Wilson Creek Catchment Area

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is sampled, and from which all rainfall and sediment are channeled into the stream down to the point where water is withdrawn for turbidity and suspended sediment analysis.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.

Approach to Wilson Creek. Photo by P. Trichilo.
Approach to Wilson Creek. (Photo by P. Trichilo)
Wilson Creek after a mild Spring storm event. Photo by P. Trichilo.
Wilson Creek after a mild Spring storm event.
(Photo by P. Trichilo)

Looking downstream at Wilson Creek. Photo by P. Trichilo.
Looking downstream at Wilson Creek. (Photo by P. Trichilo)

Wilson Creek monitoring site during the dry season.
Wilson Creek monitoring site during the dry season.
(Photo by P. Trichilo)

Friends of the Van Duzen River
PO Box 315
Carlotta, CA 95528