Geology

A series of marine terraces overlay the Salinian Block to create broad steps of coastal plains. Shown here are marine terraces north of the City of Santa Cruz, with the lowest marine terrace in agricultural production.

The character of streams is influenced by the local geology. A combination of geologic and hydrologic forces develops, maintains and sometimes destroys salmonid habitat. For example, while eroding sediments are needed to create spawning gravel, the amount and type of sediments entering stream systems can greatly affect stream dynamics. Certain types of land use such as farming and residential, commercial and industrial development, coupled with certain geologic conditions such as highly erodible soils, can result in large quantities of fine sediments entering streams, filling in deep pools, and covering spawning gravel. These sediments create shallower, more homogeneous environments that are not conducive for salmonid spawning or rearing. Natural geologic movement such as earthquakes may also impact stream conditions. Natural earth movement triggers mass wasting, smaller landslides, and debris slides. Understanding the geologic formations of the Central California Coast and the effects of human activities on geomorphic and hydrologic processes is necessary for successful watershed management and salmonid recovery.

Geologic Formations

San Mateo and Santa Cruz Counties are located within the southern Coast Range, part of a larger northwest-trending mountain range formed about 30 million years ago by the sea floor – the Pacific Plate – folding along the margin of the North American Plate. The resulting mountain range is composed primarily of crushed, crumpled, folded, and uplifted sea floor sediments. Today, the boundary between these two plates occurs within the Coast Ranges, with the Pacific plate moving northward at a rate of about 2–3 inches per year (Alt and Hyndman 2000). The boundary occurs near the ridgeline of the Coastal Range at the infamous San Andres Fault and along a system of parallel faults. In San Mateo and Santa Cruz Counties, these faults occur in the vicinity of the ridgeline of the Santa Cruz Mountains and the valleys below. Occasionally, sudden slips occur in these faults and produce damaging earthquakes such as the 1906 earthquake that destroyed San Francisco and the more recent 1989 Loma Preita earthquake. The Loma Preita earthquake caused substantial damage to Santa Cruz, San Mateo, and San Francisco Counties including triggering and reactivating numerous large land slides (Press and Siever 2000). For more information on California's coastal mountains, see this excerpt from the California Coastal Commission's California Coastal Resource Guide.

The United States Geological Survey provides geomorphology data (too many geologic units to list all of them here) for the entire United States. Such information can be helpful for assessing potential sediment production, which is a limiting factor to salmonid survival.

The geological formations to the east and west of the San Andreas fault system in San Mateo and Santa Cruz Counties differ in origin and composition: east of the San Andreas fault is the Franciscan Complex on the North American plate; and to the west is the Salinian Block, moving north with the Pacific plate (Alt and Hyndman 2000).

The Franciscan Complex: The Franciscan complex is one of two major groups of rocks that comprise the coast range (Alt and Hyndman 2000). The Franciscan Complex occurs in Santa Cruz or San Mateo Counties as an assortment of sedimentary rock originally deposited in seawater at different depths, times, and locations, mixed in with portions of the basalt ocean floor.

The Salinian Block: The Salinian block is the small piece of California that is moving northwest along the west side of the San Andreas Fault. The Salinian formation is slowly moving northward with the Pacific plate; the granite in Santa Cruz and San Mateo counties has traveled north from its source at the base of the Sierra Nevadas in Southern California or Mexico (Alt and Hyndman 2000). A basement of Salinian granite lies beneath sedimentary formations deposited in shallow seawater during the Tertiary period. The sedimentary formations are mostly soft and composed of marine sandstone, mudstone, and shale (Alt and Hyndman 2000). Although granite resists erosion processes, the overlaying sedimentary formations, including mudstone and sandstone, are prone to erosion and mass wasting.

Sea level fluctuations caused by ice ages during the past two million years coupled with uplift of the Salinian Block resulted in a series of marine terraces along the coast (Alt and Hyndman 2000). The marine terraces are now prime agricultural land in both San Mateo and Santa Cruz Counties. Alluvial deposits created the flat land between Watsonville and Marina. The land was a deep embayment at the end of the last ice age, but over the millennia, it was blocked by sand spits and filled with sediment deposited by the Pajaro and Salinas Rivers (Alt and Hyndman 2000). This alluvial land supports the extensive agriculture around Watsonville and the Pajaro Valley (see the Resource Use section for more information).

Sediments

Uplift and seismic activity greatly influence the geologic formation in the Coast Range. Here the layers of granite and mudstone have been uplifted and tilted. Erosion and mass wasting processes provide both granite and mudstone sediments to the stream system.

The Franciscan formation and the sedimentary formations overlaying the Salinian Block, especially the mudstones, sandstones, siltstones, and shale, are highly erodible and mechanically weak. These sediments have been further weakened through fracturing and shearing caused by frequent fault movements. Coastal uplift, deep valleys eroded by streams, and seismic activity in these relatively weak rocks result in landslides and other forms of mass wasting (Alt and Hyndman 2000). Although mass wasting is a natural process, the rate and source of sediment is affected by human disturbance within all of the watersheds in Santa Cruz and San Mateo Counties. The amount and type of sediment entering stream systems can greatly affect stream dynamics and may have detrimental impacts on salmonid populations (FISRWG 2000).

Siltstone and mudstone rocks fracture easily, so they tend to break apart during the sediment transport process. The break up of siltstones and mudstones releases fine sediment into the stream. These types of sedimentary rock have a lower value as a spawning substrate than igneous or metamorphic rocks and they are a source of fine sediment, which can be detrimental to salmonids. Therefore, even undisturbed watersheds dominated by sedimentary geology tend to produce lower quality salmonid habitat than a similar watershed dominated by either igneous or metamorphic geology.

The Pescadero-Butano Watershed Assessment (Environmental Science Associates 2004) makes the following conclusion of fish habitat in the Pescadero Creek watershed.

Overall, the condition of the watershed's fishery is good: adequate, but generally not excellent habitat exists throughout a relatively large area of the watershed that is accessible to anadromous fish.

Most of the tributaries of Pescadero Creek are dominated by siltstones, mudstones, and weakly consolidated sandstones. In contrast, San Vicente Creek is widely considered to have high quality salmonid habitat and its watershed has a high percentage of igneous and metamorphic geology. Thus, watersheds with a siltstone, mudstone, or weakly consolidated sandstone can provide adequate salmonid habitat, but are less likely to provide excellent salmonid habitat. On the other hand, watersheds with denser, more durable rocks from igneous and metamorphic formations tend to have a higher percentage of excellent salmonid habitat than sedimentary watersheds.

In addition, land management practices are more likely to lead to higher erosion rates in watersheds with siltstone, mudstone, and weakly consolidated sandstone geology than watersheds underlain by igneous or metamorphic formations.

References

Alt, D. , and D.W. Hyndman. 2000. Roadside Geology of Northern and Central California. Missoula, Montana: Mountain Press Publishing Company.

Environmental Science Associates. 2004. "Pescadero-Butano Watershed Assessment." Final Report, 248 pp. View document (PDF).

Federal Interagency Stream Restoration Working Group (FISRWG). 1998. "Stream Corridor Restoration: Principles, Processes, and Practices." Federal Interagency Stream Restoration Working Group (FISRWG). GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN 3/PT.653. ISBN-0-934213-59-3. View on-line document.

Press, F., and R. Siever. 2000. Understanding Earth. 3rd ed. New York: W. H. Freeman.

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