was in HY08 at 1,937 CFS with an average of 221 CFS,
maximum turbidity (HY08) was 3,220 NTU with an average of 133
NTU. This translates to an average of 1,124 tons of suspended
sediment per square mile per year.
Grizzly Creek runs year round at the upper monitoring
site, and is therefore considered to be a perennial stream (as
opposed to an ephemeral stream, which runs dry in the summer),
although it very nearly runs dry near its convergence with the
main stem during late summer months. Maximum weekly average temperature
(MWAT) in the summer of 2007 was 17.69 C, and in 2008 it was 16.65
C. Overall, temperatures were somewhat high when compared to other
streams sampled within the lower basin project area, and higher
than both Hely Creek and Cummings Creek, which typically registered
the coldest temperatures of all the streams in the lower basin.
Temperatures at Grizzly Creek were even higher than Wolverton
Gulch. Grizzly Creek has a road density of about 6.9 miles of
roads per square mile of watershed. As with all of the catchment
areas within the lower basin, this density of road networks receives
a rating of extremely high. In the 17-year period from 1991 through
2007, the proportion of the area harvested for timber equaled
43.4%, with clear cutting accounting for 17.7% of the total watershed
area.
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