Watershed-scale evaluation of Humboldt Redwood Company’s Habitat Conservation Plan timber harvest best management practices， Railroad Gulch， Elk River， California

The objective of this study is to test the effectiveness of California Forest Practice Rules and additional best management practices implemented as part of Humboldt Redwood Company’s Habitat Conservation Plan and Watershed Analysis prescriptions， in limiting the delivery of management-derived sediment. A paired watershed study format is being utilized to evaluate sediment sources: road surfaces， watercourse crossings， landslides， channel incision and bank erosion， and tributary channel head-cutting. The study compares the West Branch (1.48 km²， 365 ac) and the East Branch (1.28 km²， 314 ac) of Railroad Gulch， a tributary to the Elk River (152 km²) which flows into Humboldt Bay just south of Eureka， California. The watershed has been intensely logged in the past and is underlain by two erodible geologic terranes， the Pleistocene age Hookton Formation and the late Miocene to middle Quaternary age undifferentiated Wildcat Group. Forty seven percent of the East Branch was logged in the summer of 2016 under Timber Harvest Plan 1-12-110 HUM， with 32 ha (80 ac) of single tree selection， 18 ha (45 ac) of group selection， and 10 ha (24 ac) of no-cut zone left as buffer strips along Class I and II watercourses. A new native surfaced seasonal road was constructed in the summer of 2015. Several existing roadways were reopened during the same time period. All of these roads are appurtenant to the plan and were utilized for hauling throughout the summer of 2016. Cable yarding systems were used. No timber operations will occur within the West Branch， which will serve as the study control. Methods used to evaluate prescription performance include: pre-and post-construction road inventory and characterization， turbidity synoptic sampling during storm events， landslide inventories， channel cross-section surveys， pebble counts， continuous turbidity， stage， and rainfall monitoring， peak flow analysis， and an analysis of Beryllium-10 (Be-10) isotopes to estimate for long-term (e.g.， 1000 to 10，000 year) erosion rates. Data collection for the study began in earnest in 2014， with limited streamside landslide data collected in 2013. Sediment loads were well correlated between the two branches during an extremely dry water year (WY) and a below average water year. In WY 2014 the total annual suspended sediment load equaled 49 Mg (metric tons) km-² (0.22 t ac-¹) in the East Branch and 38 Mg km-² (.017 t ac-¹) in the West Branch. Loads were tenfold higher in WY 2015 in conjunction with a single large storm event which triggered several debris torrents in addition to several streamside failures. The WY 2015 total annual sediment load equaled 861 Mg km-² (3.8 t ac-¹) in the East Branch and 716 Mg km-² (3.2 t ac-¹) in the West Branch. Historically active debris slides and earthflows cover approximately 6 percent of the study basin. Five active upland failures have been detected during the project period; two in WY 2014 and three in WY 2015. Several of these active landslides are hydrologically connected and at selected sites were observed to strongly influence downstream turbidity. Cross-sectional surveys indicated that channel banks remained stable with limited thalweg scour between 2014 and 2015. Post-harvest monitoring will continue through 2019. Citation: Stubblefield， Andrew; Beach， Shane; Harrison， Nicolas M.; Haskins， Michelle. 2017. Watershed-scale evaluation of Humboldt Redwood Company’s Habitat Conservation Plan timber harvest best management practices， Railroad Gulch， Elk River， California. In: Standiford， Richard B.; Valachovic， Yana， tech cords. Coast redwood science symposium—2016: Past successes and future direction. Proceedings of a workshop. Gen. Tech. Rep. PSW-GTR-258. Albany， CA: U.S. Department of Agriculture， Forest Service， Pacific Southwest Research Station: 105-115