Assessment Components

Climate

The Mediterranean climate of the Big River Basin is characterized by a pattern of low intensity rainfall in the winter and cool, dry summers with coastal fog. Temperatures range from 20 to 100°F. Mean annual precipitation for the basin is about 50 inches and varies from about 38 inches at Fort Bragg near the western margin of the basin, to over 80 inches at the northeastern edges. Rainfall maps for the basin indicate that although annual precipitation generally increases as one moves towards higher elevations along the north and east parts of the basin, there are areas in the Inland Subbasin that are considerably drier (GMA 2001a). The North Fork drainage is noticeably wetter than either the South Fork or headwaters drainages. Precipitation is highly seasonal, with more than 97% falling between October and May. Snowfall occurs occasionally in the higher elevations of the basin but rarely accumulates. Snow does not have any appreciable effect on the basin’s hydrology.

Geology

The Big River Basin is mainly located on the coastal side of the Mendocino Range, which is the western-most mountain range of the northern California Coast Ranges Geomorphic Province. The topography of the basin varies from a relatively flat estuary and uplifted terraces, forming part of the Mendocino plateau (Fox 1983) on the western most portion, to the mountainous interior and eastern portion of the basin. The more subdued terrain of the western step-like marine terraces merges with the sharply dissected interior to the east. Erosional remnants of the plateau appear in the basin as scattered flat ridge tops and approximately accordant summits. Elevations range from near sea level in the western portion of the basin stepping up through a series of uplifted marine terraces to approximately 2,725 feet in the mountainous eastern portion.

The rocks of the Coast Ranges formed in deep ocean bottom and continental slope environments between about 140 and 28 million years ago (Harden 1998). Oceanic sediments and volcanic rocks were accreted to North America along the tectonic subduction zone that was present at that time (Blake and Jones 1974, 1981). The irregular folding and faulting of the rocks during this period of tectonic mixing created the resultant irregular relationship between varying rock types that is typical of the Franciscan Complex. Portions of the Franciscan Complex with similar geology are grouped into belts and further subdivided into terranes. The Coastal Terrane (broken formation) of the Cretaceous-Tertiary Coastal Belt of the Franciscan Complex forms the bedrock under most of basin with the eastern most portion composed of the more pervasively sheared and disrupted Jurassic-Tertiary Central Belt Franciscan mélange. Central Belt terrain generally underlies topographically subdued grassland or open forest. The Franciscan rocks are overlain by Tertiary marine sandstone in the southeastern portion of the basin.

Bedrock is locally overlain by surficial materials of marine and river terrace deposits, estuarine deposits and alluvium related to modern channel deposits, landslides, and beach and older dune sands. Several levels of alluvium and terrace deposits, present most notably in the western part of the watershed, and remnants of the Mendocino plateau in the interior indicate that much of this watershed has been uplifted relatively recently. This, coupled with the relatively flat, staircase like arrangement of terraces, incised preexisting drainages and u-shaped valleys indicate an early stage of maturity for the western portion of the watershed grading in to a fully mature topography on the eastern portion of the basin (Kilbourne 1986).

The geology and regional tectonics directly influence the nature of the slopes and the types and rates of landslides present. Landslide features are widespread in the watershed. The dominant form of mass wasting varies depending on the composition of the underlying rock. Generally, the Coastal Terrane Franciscan Complex has a greater clay component in the western part of Big River Basin than farther to the east. The degree of penetrative shearing is also more intense to the west. Finally, the cessation of watercourse incision due to sea level rise has more of an effect near the mouths of the streams than in the headwater areas. As a consequence, the slopes in the western part of the basin are less steep with more mature topography than they are to the east. Deep-seated rockslides (rotational/translational landslides) are more common in the middle and eastern portion of the basin than in the western most portions. Additionally, earthflows are more abundant in the eastern part of basin (underlain by mélange terrane) when compared with the areas to the west.

Faulting, Seismicity, and Regional Uplift

The Big River Basin is located along the coastal side of the Mendocino Range, which lies along the active boundary between the Pacific and North American plates. The Pacific plate is moving northwards at a much faster pace than the North American plate, which is moving northwest. At present, most movement between the plates consists of the plates sliding past one another. The plate boundary also has a component of convergence - along which a series of northwest trending mountain ranges and active fault zones have developed. The primary active fault zone along the plate boundary is the San Andreas fault located approximately four miles west of the mouth of Big River. This fault is a right-lateral strike slip fault and has been calculated to move 50 millimeters a year over the past three to four million years. Active uplift of the Coast Range continues at a rate of approximately 30 centimeters per 100 years in the Big River area (CGS 2004).

Hydrology

The Big River is a mid-sized coastal river with a catchment area of approximately 181.1 square miles. The mainstem becomes a fourth order stream downstream of the confluence with the North Fork Big River in the Middle Subbasin and most tributaries are intermittent or first or second order. North Fork Big River, South Fork Big River, and Daugherty Creek are third order streams.

Data from the Big Basin show that high flows during storms are of short duration, usually one to two days at most, and flows rapidly return to typical winter base flow within one week of peaks. Almost all significant runoff events occur between December and March (GMA 2001a).

Flow duration analysis indicates that the South Fork Big River only exceeds 162 cfs 10% of the time, or 36 days per year on average, while 50% of the time flows are below 10 cfs. Flows exceed 850 cfs in the South Fork Big River only 1% of the time, or 3.6 days per year on average. It is thought that relatively little sediment transport occurs below 400 cfs, thus all of the geomorphic work accomplished by the river occurs in less than 5% of the time, with most concentrated in the top 1% of the flows (GMA 2001a).

Fluvial Geomorphology

CGS mapped and compiled fluvial features in several major channels within the Big River Basin from 1984 and 2000 air photos. General improvement between these years in the mainstem of the Big River, the North Fork, the South Fork, and Daugherty Creek were noted. Improvement was indicated by an overall net decrease in streamside erosion and accumulated bedload sediment. In spite of overall improvement, lower gradient reaches of the lower mainstem channel and estuary deteriorated, gaining elevated sediment. These are sites of accumulation and presumably aggradation. CGS (2004) found that deposition in estuary reaches is likely related to stream channels re-adjusting to a more natural discharge regime after the effects of splash damming.

Vegetation

Prior to large scale timber harvest starting in the mid-1800s most of the Big River Basin supported mature coniferous forest, though original stands exist only in small areas today. Currently, redwood forests dominate the basin, but give way to Douglas-fir and oak woodlands in the upper elevations. Redwood in the Big River Basin typically occurs with Douglas-fir as a stand component, rather than occurring in pure stands. The Coastal Subbasin has the highest percentage of area in redwood-Douglas-fir stands (91%) and the Inland Subbasin has the least (68%).

Douglas-fir does occupy some pure stands and, in an inverse ecological trend to redwood, the range is from none in the Coastal Subbasin to 13% of the area in the Inland Subbasin. In the Coastal and Middle subbasins the redwood-Douglas-fir type is predominant, but in the Inland Subbasin, redwood occupies the lower portion of the gulches and changes to drier species such as Douglas-fir and the oaks and grasslands up slope. Overall, hardwoods occupy about 20% of the basin and grasslands about 4%. Blueblossom (Ceanothus spp.) and pampas grass are found in the Coastal and Middle subbasins and are usually a result of landscape disturbances.

Small sized trees that average 12-24 inches diameter at breast height (dbh) cover 62% of the basin. Stands that average greater than 24-inch dbh trees cover 31.3% of the area, pole-sized trees cover 5.5%, and sapling-sized trees cover 0.9%. The Coastal Subbasin has the most acres of stands that average greater than 24-inch dbh trees, which may be a result of higher year-round precipitation. Most of the basin has a crown canopy density of over 80%.

Land Use

The Big River Basin is dominated by private land holdings, three of the top five of which are owned by timber companies (MRC, Strategic Timber Trust, Hawthorne Timber Company) for a total of 29% of the basin. These companies are actively involved in managing the forest for silviculture. Weger is a family owned interest that also actively manages their forestland and is largest of the small landowners at 3% of the basin. Hawthorne Timber Company completed a land sale to the California State Parks system in 2002 creating the new 7,342-acre Big River State Park. State Park lands now comprise 7% of the basin. Jackson Demonstration State Forest (JDSF) occupies 19% of the basin. JDSF is owned and managed by the State of California for the purpose of demonstrating forest management principles, recreation, and environmental conservation. It was acquired by the state from Caspar Lumber Company after much of the old growth had been harvested. Fifteen percent of the basin is owned privately in parcels varying from 40 to 1500 acres; 2% of the basin is in small private lots of up to 40 acres. Other than the town of Mendocino, there is relatively little human occupation in the watershed, with only scattered ranches and residences. Most of the smaller parcels are in the upper or east end of the basin and are dominated by grass or shrub lands.
Hells Gate Splash Dam
Figure 5: Hells Gate Splash Dam on the South Fork (1912).
Photo provided courtesy of the Mendocino Historical
Society and the Held Poage Memorial Home and
Research Library (from the Collection of Robert Lee).

Splash Dams
Construction of splash dams began in about 1860 and continued through 1924. Some remained in use through 1937 (Jackson 1991) when the last raft of logs was floated down the North Fork Big River. The dams varied in size and construction methods, but ranged to as tall as 40 feet.

The last dam in the Big River was destroyed when it was burned by CDFG in 1972 or 1973 (Escola 2001). This was the Johnston dam on the upper mainstem Big River under Williams Peak. At the pleading of Escola, this dam had been preserved by the Resources Manager for Willits Redwood Company (present day Strategic Timber Trust) from destruction when they surveyed in a new road. Instead, WRC located the road above the dam. This dam was unique in that each joint in the construction of the dam had been ensured via mortise and tenons, or wood pins, so that the dam could later be easily disassembled and the logs transported to the mill and manufactured into lumber.

Effects of Historic Splash Dams
Splash dam logging was used extensively throughout the Big River Basin. The basin had 27 splash dams. When river flows were high during the winter season, dam flood gates were opened and the flood flows moved downstream and picked up logs that had been stacked in stream channels downstream. At some sites, logs were stored in the reservoir and released along with the water. Many of the dams were designed to operate in a synchronized fashion to maximize the flow of water in downstream reaches. The transport of logs downstream was called a log drive and usually occurred once per winter (GMA 2001a).

Before water was released from dams, the stream channels downstream from the dam all the way to the estuary were cleared of all obstructions and debris. Sometimes, logs moving downstream did get jammed, and one such jam on the Hellsgate reach of the South Fork Big River lasted for several years before it was cleared up. Most jams were quickly cleared, however (GMA 2001a).

These splash dam activities had a large impact upon stream channels across the Big River Basin that can still be seen today. South Fork Big River is heavily incised from flushing logs. Escola described the flushing of logs as intense snapping, popping and loud booms. In the fork where Anderson and Mettick Creeks come together, there resides a large boulder gouged by the pounding of the logs as they were flushed down the river. The Big River was “beat up the worst” (Escola 2001) of any of the coastal rivers due to the 80 years of driving logs down it.

Studies in the nearby Caspar Creek watershed of the effects of splash dams on channel geometry found post-splash damming channels to be deeply entrenched, cut down to bedrock in many places, lacking functional floodplains, and depleted of LWD. The lack of LWD is also allowing sediment to move more quickly through the stream system and thus reach the estuary in greater quantities than pre-disturbance (Napolitano 1996, 1998 as cited in GMA 2001a). Channels within the Big River Basin share these characteristics (GMA 2001a). Another common effect of splash dam logging was displacement of main-channel gravels during log drives (Sedell et al. 1991).

Fish Habitat Relationships

Flow/Water Quality

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

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.

Erosion/Sediment

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.

Riparian Condition

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.

Instream Habitat

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;

Gravel/Substrate

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.

Fish History and Status

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