Driving east on Highway 36, Main Stem (MS) Rainbow Bridge is the eastern-most monitoring site of the 11 sites within the Van Duzen Watershed Project, and lies about one mile past Grizzly
Creek State Park. MS Rainbow Bridge is located on the main stem Van Duzen River, adjacent the USGS/DWR discharge gauging station, where project volunteers collected water samples during the course of the project. The site is situated 40 o 28.825’ North and 123 o 53.440’ West, and above the site, but within the project boundary is a relatively large catchment area with approximately 44.5 miles of stream network. Elevations in this basin range from 364 feet at the base to over 3,100 feet in the upper reaches, and the monitoring site is approximately 364 feet in elevation. The catchment area for MS Rainbow Bridge is completely in private ownership with a small amount of the timber holdings and timber harvests operated by Humboldt Redwood
Company (HWC, formerly owned by PALCO) in the lower part of the watershed. Most of the timber harvest activity in the catchment area is being conducted by various private operators, including ranchers and other landowners. As this area is beyond the alluvial plain favored by coastal redwood, vegetation is predominantly mixed and evergreen species dominated mostly by
Fir and some Pine forest. 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is sampled, and from which all rainfall and sediment are channeled into the stream down to the point where water is withdrawn for turbidity and suspended sediment analysis.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. Within the boundary of the project
MS Rainbow Bridge is the third largest of the 11 catchment areas, and above this point in the river, the catchment area drains approximately 62.6 square kilometers which is equivalent to 24.2 square miles. As samples are taken upstream from VDR Weares, the MS Rainbow Bridge catchment area is contained within (overlaps) the VDR Weares catchment area. During the first year of sampling (HY07) volunteers recorded a maximum discharge of 6,930 cubic feet per second (CFS) and an average discharge of 1,369 CFS, a maximum turbidity of 820 Nephlometric
Units (NTU) and an average turbidity of 72 NTU. In HY08, there was a maximum discharge of
20,600 CFS and an average discharge of 1,636 CFS, a maximum turbidity of 2,826 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 20,600 CFS with an average of 1,527
CFS, maximum turbidity was also in HY08 at 2,826 NTU with an average of 123.
The main stem Van Duzen River runs year round through most of the lower basin, and is therefore referred to as a perennial stream (as opposed to an ephemeral stream, which runs dry in the summer). However, in recent years, the stream has begun to go dry, or goes underground in the alluvial plain before it reaches the Eel River. These conditions are directly due to the vast deposits of bedload (sand, gravel, and cobble) that have been deposited and continue to be deposited in the river over time. Maximum weekly average temperature (MWAT) in the

summer of 2007 at MS Rainbow Bridge was 22.92 C, and in 2008 it was 21.75 C. Overall, temperatures were extremely high compared to all of the other streams sampled within the lower basin project area, higher than Yager Creek, which was the warmest of all tributaries sampled within the lower basin, and even slightly higher than VDR Weares downstream. MS Rainbow
Bridge catchment area has a road density of about 5.9 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 42.7%, with clear cutting accounting for
3.5% of the total watershed area. Similar to Yager Creek and VDR Weares, a large portion of the MS Rainbow Bridge catchment area (in the eastern higher elevations) is non-forest grassland and grazing land, which therefore makes it difficult to quantify the impact of timber harvest in the forested parts of the region which is primarily Fir forest.
Captions to Photos
VDR_Rainbow1: Looking at Rainbow Bridge from a short distance downstream. Photo by P.
Trichilo.
VDR_Rainbow2: Looking downstream at the Van Duzen River from Rainbow Bridge at high water after a major winter storm event. Photo by K. Bromley.
VDR_Rainbow3: Looking upstream at the Van Duzen River from Rainbow Bridge after a winter storm event. Photo by K. Bromley.
VDR_Rainbow4: Site of a temperature data logger placed in the stream near MS Rainbow
Bridge during the summer of 2008. Photo by P. Trichilo.