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).