Suitability Discussion

CWPAP staff assessed changes in Northern Subbasin salmonid habitat using historic data collected on surveys from 1938-1990, and stream habitat typing survey data collected from 1990-2010. Data from older surveys, collected prior to the establishment of a stream survey protocol (Flosi et al. 2010), provided a snapshot of the conditions at the time of each survey. Terms such as excellent, good, fair, and poor were based on the judgment of the biologist or scientific aid who conducted the survey. The results of these historic stream surveys were qualitative and were not used in comparative analyses with quantitative data provided by habitat inventory surveys collected beginning in the 1990s. However, the two data sets were compared to show general trends.

In historic surveys, spawning habitat was generally good in Northern Subbasin streams, but siltation and habitat destruction from past land use practices and flooding was noted following the large flood events in 1955 and 1964. Barriers documented on historical surveys were primarily log jams and landslide debris, with the same large gradient barriers (waterfalls) as those identified in recent habitat typing surveys.

Where habitat data were available from both the late 1990s and early 2000s, average embeddedness and canopy density scores in Northern Subbasin streams increased considerably, and most primary pool length and pool shelter scores increased slightly over time. Although some increases in these factor values were seen, average values were below target values for all streams and these habitat factors are likely limiting to salmonid populations.

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 in most Northern Subbasin streams over time.

Pool depth and pool shelter were well below target values, and suitability in most Northern Subbasin streams was in the lowest suitability category for both of these factors. Pool shelter suitability increased slightly in Bull and Salmon creeks, but primary pool habitat was lacking. Both pool depth and pool shelter are likely limiting factors in Northern Subbasin streams.

Cobble embeddedness suitability increased on nearly all Northern Subbasin streams when comparing habitat data collected in the 1990s and early 2000s. Embeddedness scores increased significantly on Ohman and Salmon creeks, where suitability in the 1990s were in the lowest category, and by the early 2000s were in the highest suitability category. Improvements in spawning habitat conditions are due to sediment from large historic flood events moving through the system, and to restoration activities designed to reduce erosion in streams throughout the subbasin.

Summer water temperature measurements showed that water temperatures were good for salmonids in headwaters areas near Branscomb, but were stressful for salmonids at downstream sites near the confluence with the Eel River. Many of the sampling sites in poor habitats were located in the two largest streams in the subbasin, Bull and Salmon creeks, and lethal temperatures were recorded in the mainstem SF Eel River. Mainstem Bull Creek has very little canopy cover and large amounts of sediment entering from upstream sites near Cuneo Creek, resulting in increased stream temperatures from shallow pools filled in with sediment, and increased direct solar radiation from reduced riparian cover and wide channels. Warm water temperatures in mainstem Salmon Creek are due to 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 Northern 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 temporary road construction. 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.