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Driving east on Highway 36 Wilson Creek is located
between Yager Creek and Cummings Creek monitoring sites but is
actually just down the road from Yager Creek. Wilson Creek is
a tributary of Yager Creek, which drains into the Van Duzen River
in the lower alluvial plain, where numerous gravel mining operations
are visible. The Wilson Creek site is located adjacent Highway
36 and is in the town of Carlotta. The site is situated 40°
32.200' North and 124° 03.494' West, and its catchment area
encompasses approximately 5.11 miles of stream network. Elevations
in Wilson Creek range from 98 feet at its mouth to over 1,735
feet in the upper reaches, and the monitoring site is approximately
118 feet in elevation. The watershed is completely in private
ownership with virtually all timber holdings and timber harvests
conducted by Green Diamond (formerly Simpson Timber Company).
Most of these harvest plans were for clear cuts conducted on very
steep southern facing slopes of the watershed that were, unfortunately,
clearly visible from Highway 36. Vegetation is predominantly mixed
and evergreen (Redwood) forest on the hillsides, with abundant
pasture and agricultural land in the lower elevations. 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withdrawn for turbidity and suspended sediment analysis.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. Wilson Creek is
relatively small compared to other streams within the project
area, and the stream drains an area of approximately 4.574 square
kilometers which is equivalent to 1.766 square miles. During the
first year of sampling (HY07) volunteers recorded a maximum discharge
of 95 cubic feet per second (CFS) and an average discharge of
18 CFS, a maximum turbidity of 709 Nephlometric Units (NTU) and
an average turbidity of 91 NTU. In HY08, there was a maximum discharge
of 175 CFS and an average discharge of 34 CFS, a maximum turbidity
of 734 NTU and an average turbidity of 123 NTU over the winter
sampling season. Over the two seasons combined (HY07-HY08), the
maximum discharge was in HY08 at 175 CFS with an average of 26
CFS, maximum turbidity was also in HY08 at 734 NTU with an average
of 106 NTU. This translates to an average of 788 tons of suspended
sediment per square mile per year.
Wilson Creek does not run year round and is therefore
referred to as an ephemeral stream (as opposed to a perennial,
year round stream). As the stream runs dry in the summer, temperatures
during this time of the year were not measured. Wilson Creek has
the highest road density of any of the catchment areas within
the Lower VDR Basin at 10.4 miles of road per square mile of watershed.
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 42.8%, with clear cutting
accounting for 34.5% of the total watershed area.
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