What are the history and trends of the sizes, distribution, and relative health and diversity of salmonid populations in the Big River Basin?
Both historic and current data are limited. Little data are available on population trends, relative health, or diversity. According to NOAA Fisheries Endangered Species Act listing investigations, the populations of salmonids have likely decreased in the Big River Basin as they have elsewhere along California and the Pacific Coast. Coho salmon in Mendocino County are currently listed as endangered under the California and federal Endangered Species Acts and steelhead trout are listed as threatened under the federal Endangered Species Act;
Based on limited CDFG, USFWS, HTC, MRC, and SONAR presence surveys and surveys documented by NMFS, the distributions of coho salmon and steelhead trout do not appear to have changed since the 1960s;
Steelhead trout were documented in more reaches surveyed by CDFG and MRC since 1990 than coho salmon;
◾Thirty tributaries, the mainstem Big River, and the estuary had records of coho salmon and steelhead trout since 1990. Twenty additional tributaries recorded only steelhead trout.
What are the current salmonid habitat conditions in the Big River Basin? How do these conditions compare to desired conditions?
Water temperatures at all seven monitoring sites along the mainstem of the Big River were unsuitable for salmonids;
Water temperatures in tributaries across the basin showed that temperatures were generally suitable for salmonids in the Coastal and Middle subbasins and mixed in the Inland Subbasin. Temperatures in the larger tributaries in the Inland Subbasin such as the North and South forks Big River were generally unsuitable for salmonids while temperatures in the smaller tributaries were suitable;
There have been very few water quality samples taken across the basin. Some sites show indications of exceeding NCRWQCB criteria for sodium, copper, specific conductance, total dissolved solids, aluminum, zinc, or boron. However, these findings are based on few sample sites and in some cases may be artifacts of the type of sampling procedure used.
Fish passage barriers have been identified in seven surveyed tributaries across the basin and several small tributaries along the estuary are blocked to fish passage by perched culverts;
Areas of dry channel found during CDFG stream surveys may indicate fish passage problems in some tributaries during periods of low flow.
Data collected in four tributaries in the basin indicated excessive amounts of fine sediment in the sub-0.85 mm and/or sub-6.5mm size classes, which would create unsuitable conditions for salmonids. However, much of the basin has not been evaluated for sediment delivery and deposition.
Canopy cover was suitable for salmonids on all surveyed reaches within the basin except for James Creek and the mainstem Big River. The mainstem Big River has a larger, broader channel and floodplain and is expected to have reduced canopy levels.
In general, a high incidence of shallow pools, and a lack of cover and large woody debris indicate simplification of instream salmonid habitat in surveyed tributary reaches and the estuary;
Cobble embeddedness values in many CDFG surveyed reaches were unsuitable for salmonid spawning success. Of surveyed pool tails, only 17.2% had cobble embeddedness less than 26%. In addition, the MRC characterized spawning gravels as fair quality on segments they surveyed;
Permeability sampling in four locations throughout the basin indicated low to moderate amounts of fine material. This could indicate suitable to somewhat unsuitable conditions for salmonid in these sample sites. Refugia Areas
Salmonid habitat conditions in the Big River Basin are generally best in the Coastal Subbasin tributaries where they have generally been rated as high potential refugia. Conditions in the Middle and Inland subbasins are mixed and generally rated as medium potential refugia.
What are the impacts of geologic, vegetative, fluvial, and other natural processes on watershed and stream conditions?
The geology of the Big River Basin is primarily comprised of Coastal Belt Franciscan Complex. This portion of the Franciscan complex is relatively stable compared to the mélange terrane of the Central Belt, which is found only in the upper parts of the watershed. A small portion of Tertiary age sandstone is found in the Greenough Ridge - Montgomery Woods State Reserve area (EPA, 2001);
The Coastal and Middle subbasins have much lower relief and longer slopes than the Inland Subbasin, which has a high percentage of area in higher slope classes;
Redwood and Douglas fir forests have historically and continue to dominate the basin. Additional vegetation includes tan oak, madrone, alder, bishop pine, pygmy cypress, willow, grass, oak, bay laurel, alder oak, and blueblossom. Pre-European forests consisted of mostly large old-growth trees;
A long history of wildfire has influenced the current vegetation of the Big River Basin, although the specifics of fire practices and history are unknown. However, fire was a natural and frequent occurrence. Prior to European settlement, the Mendocino Coast experienced a fire every 6-20 years during the last 200-400 hundred years (Brown 1999). In 1931, the Comptche fire swept across the eastern part of the basin, burning 10,733 acres, 9% of the basin.
The basin has experienced a variety of natural disturbances such as earthquakes, flooding, droughts, and decadal climate shifts. Examples include a moderate earthquake that originated about two miles south of the Albion Basin during the mid to late 1800s, another strong earthquake that originated near Fort Bragg in 1898, and the distant San Francisco earthquake in 1906. Earthquakes often trigger landsliding;
Landsliding has occurred across the entire basin. More landslides and more volume from landslides by area are found in the Inland Subbasin than the other two subbasins;
Many of the tributaries in the basin are intermittent in their upper reaches and usually have summer and fall flows of less than 1 cfs;
How has land use affected these natural processes?
Historic timber harvest activities reduced riparian canopy, 86% of the basin has experienced one or more timber harvests. However, canopy is currently suitable along most surveyed tributary reaches across the basin;
As a result of timber harvest, the current landscape is comprised of smaller diameter forest stands than in pre-European times (61% of trees in 75-100 feet wide watercourse buffer zones have diameter at breast height (dbh) less than 24 inches). The small diameter of near stream trees across the basin limits the recruitment potential of large woody debris to streams and contributes to the lack of instream habitat complexity;
Splash dam logging involving 27 splash dams across the basin before 1920 likely greatly accelerated erosion and widened stream channels across the basin. However, significant bed lowering along the lowermost reaches of Big River associated with splash dams is unlikely;
Post splash damming channels are deeply entrenched, cut down to bedrock in many places, lacking functional floodplains, and depleted of LWD and gravel;
Early splash dam and barrier removal projects, starting in the 1950s, cleared many streams across the basin of timber-related woody debris. The lack of instream complexity seen today likely results from these past practices;
A lack of LWD throughout the Big River Basin also allows sediment to move more quickly through the stream system and move downstream in greater quantities than pre-disturbance;
CGS found that channel narrowing, floodplain growth, and encroachment of forest vegetation on marshes seen since 1900 along the estuary is likely the result of a river channel reclaiming itself after the multiple decades of channel clearing, splash dam flooding, and battering by logs in transport;
Historic sawmill complexes on the Big River flats reduced wetland habitat;
Construction of near stream railroads in the Coastal and Middle subbasins and North Fork Big River and roads throughout the basin used fill that constricted stream channels and destabilized streambanks;
From 1937 to 2000 the rate of landsliding across the basin was 664.3 tons/square mile/year (approximately 332 cubic yards or 33 truck loads). Rates were highest in the Inland Subbasin, followed by the Middle and Coastal subbasins, respectively;
CGS photo mapping of stream channels in 1984 and 2000 found that negative channel features increased in the Mouth of Big River PW and decreased in the North and South forks Big River and Daugherty Creek, as expected between source and depositional reaches. The greatest reductions in negative channel features were seen in Daugherty Creek;
There has been a significant increase in road building since 1989 across the basin, especially in the Coastal and Middle subbasins. However, new roads have been built to higher standards, on ridge-tops, and are paved; thus creating less of a sediment source;
Roads and timber harvesting are listed in the NCRWQCB TMDL report as major sources of human-related sediment into the stream system. The effects from these activities are often spatially and temporally removed from their upland sources;
County culverts located on three tributaries in the Inland Subbasin have been identified as total salmonid passage barriers by a Mendocino County roads study. Additionally, perched culverts have blocked fish passage to small tributaries along the estuary.
The recent purchase of a large portion of the estuary and transfer to DPR for management as a park also will likely improve temperature and sediment conditions in the Coastal Subbasin as planned management improves roads and riparian zones.
Based upon these conditions trends, and relationships, are there elements that could be considered to be limiting factors for salmon and steelhead production?
Based on the information available for this assessment, it appears that salmonid populations are currently being limited by:
Low summer stream flows in tributaries in the Inland Subbasin;
High water temperatures in the mainstem Big River;
Fish passage barriers;
Embedded spawning gravels;
Reduced habitat complexity.
Flow and Water Quality Improvement Activities
To minimize and reduce the effects of water diversions, take action to insure compliance with state water laws to address seasonal diversion, off-stream reservoirs, bypass flows protective of coho salmon and other anadromous salmonids and the normal hydrograph, and avoidance of adverse impacts caused by water diversion;
Discourage instream flow diversions in tributaries with cooler water temperatures for thermal refugia delivered to the warmer North and South Forks and mainstem Big River in the summer;
Land managers should work to reduce the temperature of water flowing into the Middle and Coastal subbasins. In order to do this, they should maintain and/or establish adequate streamside protection zones to increase shade and reduce heat inputs to Big River and its tributaries throughout the basin;
Follow the procedures and guidelines outlined by NCRWQCB to protect water quality from ground applications of pesticides.
Consider modifying debris accumulations to facilitate fish passage where necessary;
Adequately fund prioritization and upgrading of culverts to provide fish passage within the range of coho salmon and to pass 100-year flows and the expected debris loads;
Erosion and Sediment Delivery Reduction Activities
To reduce sediment delivery to Big River, land managers should continue their efforts such as road improvements, good maintenance, and decommissioning and other erosion control practices associated with landuse activities throughout the basin. Thirty-six CDFG stream surveys had road sediment inventory and control as a top tier tributary recommendation;
Support and encourage existing and active road management programs undertaken by landowners throughout the basin;
Map unstable soils and use soil mapping to guide land-use decisions, road design, THPs, and other activities that can promote erosion;
Sediment sources from eroding streambanks and adjacent hillslopes should be identified and treated to reduce sediment generation and delivery to creeks;
Limit unauthorized and impacting winter use of unsurfaced roads and recreational trails to decrease fine sediment loads;
Develop erosion control projects similar to the North Fork Ten Mile River erosion control plan (Mendocino Department of Transportation 2001).
Riparian and Instream Habitat Improvement Activities
Improve instream structure for juvenile ambush escape and cover. Thirty-one CDFG stream surveys and the mainstem Big River have increase escape cover as a top tier tributary recommendation;
Add LWD to stream channels where appropriate/feasible to develop habitat diversity and to increase shelter complexity. In addition, there is a need to leave large wood on stream banks and in estuarine channels for potential recruitment into stream channels and the estuary;
Maintain and improve existing riparian cover where needed;
Encourage growth and retention of nearstream conifers;
Ensure that any land management activities include protection and preservation of stream and riparian habitats and maintain or improve ecological integrity within the basin;
Ensure that high quality habitat is protected from degradation. Salmonid habitat conditions in the Big River Basin are generally best in the Coastal Subbasin, and mixed in the Middle and Inland subbasins;
Consider the use of management strategies such as conservation easements to maximize potential benefits to aquatic habitats from near-stream forest protection.
Education, Research, and Monitoring Activities
State Parks, DFG, MRC, and HTC should continue and expand existing monitoring of anadromous salmonid populations to include some winter and spring fish sampling;
Support stream gage installations and maintenance to establish a long term record of Big River hydrologic conditions;
Additional investigations of the physical characteristics of Big River are needed to re-evaluate the Sediment Source Analysis. A regional curve of bankfull dimensions vs. drainage area should be developed for Mendocino County and used to validate CGS (2004) bankfull discharge estimates for Big River;
Hillslope and in-stream monitoring proposed by the MRC in their Watershed Analysis (2003) should be carried out and additional monitoring programs throughout the basin should planned with respect to to MRC techniques;
A study examining how sediment plugs moved downstream from historic splash dam locations over time on air photos is recommended;
Continue water temperature monitoring at current locations and expand these efforts where appropriate;
Further study of timberland herbicide use is recommended.