Suitability Discussion

In historic surveys (prior to 1990), spawning habitat was generally good in Western Subbasin tributaries. High water temperatures were noted in the lower reaches of the mainstem SF Eel River. Log jams were the most common barrier type, but most were not classified as total barriers.

Average canopy density and embeddedness scores in Western Subbasin streams increased over time when comparing data collected in the 1990s with data from the early 2000s. Most primary pool length scores were in the lowest suitability category during both time periods, and pool shelter scores decreased slightly over time. Although some increases in factor values were seen, average values were below target values for all streams except canopy density in the 2000-2010 sampling period. Embeddedness, primary pools, and pool shelter are likely limiting to salmonid populations in this subbasin.

Canopy density was suitable on most surveyed creeks. However, overall canopy density measurements do not take into account differences between smaller, younger riparian vegetation and the larger microclimate controls that are provided by old-growth forest canopy conditions. CWPAP staff considered the contribution of coniferous and deciduous components in the canopy, and found that the average percent of coniferous vegetation increased and percent open canopy decreased considerably in most Western Subbasin streams over time.

Primary pool length was in the lowest suitability category for nearly all streams during both sampling periods. The headwaters of the SF Eel River was the only stream sampled that showed improvement in the length of primary pool habitat over time.

Pool shelter was in the lowest suitability category in most Western Subbasin streams, and in streams that were sampled during both time periods, shelter suitability decreased from suitable to unsuitable in 7 streams (Bear Wallow, Butler, Doctors, Huckleberry, Low Gap, Moody, and SF Redwood creeks). Both pool habitat and pool shelter are likely limiting factors in Western Subbasin streams.

Cobble embeddedness suitability increased in most Western Subbasin streams over time, and went from the lowest to the highest suitability category in Michaels and Doctors creeks. Embeddedness values increased throughout the Hollow Tree Creek drainage over time. This improvement is most likely due to changes in timber harvest regulations, road decommissioning, numerous restoration and instream habitat improvement projects completed in this basin, and sediment from historic floods moving through the system. Although embeddedness suitability scores increased in many streams, average values were still below target values during both sampling periods.

Summer water temperature measurements showed that water temperatures were good for salmonids in headwaters areas above Branscomb (RM 95), but were stressful for salmonids at downstream sites and in larger tributaries. Sampling sites in tributaries with poor temperatures were located in the lower reaches of the largest streams in the subbasin (Hollow Tree and Sproul creeks) and in the mainstem SF Eel River from RM 25-86. Lethal temperatures were recorded in the mainstem SF Eel River at Piercy (RM 54) and Sylvandale (RM 25). Lower Hollow Tree and lower Sproul creeks are wide channels with little riparian canopy cover, and increased direct solar radiation results in higher stream temperatures. Warm water temperatures in Redwood Creek (Redway) are caused by reduced riparian canopy and increased water diversion for residential use and industrial marijuana cultivation operations. Water temperature is likely a limiting factor for salmonids in surveyed streams in this subbasin, and cold water seeps where springs or tributaries enter the mainstem may provide important patches of cooler water for salmonids during late summer months.

Sediment loading in the Western Subbasin is extremely high, and primary input sources include natural landslides and earthflows, road erosion and failure, and logging related erosion from skid trails and road construction. This subbasin has a very high road density, and road decommissioning projects have resulted in decreased fine sediment input at most treated sites, however, considerable erosion control measures will be required to meet the established TMDL and loading capacity. Sediment loading and turbidity conditions may be limiting factors for salmonid production.