Suitability Discussion

CWPAP staff assessed changes in Eastern Subbasin salmonid habitat using historic data collected on surveys from 1938-1990, and stream habitat typing survey data collected from 1990-1999 and 2000-2010. 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 (1934-1990), spawning habitat, invertebrate food, and shelter were good in Cedar, Grapewine, Rancheria, and Red Mountain creeks. High water temperatures were noted in Red Mountain Creek and in the mainstem and East Branch SF Eel River. Low summer flows were also mentioned in many Eastern Subbasin stream reports. Diversions were a concern and were noted beginning in the 1950s in Cahto, Mill, and Taylor creeks. Log jams and waterfalls were the most common barrier type, and many of the waterfalls were considered total barriers to fish passage.

Using recently collected (1990-2010) habitat typing data from Eastern Subbasin streams, canopy density suitability was generally good except in Tenmile, Milk Ranch, Mud, and Cahto creeks. Canopy density suitability did not change in most streams between the two time periods, except for slight decreases in Cahto Creek and substantial increases in Milk Ranch and Mud creeks.

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 and deciduous vegetation increased slightly in Milk Ranch, Mud, SF Bear Canyon, and Tenmile creeks over time.

Primary pool length decreased dramatically in nearly all Eastern Subbasin streams surveyed, and was in the lowest suitability category for nearly all streams during the 2000-2010 sampling period. Tenmile Creek was the only stream surveyed that showed improvement in the length of primary pool habitat over time.

Pool shelter was in the lowest suitability category in most Eastern Subbasin streams during both time periods. Pool shelter values were only suitable in Tenmile Creek in 1996. Both pool habitat and pool shelter are likely limiting factors in Eastern Subbasin streams.

Cobble embeddedness suitability increased slightly in most Eastern Subbasin streams over time, but was only in the highest category in Streeter Creek in 2009. 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 there were more Eastern Subbasin sites with poor stream temperatures than good or fair sites. Temperatures were good for salmonids in the mainstem SF Eel River and tributaries above Branscomb (RM 95), but were stressful for salmonids at downstream sites and in larger tributaries. Lethal temperatures were recorded in the mainstem SF Eel River at Piercy (RM 54) and Sylvandale (RM 25), and in the East Branch SF Eel River and lower Tenmile Creek. These streams are wide channels with little riparian canopy cover and increased direct solar radiation, resulting in higher stream temperatures than smaller, shaded streams. Stream temperatures are also higher in tributaries where water is diverted for residential use and 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 refugia areas with cooler water for salmonids during late summer months.

Sediment loading in the Eastern 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 high density of roads, 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.